Pen-Mating Female Pigs: Problems and Possible Solutions

1. CHECKLIST SUMMARY OF PROBLEMS AND MANAGEMENT IDEAS
to increase the odds of successful reproductive performance when pen-mating

 

 

2. INTRODUCTION

 

Pen-mating, one of three mating systems available to pork producers, (Table 1) is the process of putting boars and females in the same pen for a designated period (u sually 21 to 42 days) using an unsupervised mating process. Scientific experiments have evaluated the influence of penmating on reproductive performance. Although pen-mating is utilized as a labor-saving strategy for breeding management, farrowing rate (number of females farrowed per number of females exposed) of females penmated at first estrus after weaning varies substantially (Table 2). A comparison between breeding pens in Table 2 cannot be made because of differences in the age of boars, number of boars per pen, and number of sows in each pen.

 

It is important to have a high farrowing rate because farrowing rate is significantly correlated with the number of live pigs born per bred female per year. An analysis of hand-mating data from North Carolina State University’s Swine Development Center found that the number of live pigs born per litter increased by 0.5 piglets per each 10 percent increase in farrowing rate (Figure 1). An analysis of hand-mating data gathered in England found that the number of live piglets born per litter increased by 1.3 piglets per each 10 percent increase in farrowing rate. Many times, a decrease in farrowing rate and/or litter size born live occurs because various biological and sociological events in the pen-mating environment cannot be adequately controlled or managed.

 

While it is recommended that pork producers use hand-mating and/ or artificial insemination, we recognize that some producers elect to use pen-mating. A survey conducted by the United States Department of Agriculture during 2006 found that 62.5 percent of sows and 75.4 percent of gilts were pen- mated on farms with fewer than 250 sows (Table 3). This publication discusses various pen-mating issues and their possible solutions.

 

3. COMMON PEN-MATING ISSUES

 

To ensure the best results from pen-mating, it is important to understand the biology and behavior of boars and sows while they are involved in a pen-mating system. The key issues are:

• Accumulation of estrous females
• Boar fertility
• Boar mounting efficiency
• Inadequate sexual behavior of boars
• Improper time mated
• Females not being mated
• Injured females
• Multi-sire competition for a sexual partner
• Inadequate production records to use to make appropriate decisions

 

Table 1. Comparison of mating systems for swine.

 

Table 2. Farrowing rate of sows from first estrus after weaning when pen-mating on a commercial farm in Nebraska (D. G. Levis, unpublished data).

All females were weaned into a boar pen and remained in the pen with the boar until farrowing.

 
3.1 Accumulation of estrous females
The best method available to shorten the duration of time spent breeding sows is batch weaning, which can be thought of as the “master control switch” for a swine operation. However, weaning all sows on the same day can result in several sows being in estrus on the same day. After a group of females is weaned, management has essentially no control over the cycling pattern of the female group. Currently, there is no U.S. Food and Drug Administration approved product to control estrus in weaned sows. The following three factors have major influence on the number of females in estrus on each day after weaning: (1) the number of sows weaned and placed in one breeding pen on the same day, (2) the total number of sows coming into estrus on each day after weaning (weaning-to-estrus interval), and (3) the duration of estrus for each sow. Duration of estrus is the number of hours each estrous female is receptive to boar stimuli. These three factors cause estrous sows to accumulate on various days/hours after weaning (Figure 2).

 

3.1.1. Weaning-to-estrus interval. The majority of recently weaned females should cycle four to seven days after weaning (Figure 3); however, the distribution of females first detected in estrus on each day after weaning can be quite variable (Figure 4). The following can influence the weaningto-estrus interval: genetics, parity, amount of feed consumed during lactation, season, length of lactation, suckling frequency of piglets, photoperiod, ambient temperature, body condition at weaning, and mycotoxins in the feed. As the weaning-to-estrus interval increases from three to five days, the average duration of estrus decreases (Figure 5).

Table 3. Percentage of sows and gilts pen-mated according to number of sows and gilts on inventory (USDA, 2007).

 

Table 4. Variation in duration of estrus ( Steverink et al., 1999).

 

 

 

 

 

 

3.1.2. Duration of estrus. Table 4 indicates that a substantial amount of variation occurs in the duration of estrus. Figure 6 depicts the frequency distribution for the percentage of females standing for 8 to 152 hours . Although the weaning-to-estrus interval influences the duration of estrus, there is a substantial amount of variation in the duration of estrus within a specific weaning-to-estrus interval (Figure 7). In Figure 7 the duration of estrus ranged from 24 hours to 72 hours for a group of sows with a weaning-to-estrus interval of 4.5 days.

 

3.1.3. Time from onset of estrus to ovulation. Research has shown that as the weaning-to-estrus interval increases, the duration of estrus decreases and the time from onset of estrus to ovulation decreases. However, a substantial amount of variation occurs in the time of ovulation after onset of estrus within a specific duration of estrus. As indicated in Figure 8, the time of ovulation after onset of estrus ranged from 22 hours to 46 hours for a group of sows, with duration of estrus of 56 hours . Optimal fertilization of ova results when females are mated O to 24 hours before ovulation. When females are mated outside this range, as can easily occur when pen-mating females, the fertilization rate of the ova decreases, thus reducing litter size.

 

By combining the weaning-to-estrus interval with the duration of estrus effects, it is easy to see how in pen-mating situations the number of females in estrus accumulates daily during the breeding period. Figure 2 illustrates how the number of females in estrus accumulates on each day when 20 females are weaned on the same day and placed in one breeding pen. It is possible to have 75 percent of the weaned females in estrus on day six after weaning. Although this pattern varies between each group weaned and between farms, the principles shown can be applied to any weaning system to estimate the number of females in estrus on each day.

 

3.2. Boar fertility

 

When the number of estrous females accumulates as shown in Figure 2, overworked boars may result in a boar fertility problem. Some overworked boars have a decrease in sexual behavior and do not breed very many different females. Another possibility is that an aggressive boar may continue to breed females after his sperm supply is substantially reduced or depleted.

 
Boars need sexual rest for sperm replenishment because sperm output decreases rapidly. A research study indicated that after five days of sexual rest and the boars were collected every 24 hours (Figure 9):

• The second ejaculation contained 46.9 percent fewer sperm cells than the first ejaculation.
• The third ejaculation contained 51. 7 percent fewer sperm cells than the first ejaculation.
• The fourth ejaculation contained 55.8 percent fewer sperm cells than the first ejaculation.
• The fifth ejaculation contained 67.9 percent fewer sperm cells than the first ejaculation.
• The sixth ejaculation contained 80.2 percent fewer sperm than the first ejaculation.

 

Generally, when on a 12- or 24hour mating interval, the sperm output number tends to stabilize after five matings. The important question is, “What level of sperm output is being reached at stabilization – fertile, subfertile, or infertile?” It is believed 3 to 6 billion motile sperm are needed to adequately fertilize ova. However, the number of motile sperm required for good fertility varies from boar to boar.

 

 

 

 

 

Table 5. Fertility of sows after a natural mating by boars ejaculating either one or four times per day (Paquignon et al., 1984; Nowak et al., 1988).

 

The major problem with penmating is that boars do not pace themselves to mate females on a 12- to 24-hour interval. For example, an aggressive boar may mate the first estrus female five times during the first 24 hours he is in the weaned female pen. Obviously, a boar that has mated five times the first day will have a substantial reduction in sperm count for the remaining females he will mate (Figure 10). The pen-mating example indicated in Figure 10 weans four females on the same day and houses the females with one boar that is not rotated for sexual rest. It is assumed that onset of estrus occurs on Monday (one female), Tuesday (two females), and Wednesday ( one female) after being weaned the previous Thursday. The assumed duration of estrus is the same as indicated in Study 1 of Figure 5.

 

If the boar only mates once every 24 hours after the first five matings, there is a risk that some females will: ( 1) not be mated because there is more than one female in estrus on Tuesday, Wednesday, and Thursday, (2) only be mated once, (3) be mated at the wrong time with respect to time of ovulation, especially, if only mated once, and (4 ) be mated more frequently than other females.

 
If the boar maintains a high level of sexual activity and mates estrous females numerous times, his fertility will be substantially diminished. The effect of boars mating too frequently on decreasing farrowing rate was clearly demonstrated by scientists in France. The researchers controlled the mating frequency by using hand-mating. Boars that only mated once per day had a higher pregnancy rate compared to boars that mated four times per day (Table 5). Mating frequency did not influence embryonic survival. It did influence the mating behavior of six boars ( 10 months of age at start of experiment) during five consecutive days on each ejaculation frequency of one, two, three, or four times per day:

• 100.0 percent ejaculated at a frequency of one time per day,
• 91.6 percent ejaculated at a frequency of two times per day,
• 58.3 percent ejaculated at a frequency of three times per day, and
• 41.5 percent ejaculated at a frequency of four times per day.

 

An ejaculation frequency of two times per day (12-hour interval) for four days has resulted in defective sperm maturation and abnormal development of sperm motility.

 

3.3. Boar mounting efficiency and quality of mating

 

When pen-mating, the number of successful matings is very low when compared to the large number of mounts performed by a boar. The number of mounts, number of matings, number of mounts per mating, and mating efficiency (number of matings divided by number of mounts times 100) observed in one pen-mating study is shown in Table 6.

 

Each mating pen contained between two and four one-year-old boars and four estrous females. The boars in each breeding pen were full siblings from the same litter and had been reared and used for natural mating together. Mating efficiency ranged from 1.4 percent (t wo matings per 138 mounts) to 57.1 percent (four matings per seven mounts). Mating efficiency is substantially reduced when large differences exist in body size between a boar and female. Two females (12 and 15) were mounted 72 and 59 times, respectively, but they were never mated by any boar in the pen (Table 7). These two females were 139 and 86 pounds heavier than the average body weight of the boars in the pen. These results show that despite the low number of females to boars in each of the pens, there is no guarantee that each estrous female in a pen- mating situation will be mated satisfactorily.

 

Researchers in England investigated the reproductive behavior of 80 gilts and 14 boars when mating occurred in a dynamic mating system. Each mating pen contained four to five boars and 20 gilts. The two mating pens were 59 feet x 33 feet. Half of the pen was deep straw and the other half was a dunging and feeding area. Each week the four oldest gilts were removed from each pen and replaced by four new gilts. The overall quality of each observed mating activity (MA) was scored according to the following quantitative behavioral criteria: ( 1) the total time [minutes] that a boar spent on the gilt’s back [TTGB], (2) number of successful mounts that lasted at least 10 seconds on the gilt’s back [SM], (3) the mounting efficiency [(SM/AM) xlOO], and ( 4) the ratio TTGB/SM.

 

According to these criteria, the observed MA were classified as very poor, poor, fair, good, or very good (Table 8). Of the 933 mating activities observed in the study, 59 percent of the matings were classified as poor and very poor (Table 8). The reasons for termination of matings were interruptions by another boar (29 percent), gilt lying down (27 percent), gilt moving away (18 percent), boar dismounting ( 17 percent) and interruptions by another gilt (1 percent). Gilts were kept with the boars for about 33 days and pregnancy was diagnosed using ultrasonic scanning at time of removal from pen. Of the 80 gilts, 84 percent had a positive pregnancy diagnosis. Gilts with a positive pregnancy diagnosis had received more total mating activity and more total good + very good mating scores.

 

Table 6. The number of mounts, number of matings, number of mounts per mating, and mating efficiency when pen-mating four females per pen (Tanida et al., 1989).

^aMating behavior was observed for 72 continuous hours. ^bLW is Large White; H is Hampshire; LR is Landrace ; D is Duroc. ^cTable 7. The number of mounts, number of matings received per female, mating efficiency, and proportion of females mated during a 72-hour period of pen-mating (Tanida et al., 1989).

^aAll sixteen females were in estrus. ^bNumber of matings divided by number of mounts x 100.

 

Table 8. Percentage of gilt matings that were scored as very poor, poor, fair, good, or very good (Grigoriadis et al., 2000).

^aTotal time the boar spent on the gilt’s back
^bMounting efficiency= number of successful mounts (SM)/number of attempted mounts (AM) x 100

 

3.4. Improper time mated

 

Figure 11 shows that for optimum farrowing rate and litter size, gilts should be bred 11 to 44 hours after standing estrus starts. When too many estrous females are in the same pen, they may be bred only one time or not at all. It is very possible that a single mating may occur too early or too late during estrus. When gilts or sows are only bred once at O to 11 hours after onset of estrus, a lower conception rate and litter size occurs because sperm cells start to die before ovulation takes place. When gilts or sows are only bred once sometime between 44 and 66 hours after onset of estrus, pregnancy rate and litter size can sharply decrease because ova have died before the arrival of sperm cells or polyspermy (more than one sperm fertilized the ova) occurs, which results in cellular problems in the development of fertilized ova. In pen-mating situations, there is no control over mating time.

 
It is important that sows and gilts are mated multiple times. Optimal fertility occurs when an adequate number of viable spermatozoa are present in the oviduct just prior to ovulation. The influence of natural mating frequency on farrowing rate and number of piglets born live per litter from outdoor breeding units is indicated in Table 9. Farrowing rate did not differ between single- and double-mated sows; however, the number of live-born piglets per litter was significantly greater for sows receiving two matings compared to sows receiving one mating.

 

Table 9. Influence of number of natural matings (single or double) on farrowing rate and number of piglets born live per litter (Walker and Bilkei, 2002).

¹Fecundity index= (Farrowing rate x litter size) x 100

 

Table 10. Number of matings per boar and number of different estrous females mated by each boar during pen-mating (Levis et al., 1997).^a

^aThe boar was placed in a pen that contained two females in estrus and one female not in estrus for 4. 7 days.

 

3.5. Females not being mated

 

In pen-mating situations some females are never mated even though they are in estrus. This was demonstrated in one study where two to four boars were placed in a breeding pen with four estrous females. The overall percentage of females mated was 75 percent (12 of 16 females), with the number of matings per female ranging from zero to eight (Table 7). Mating efficiency ranged from Oto 37.5 percent (t hree matings per eight mounts).

 
Some pork producers have tried to improve results from pen-mating by putting one boar in a pen with two weaned females. However, the farrowing rate ( calculated on the first service after weaning) achieved with this method rarely exceeds 70 to 75 percent. Results from a research trial in which one boar was placed in a pen with three females (t wo estrous females, one anestrous female) showed the overall percentage of females mated was 61.1 percent (Table 10). Only three of the nine boars (33 percent), each of which were known to have a moderate to high level of sexual behavior, mated both females that were in estrus in their pen. The other six boars also mated females but they mated the same female many times. Other research also has shown some boars select a particular female with whom they spend a preponderance of courtship time. It appears there is no simple way to pen-mate females to be absolutely sure all females are bred during the same estrous period.

 

3.6. Inadequate sexual behavior of boars

 

Inadequate sex drive in boars, such as lack of willingness or eagerness to pursue, mount, and attempt service of females when pen- mating, can be influenced by high environmental temperatures, domination by other boars or larger females, bad past breeding experiences (psychological), over-use (sexual fatigue), sickness, inexperience, immaturity, excessive body condition, and old age.

 

3.7. “Injured” females

 

Pen-mating can result in estrous females getting injured because too many boars are in the pen when the first female stands for mating. Being mounted excessively increases the risk that females will experience muscle tears, fractures, or spinal cord injuries.

 

Table 11. Frequency distribution for the total number of copulations (all durations of time) per sow and number of copulations lasting longer than two minutes per sow in a multisire mating pen (Kongsted and Hermansen, 2008).^a

^aNumber of sows in each group varied from five to nine and the boar to sow ratio ranged from 1:3 to 1:1
on observations days of three to seven after weaning. A total of 47 sows were observed.
^bThree of the 13 sows not mated had shown signs of estrus. Ten sows did not express estrus.
^cAlthough 18 sows had shown signs of estrus, they had no copulations lasting two minutes or more.

 

Table 12. Frequency distribution for the total number of copulations (all durations of time) per boar and number of copulations lasting more than two minutes per boar in a multisire mating pen (Kongsted and Hermansen, 2008).^a

^aNumber of sows in each group varied from five to nine and the boar to sow ratio ranged from 1:3 to 1:1
on observations days of three to seven after weaning. A total of 31 boars were observed.

 

3.8. Multi-sire competition for sexual partner

 
In a multi-sire mating system the sows are placed in either large indoor or outdoor breeding pens at weaning with a group of boars. Reproductive performance in a multi-sire mating system depends to a large degree on appropriate mating behavior of the sows and boars. The duration of ejaculation in the boar varies from 3 to 20 minutes. The ejaculation process of a boar occurs in four phases: First phase – pre-sperm (clear fluid); Second phase – sperm-rich (contains 80 percent of total sperm cells ejaculated); Third phase – sperm-poor (mainly consists of fluids from the vascular glands), and Fourth phase – gelatinous plug. Approximately 80 percent of the total number of sperm cells ejaculated during mating occurs within the first 1.5 to 2.0 minutes of copulation. Thus, the boar should mate the female for at least two minutes to increase the chances that the sperm-rich fraction has been deposited into the sow’s reproductive tract.

 

Table 13. Percentage of copulations interrupted, percentage of sows showing estrus, and percentage of sows diagnosed pregnant (Kongsted and Hermansen, 2008).

^aOne sow was culled before pregnancy diagnosis and two sows were impossible to diagnose.

 

When a group of sows are being mated with a group of boars (multi-sire mating system), a specific copulation can be interrupted by another boar or another sow, or the sow collapses due to the weight of the boar; thus, the duration of copulation can be less than two minutes. The frequency distribution for the total number of copulations and number of copulations lasting more than two minutes per sow in a multi-sire mating pen experiment in Denmark is indicated in Table 11. Eighteen of 47 sows (38.3 percent) did not receive a copulation lasting more than two minutes. Three of the 13 sows not mated had shown signs of estrus. Eleven of 47 sows (23.4 percent) received one mating lasting more than two minutes. The frequency distribution for the total number of copulations and number of copulations lasting more than two minutes per boar in a multi-sire mating pen is indicated in Table 12. Two boars had no copulations. Eight of 31 boars (25.8 percent) had no matings lasting for two minutes or more. The percentage of copulations interrupted, percentage of sows showing estrus, and percentage of sows diagnosed pregnant is indicated in Table 13. The percentage of all copulations interrupted was 56 percent for pens with large sows and 72 percent for pens with small sows. The majority of the interruptions were due to another boar. The percentage of all weaned sows found pregnant at four weeks after weaning was 55 percent.

 

3.9. Inadequate production records

 
Many pork producers who pen-mate gilts and sows do not have any records to document the reproductive performance of the sow herd. The management adage, “You cannot manage what you do not measure” applies to pork producers. Using a pen-mating system is thought to involve low fixed cost and a low amount of labor and management. Pork producers can make better informed decisions about implementing various types of management procedures to improve reproductive performance when using accurate production records.

 

4. POSSIBLE SOLUTIONS

 

Because pen-mating involves unsupervised natural mating between sows and boars, it is impossible to guarantee that all management practices implemented in a pen-mating system will control boar fertility and ensure that all gilts and sows are mated. However, the following management procedures will help increase the possibility that all estrous gilts and sows are mated with a fertile boar.

 
4.1. Controlling accumulation of estrous females

 
One way to reduce the excessive number of estrous females accumulating when pen-mating is to use two, three, or four breeding pens or wean sub-groups (split-wean) every two to three days into two, three, or four breeding pens. However, there are disadvantages of split weaning: (1) the farrowing area is used over an extended period of time; thus, it is more difficult to use an all-in-all-out facility management system, (2) establishing social hierarchy among sows takes longer to establish, and (3) managing the nursery facility can be more difficult due to the wider age range of piglets. In addition, the period of time the farrowing area is used can be extended due to the variation in length of gestation. The length of gestation can range from 108 to 123 days (Figure 12).

 

Figure 13 illustrates the estimated number of females in estrus on each day when 20 females are weaned on the same day and pen-mated in one, two, three, or four breeding pens. The number of females in estrus still accumulates in each breeding pen; however, there are fewer estrous females per pen on each day. A possible advantage to having fewer estrous females per pen per day is that the boar( s) might be able to more easily identify and breed different females. Thus, there is a better chance that farrowing rate and litter size will be improved.

 

It is well known that fighting occurs between sows that are unfamiliar with each other. Fighting establishes a relative social ranking; therefore, reduce the need for aggression to settle future disputes between sows. Research has indicated that relative stability in social rank is obtained in groups of sows after two to three days. Factors that can influence the number and severity of agonistic interactions between sows after mixing include housing animals outdoors or indoors, space allowance, pen design, group size, composition of the group, and feeding method.

 

Figure 14 indicates the relationship between the occurrence of fighting and estrous activity when five females per group are weaned four days apart into one breeding pen. Duration of estrus is assumed to be 61, 53, 49, and 38 hours for sows that first express estrus on days three, four, five, and six after weaning, respectively. There will be a substantial amount of fighting when the first sub-group is expressing estrus; however, there will be little fighting for social dominance when the second group of sows are expressing estrus.

 

Figure 15 shows the relationship between the occurrence of fighting and estrous activity when five females per group are weaned every two days into one breeding pen (90 percent cycle). There will be a substantial amount of fighting when sows in groups one, two, and three are expressing estrus. The number of females expressing estrus on days 5 to 11 after weaning the first subgroup of females ranges from 5 to 8.

 

 

 

 

 

 

Figure 16 indicates the relationship between the occurrence of fighting and estrous activity when five females are weaned every two days. The first two groups (group 1 and 2) weaned are placed in breeding pen 1 and the second two groups (group 3 and 4) are placed in breeding pen 2.

 

Within breeding pen 1 there will be substantial amount of fighting during days three and four after weaning the first sub-group of sows; plus, this period of time is when the highest percentage of group 1 will be expressing estrus. An identical situation occurs in breeding pen 2. The number of sows expressing estrus in pen 1 ranges from six to eight on days five to seven after the first sub-group is weaned.

 

Figure 17 indicates the relationship between the occurrence of fighting and estrous activity when five females are weaned every two days between two breeding pens. The breeding pens are alternated every two days for sows entering the pen (pen 1 – 90 percent cycle, pen 2 – 80 percent cycle). Although the number of sows expressing estrus on an individual day within a breeding pen is reduced compared to the situation in Figure 16, there is the possibility of a substantial amount of fighting when all the sows are expressing estrus.

 

4.1.1. Effects of stress on reproduction. A sow can encounter physical activities and environmental conditions that increase the level of stress when pen-mating and gestating sows in a group-housing environment. The effects of stress on reproduction depends on the critical timing of stress in relation to the stage of the estrous cycle, the genetic capability of sows to handle stress, and type of stressor. The two main types of stress involved with pen-mating are physical aggression and insufficient feed intake. Lack of feed intake occurs when sows are not individually fed. A sow that does not cope with these stressful situations may have a reduced well-being and impaired reproductive performance. During the time of pen-mating the continuous interaction among sows and boars occurs when significant biological functions are happening that can influence reproductive performance. Researchers have investigated whether stress that occurs from weaning to 35 days after mating has detrimental effects on hormones involved with reproduction, onset of estrus, estrous expression, duration of estrus, transport of sperm cells to oviduct, onset of ovulation, ovulation rate, duration of ovulation, transport of ova through the oviduct, number of sperm cells attaching to the ova, early stages of embryonic development, implantation of embryos, embryonic survival, farrowing rate, and number of piglets born per litter.

 

Nutritional stress. When sows are group fed without the use of individual feeding stalls, a substantial amount of fighting occurs. Stress created by food deprivation for 48 hours in sows elevates the synthesis of cortisol and prostaglandin F2 alpha, and affects motility of the oviduct. This leads to a prolonged transport time of the ova as well as a decrease in both the cleavage rate of the embryos and the number of spermatozoa attached to the ova. Food deprivation during days 10 to 14 of pregnancy did not affect total number of fetuses or fetal survival rate at day 30 of gestation.

 

Physicals tress. A stress response is dependant on the nature of stress, intensity of stress, and duration of stressful event. There is a large variation among sows in their ability to cope with stress. Stress levels in a newly formed group of sows may persist for about two days until a dominance order is established. In large groups of sows, physical aggression may continue for 10 to 12 days. The effect of grouping sows on the weaning-to-estrus interval depends on both the level of aggression in the group and the social dominance position of the individual sows. Dominant sows in a group mount lower ranking sows; whereas, the subordinate sows seldom mount dominant sows. Sows that receive the highest amount of aggression have longer weaning-to-estrus intervals and a shorter duration of estrus. Simulated stress induced by injections of ACTH ( adrenocorticotropic hormone) during pro-estrus caused a delay in the onset of estrus and the development of cystic follicles in sows. Sows given ACTH from onset of estrus to 60 hours after ovulation had a tendency to have a shorter duration of estrus, larger number of sperm cells in the utero-tubal junction, greater number of sperm cells in oviduct, and a faster transport of embryos to the uterus compared with nonstressed sows. Simulating stress during estrus did not effect early embryo development. A study in the Netherlands reported that acute stressors (nose-sling for five minutes or unpredictable feeding time) during the follicular phase and early pregnancy did not affect follicle development, embryo development, or embryo survival.

 

When using multiple breeding pens to reduce the number of sows in estrus on the same day in the same breeding pen, the sows will most likely be mixed at a later date. Researchers have investigated the effects of mixing bred sows during various stages of gestation. Because exact breeding dates are not known and the breeding period is at least 21 days when pen-mating, the time of mixing sows is less of a concern compared with a weekly breeding period. Mixing bred sows at 28 days or more of gestation is not detrimental to fecundity index value compared to mixing during the first 14 days of gestation (Table 14).

 

Table 14. Influence of day of mixing after breeding on reproductive performance (Cassar et al., 2008).

^aFecundity index = (farrowing rate x number of piglets born alive) x 100.
 

Boar fertility can be influenced by many environmental factors such as season, breed, age, photoperiod, ambient temperature, feed intake, testis size, and ejaculation frequency. The two most important factors influencing reproductive efficiency are ejaculation frequency and ambient temperature.

 

4.2 Controlling boar fertility

 

4.2.1. Ejaculation frequency. An ejaculation frequency of seven times per week has been shown to decrease motility of sperm cells, decrease total sperm cells, decrease farrowing rate, decrease number of live piglets born, and increase percentage of abnormal sperm cells. Generally, the solution recommended to enhance boar fertility is rotating boars for sexual rest. Boars are easily trained to a daily rotation system: ( 1) do not feed the boar until the boar has moved from a breeding pen to a pen for sexual rest, and (2) the boar completes eating in his resting pen before moved to breeding pen.

 

Group-housed boars should be sexually rested out of sight and sound of estrous females and working boars. This procedure substantially reduces homosexual activity, which also can deplete sperm reserves during the resting period. Ideally, boars should be housed individually; however, this requires they be used individually in the breeding pen.

 

4.2.2. Ambient temperature. Boars exposed to ambient temperatures in excess of 82°F have lower sperm output, poorer sperm motility, and increased morphological abnormalities of the sperm cells. Because of these detrimental effects on semen quality, farrowing rate and litter size can be reduced. In addition, hot and humid weather can decrease the level of a boar’s sexual behavior, especially during the daytime. If a boar is subjected to heat stress sporadically over the summer months, it is possible that the fertility of his semen could be reduced during this period as well as a six to seven week period after the heat stress has ended. After a boar has been heat-stressed, it takes 42 to 49 days for a new group of sperm cells to mature within the boar’s testicle and epididymis.

 

It is difficult to keep the boars cool when pen-mating outdoors. To effectively cool, boars need to simultaneously have an adequate shaded area, have water sprinkled on them, and have adequate air movement across their body. One way to increase the possibility of boars being kept cool is to house them indoors during the daytime. When the weather is extremely hot, the boars should only be allowed to work during the evening, night, and early morning hours. This practice can be implemented by not letting the boars eat in sow pen.

 

4.3. Evaluating sexual behavior and mating ability of boars

 

The first objective when attempting to solve a perceived problem that a boar has a low level of sexual behavior is to determine whether the boar is truly sexually inactive or has poor mating ability.

 

The highest level of courtship behavior occurs immediately after a sexually rested boar comes in contact with females; thus, a suspect boar should be individually housed overnight out of the sight and sound of estrous females. The following morning, after the boars and females have eaten, the suspect boar should be evaluated for sexual behavior with an estrous female of similar body size for 15 minutes. The boar should be evaluated on his ability to pursue and mount a female, obtain an erection, gain entry into the vagina, and successfully copulate for more than two minutes. A less-effective method to evaluate male sexual behavior is to move a boar from his current breeding pen to another breeding pen. This method of evaluation is less effective because the boar may have recently completed an unobserved mating; thus, diminishing his sexual desire to mate.

 

It is best to observe boars for sexual behavior early in the morning, especially during hot weather. When the ambient temperature exceeds 86°F, boar sexual behavior starts to diminish. During extremely hot weather, some boars will cease to mate about midmorning and will not resume sexual activity until dark. The majority of courtship behavior and mating occurs between 2:00 a.m. and 11 :00 a.m. (Table 15).

 

4.3.1. Dominate boars. Domination by one boar during courtship and mounting activities in a multisire breeding system (t wo to four boars in same pen) may or may not be a problem. Research has indicated that dominate boars in the pen were responsible for 82 percent of all interrupted copulations in a multi-sire pen-mating system. Subordinate boars are only responsible for 5 percent of all interruptions. Domination by one boar in a multi-sire breeding pen has been shown to be less of a problem when boars are reared together. When domination is not a problem in a multi-sire breeding pen, sexual activity in the breeding pen is enhanced. It appears that when one boar starts expressing sexual activity, the other boars commence sexual activity. Excessive fighting among boars can be a problem when there are not enough estrous females available for all sexually stimulated boars or all boars in the pen prefer the same female. Fighting among boars within a multi-sire breeding system is an inherent problem that cannot be avoided. The only method to prevent fighting among boars is to house them individually and rotate them on a schedule either into or out of the breeding pen. Rotating boars is discussed in section 4.2 and 4.8.

 

4.4. Increasing the probability of mating at proper time

 

Rotating sexually rested boars among breeding pens may or may not increase the probability of all estrous females being bred at the proper time. Often, the new boar in the pen mates the same female the previous boar mated; thus, the probability that the female is bred at the proper time is increased. On the other hand, other estrous females may not be mated.

 

4.5. Reducing injured females

 

Reducing the number of females injured due to excessive sexual behavior by boars can be prevented by not placing the boars in the breeding pen until the afternoon of the fourth day after weaning. This practice increases the number of estrous females available for boars to court and mate.

 

Table 15. Courtship behavior time and number of matings by 18 boars within each 60-minute period of a 24-hour day (Levis et al., 1997).^a

^aThe observations were taken for 113 continuous hours when females were coming into estrus, were in estrus, and going out of estrus. ^b24-hour clock times; 2400 is midnight; 1200 is noon.

 

Table 16. Proportion of gilts in standing estrus at various times when continuously exposed to mature boars for 21 minutes during heat-checking (Levis and Hemsworth, 1995).

 

4.6. Checking bred sows for return to estrus

 

Pork producers who pen-mate females can easily increase reproductive efficiency (pigs sold per female maintained) by checking for returns to estrus in gestating females. Open or nonpregnant females need to be identified as soon as possible after their first service and either remated or culled. When deciding whether to remate or cull females, consider that the farrowing rate for females rebred at their first post-breeding return to estrus was 65.6 percent, and 51.0 percent for females rebred at their second post-breeding return to estrus, according to one study. The farrowing rate of females mated at first estrus after weaning was 83.6 percent.

 

If a group of females is weaned on the same day, they can be checked for estrus every other day from 25 to 45 days (first return to estrus after breeding) and 46 to 66 ( second return to estrus after breeding) days after weaning. It is assumed that the females are bred during a 21-day breeding period at first estrus after weaning; plus, they start cycling four days after weaning. The majority of bred sows returning to estrus should occur on days 25 to 36 after weaning (18 to 35 days after mating). The key point to remember: do not let females receive boar stimuli (sight, sound, or smell) for one hour before checking for estrus. It has been demonstrated that estrous females become refractory (w ill not stand in presence of a boar) to boar stimuli within 5 to 10 minutes after exhibiting a standing response (Table 16). Boars that are housed continuously with the sows have to be removed the night before heat checking the next morning.

 

4.7. Evaluating physical and environmental stresses on bred sows

 

Generally, all systems that house sows in groups will require regrouping. Regardless of whether sows are housed indoors or outdoors, some degree of aggression is inevitable when unfamiliar sows are mixed. Aggression among sows at the time of grouping is considered a negative aspect of group housing. It takes two to seven days to reach a relative stability of social hierarchy in a group of newly mixed sows. The presence of a boar at the time of mixing sows has been suggested as a management tool to reduce aggression among sows; however, the results from different scientific studies are contradictory.

 

A study in England found that boar presence led to a reduced frequency of aggressive interactions and skin lesions of sows during the first eight hours after mixing newly weaned sows. A study in Germany found that having a mature boar ( about 2 years of age) in the pen did not affect the total number of aggressive interactions per sow during the first 48 hours following the mixing of newly weaned sows; however, boar presence significantly decreased the number of fights ( 3 .6 vs 6.4; P < 0.01) and significantly reduced the duration of fights between sows (14.9 seconds vs 39.6 seconds; P < 0.01). A Canadian study found that having a mature boar (a bout 2 years of age) in the pen did not reduce aggression between females during the first 48 hours after mixing. The sows were mixed at 2 to 28 days after mating.

 

The welfare and productivity of sows improves when sows are exposed to less physical and environmental stress. Extreme levels of stress may be caused by high stocking density, new social grouping, poor environments, thermal extremes, and human-animal interactions which cause physical and (or) psychological trauma to animals. All types of physical and environmental stress should be minimized for 28 days after mating. Some of the reasons to reduce stress include:

• After mating, the fertilized eggs are retained within the oviduct for about two days before they are released into the uterus. If stress causes the eggs to be released into the uterus too early, they will die because the uterus is a hostile environment prior to the normal time the eggs enter the uterus.
• About 10 to 12 days after mating, the blastocysts start to develop into a 39-inch long, extensively folded structure that signals the female to maintain pregnancy. Thus, the level of stress should be minimal when the sow is receiving the pregnancy signal.
• Attachment of the blastocyst to the uterine wall starts around 12 to 14 days after mating and continues until about 28 days. Research studies have found that sows regrouped every two or three weeks during pregnancy tend to have lower farrowing rates compared to sows not regrouped.

 

Most likely, sows that are group-housed indoors will encounter more stress because they have less space to escape when fighting compared to sows housed outdoors. The distance required for a subordinate sow to escape from an aggressive sow is very important. One study using a dynamic grouping system found the distance over which a subordinate sow is pursued following aggressive interactions can vary from Oto 66 feet, with 75 percent of the encounters resulting in chase distances of less than 8 feet.

 

4.8. Estimating number of boars needed

 

There has been no research conducted to evaluate the influence of various boar-to-weaned female ratios when pen mating, on the percentage of estrous females mated, or number of copulations received per female. Therefore, it is difficult to make absolute recommendations for boar requirements when pen-mating. Generally, the number of females cycling and receptive to the boar on each day of the breeding period and the number of ejaculations by each boar per day are unknown. The percentage of females pregnant at 30 to 35 days after mating has been reported to be 92 percent for females bred by boars ejaculating once per day for five days and 58 percent for females bred by boars that ejaculated four times per day for five days. It is not uncommon for boars to ejaculate four or more times per day when pen- mating. Therefore, a conservative approach must be taken when estimating the number of boars to use when penmating, especially when it is important that the farrowing rate and litter size born live remains high. The following suggestions will help in determining boar requirements when pen-mating:

 

4.8.1. All females are weaned the same day. Because the number of females in estrus accumulates over time (Figure 2), it is important to answer the following questions:

• How many females are being weaned?
• How many pens are being used for breeding?
• How many weaned females are being placed in each breeding pen?
• Are some boars going to be moved out of the female pen for a short period while other boars take their place?, and
• How much time are the boars given for sexual rest?

 

An example of calculating the number of boars needed when pen-mating females that are all weaned the same day is shown in Table 17 ( one boar per two females bred) and Table 18 ( one boar per four females bred) . Regardless of the boar-to-female ratio used, there is no guarantee all females will be satisfactorily mated during their first estrus after weaning. However, when following the example in Table 17, a higher farrowing rate and improved litter size may be achievable because more boars are used relative to the number of females to be bred. Having more boars available at the time when groups of females are in estrus increases the chance that a sow in heat will have a satisfactory mating and increases the chance of her becoming pregnant.

 

A minimum of five boars would be needed to breed the distribution of 20 females shown in Figure 13 when using the following assumptions: ( 1) Boar-to-female ratio is one boar to four females to breed (Table 17), (2) two breeding pens are used and (3) each boar is rested for one day. However, because of the many uncontrollable variables associated with pen-mating, there is no guarantee that all 20 females will be mated satisfactorily during the first estrus after weaning.

 

4.8.2. Female group is split-weaned. The most sensible method for estimating the number of boars needed when pen-mating a group of split-weaned females is to estimate the accumulated number of females in estrus on each day during the breeding period. The factors to consider are: ( 1) number of days between weaning each sub-group offemales, (2) number offemales per sub-group, (3) number of breeding pens, (4 ) distribution of weaned females among breeding pens, (5) rotating boars into and out of breeding pens for sexual rest, (6 ) length of time for sexual rest by boars, (7) percentage of females first found in estrus on each day and ( 8) the length of time a female is in estrus.

Table 17.An example of calculating the number of boars needed when pen-mating females that are all weaned the same day.

^aAssumes boars are sub-divided into groups to allow them to be rotated for a one or two-day sexual  rest.

 

4.9. Development of production records

 

Production records used when pen-mating do not have to be highly sophisticated or computerized. Measurements to evaluate critical success factors are referred to as Key Performance Indicators (KPis). Although many things are measurable, that does not make them key to the success of a pen-mating program. In selecting KPis, it is critical to limit them to those factors that are essential to enhancing reproductive performance of the sow herd. Some of the critical success factors that have to be done to accomplish an acceptable reproductive performance of the sow herd include:

• getting a high percentage of sows and gilts successfully bred,
• getting a high percentage of sows and gilts successfully farrowed,
• getting a high number of piglets born live per litter, and
• getting a high number of piglets weaned per litter.

 

Table 18. An example of calculating number of boars needed when pen-mating females that are all weaned the same day.

^aAssumes boars are sub-divided into groups to allow them to be rotated for a one or two-day sexual rest.

 

It is essential to have production records that measure the previously mentioned KPis. A suggested simple data sheet of measuring the KPis when pen- mating is shown in Table 19. A gradual but continual improvement in reproductive performance of the sow herd can be accomplished by using a continuous improvement cycle of planning, implementing the plan, evaluating the KPis, and taking action to make needed changes. It is important that pork producers who pen-mate sows keep good records related to:

 

(I) farrowing rate for sows mated at first service after weaning; (2) total number of piglets born and number of piglets born alive per litter for sows mated at first service after weaning; (3) number of boars and age of boars used for breeding; (4 ) management procedure used for breeding sows, such as, rotation of boars for sexual rest, number of days boars are sexually rested, number of boars simultaneously used in the breeding pen, number of sows in the breeding pen(s), and procedure for weaning sows into the breeding pen; (5) dates the breeding period occurred, and (6 ) whether boars are run with the sows to breed sows that return to service during a 21-day period after their first service. This data can help pork producers make improvements for increasing reproductive performance of the sow herd.

 

5. OUTDOOR PEN-MATING FACILITY DESIGN

 

The layout of breeding facilities for pen-mating outdoors will vary enormously according to the number of sows in the breeding herd, number of groups of sows, number of sows per group, soil type, topography, amount of land area available, geographic location, and various management procedures.

 

The main principles to consider when designing and managing an outdoor pen-mating facility include: (1) method of housing boars, (2) method for ease of rotating boars into and out of a breeding pen, (3) number of recently weaned sows per breeding pen, (4 ) amount of feeding space per sow and ease of feeding sows either individually or in a group, ( 5) requirements for maintaining fences and ground within the pen, ( 6) ease of administering vaccinations and medications for sows, gilts, and boars, (7) ease of accurately detecting open sows and gilts after mating, (8) ease of removing sows and gilts from a breeding pen, ( 9) method for watering sows, gilts, and boars, (IO) method for cooling sows, gilts, and boars during hot weather, and ( 11) method of providing bedding to sows, gilts, and boars.

 

An example of an outdoor penmating facility is shown in Figures 18 and 19. The facility is designed to individually house boars, easily move boars into and out of a breeding pen, and easily and safely feed sows and boars on a concrete pad. The boars are trained to a daily rotation procedure by always feeding them in their home pen. The feeding pad allows an easy way to catch the sows for medical treatment or loading them for transport to a gestation pen. Be sure all gates and gate latches are designed for easy opening and closing. Make sure all gates swing in an appropriate manner and direction. As seen in Figure 19, overhead braces are used to make sure the gates easily swing and lock. Some pork producers use a more permanent procedure to reduce the requirements needed to maintain a fence by placing a steel post in a concrete footing (Figure 20).

Table 19. Breeding group record for pen-mating of sows and gilts.

 

 

 

The amount of land needed for the breeding pens depends on soil type, drainage, slope, number of animals per pen, and whether pasture is used. It has been recommended that five sows and one boar on pasture require 3,588 square yards. Some pork producers have moved their sows indoors because of the ongoing requirement for maintaining fences, mud lots, and serious problems with snow and ice (Figure 21). Indoor pen-mating facilities should be carefully designed and constructed to provide animal comfort, ease for moving animals, individual housing of boars, and enhancement of reproductive performance.

 

6.1. Boar housing

 

Ideally, boars should be housed individually rather than as a group. The benefits are: ( 1) reduction of injuries from fighting and riding, (2) stimulation of sexual behavior, (3) simplification of moving them in and out of their living space, ( 4) elimination of homosexual activity, and ( 5) allow for feed intake adjustments to maintain proper body condition. These benefits extend the useful life of the boars and reduce costs due to the need for fewer replacement boars.

 

6.1.1. Floors pacea nd surface. The amount of floor space allowed per boar is generally 35 to 50 square feet. The floor should be partially or totally slatted to prevent a mucky environment . When using a partially slatted floor, the boars are fed on the floor. The alley is raised 4 inches to prevent the boars from moving feed into the alley.

 

6.1.2. Pen dimensions and gate latch. The minimum width of the pen is 5 feet, however, 6 feet is preferred. The pen partitions should be 46 to 48 inches high and constructed of vertical pipe to prevent climbing. Vertical pipe should be placed 4 to 5 inches apart. The bottom horizontal pipe should be no more than 8 inches off the floor between adjacent pens and only 6 inches off the floor next to the feed alley. Boar pens should have a gate latch that can be opened and shut quickly, yet not be opened by the boar.

 

6.1.3. Ventilation, heating, and cooling. The resting area for individually housed boars should be totally enclosed because there is not enough body mass to generate sufficient heat to warm the boars. The boar living area is not large; therefore, the cost of installing and operating a mechanical ventilation system is reasonable. During warm seasons, boars are generally cooled with an intermittent spraying system combined with fans. It is the evaporation of water that cools the boars; therefore, it is best to spray the boar until wet, set the timer to turn off the water until the boar dries off, and then repeat the cycle. Moving air across the boar with a fan helps ensure that moisture is being removed, and increases the evaporation process during extremely hot weather.

 

6.2. Sow housing

 

When pen-mating indoors, the number of weaned females per pen should be kept small, about 4 to 10 sows. The floor plan indicated in Figure 22 houses seven females with one boar (18 square feet per animal).

 

6.2.1. Floor space and surface. The floor space allowed per female is generally 18 to 20 square feet in the breeding pen and 16 square feet in gestation (14 square feet for gilts). Normally, females are pen-mated on a partially slatted floor. Imprinting a diamond pattern (4- to 5-inch o.c., one-half inch deep) in the breeding pen floor helps prevent injuries when females are mounting or being mounted by other females or a boar. Feeding females on the floor helps keep it clean and dry. Raising the alley 4 inches above the breeding pen floor prevents the females from moving feed back into the alley.

 

6.2.2. Pen partition and gate latch. Normally, a 48-inch high pen partition is used to prevent females and boars from getting into an adjacent pen. In a partially slatted facility, a solid pen partition is used on the solid portion of the floor to enhance a good dunging pattern; however, a solid pen partition interferes with air flow. To enhance air flow and still have an effective 48-inch high partition, set a pipe or steel rod panel on top of a 32-inch concrete wall (Figure 23). Breeding and gestation pens also should have a gate latch that can be opened and shut quickly and yet cannot be opened by animals. Open partition gates are used along the alley, so estrus detection can be easily accomplished during the gestation phase.

 

6.2.3. Ventilation, heating, and cooling. The facility shown in Figure 22 is a gable or ”N.’ roof building. The building is operated as a nonmechanically ventilated, modified open-front facility.

 

 

7. HAND-MATING

 

Pork producers who change from pen-mating to hand-mating find the time spent hand-mating more than pays for itself. Reproductive performance of a farm that changed from pen- mating to hand- mating is shown in Table 20.

 

During a 12-month period this farm had pen- mated the first four months, hand-mated the middle four months, and pen-mated the last four months. When the reproductive data was summarized, the farrowing rate and litter size was lower for penmating compared to hand-mating. When a 12-month period of handmating was compared to an earlier 12-month period of pen-mating, the improvement in reproductive performance when hand-mating was:

• 0.2 percent increase in pigs weaned per litter
• 15 percent increase in pigs born live per litter
• 30.5 percent increase in total number of pigs weaned
• 36.3 percent increase in total number of pigs born live
• 49.4 percent increase in litters per female per year
• 51 percent increase in pigs weaned per female per year

 

Table 20. Reproductive performance on a commercial farm that changed from pen-mating to hand-mating (Levis – unpublished data). Avg. number of Number Method of females on females Year Mating inventory farrowed

 

The increase in reproductive performance when hand-mating was done with a 20.7 percent decrease in average number of females on inventory. Some factors that improved reproductive performance were: better control of boar fertility; a tighter production schedule due to knowledge of exact breeding dates; bred females regularly heat-checked to identify open females; and females returning to estrus after their second mating were culled.

 

8. PRODUCTION SCHEDULING

 

Most pork producers who pen-mate sows have a diversified farm; thus, they cannot devote total time and management to the swine enterprise. These pork producers seasonalize the production of pigs around their cropping plans. The development of a yearly production schedule for the sow herd helps optimize the use of production facilities to enhance control of diseases ( especially in the farrowing and nursery buildings), labor, and feed. In addition, the periods of time when the sows and gilts are to be bred are clearly identified; thus, the date the boar(s) should be removed from the breeding pen is established. Batch breeding, farrowing, and weaning enables small farms to produce larger groups of pig at regular intervals instead of small groups of pigs. A disadvantage of interval batch breeding is that females failing to conceive and rebred accumulate more nonproductive days than females in a conventional weekly breeding scheme. These females return to heat at the wrong time according to the breeding schedule. Two publications to help pork producers develop a swine production schedule and calendar of management activities for their particular situation can be found in the Pork Industry Handbook (2001 CD-ROM Edition). The title of the publications is: 1. Calculating Swine Schedules, and 2. Pork Production Systems with Business analyses: Two Groups of Sows Farrow-to-Finish. The Iowa Pork Industry Center at Iowa State University has a computer template available that calculates production schedules. This computer template can be used to production schedule one group of sows to farrow twice per year.

 

References

Arey, D. S. 1999. Time course for the formation and disruption of social organization in group-housed sows. Applied Animal Behaviour Science 62:199-207.
 
Arey, D. S., and S. A. Edwards. 1998. Factors influencing aggression between sows after mixing and the consequences for welfare and production. Livestock Production Science 56:61-70.
 
Arey, D. S., J. Messinger, and M. Nobre. 1999. Agonistic and mating behaviour in two loose-housing sys­ tems for sows. Proceedings British Society of Animal Science. Page 184 (abstract).
 
Barnett, J. L., G. M. Cronin, T. H. McCallum, and E. A. Newman. 1993. Effects of ‘chemical intervention’ techniques on aggression and injuries when grouping unfamiliar adult pigs. Applied Animal Behaviour Science 36:135- 148.
 
Belstra, # A., W. L. Flowers, and M. T.See. 2002. Effect of season on duration of estrus, time of ovulation, and fertility of sows in a commercial herd. Annual Swine Report. North Carolina State University. http://www. ncsu.edu!project/swineextension/swinereports/2002/belstra2.htm. Accessed: March 4, 2010.
 
Berger, T., and B. M. Roberts. 2009. Reduced immunolabelling of a porcine oocyte membrane protein reflects re­ duced fertilizability of porcine oocytes following elevated ambient tempera­ ture. Reproduction of Domestic Ani­ mals 44:260-265.
 
Borberg, C., and S. Hoy. 2009. Mixing of sows with or without the presence of a boar. Livestock Science 125:314-317.
 
Brandt, Y., A Madej, H. Rodnguez­ Martmez, and S. Einarsson. 2007.
 
Effects of exogenous ACTH during oestrus on early embryo development and oviductal transport in the sow. Reproduction of Domestic Animals 42:118- 125.
 
Britt, J. H., and D. G. Levis. 1982. Effect of altering suckling intervals of early-weaned pigs on rebreeding performance of sows. Theriogenology 18:201-207.
 
Cameron, R. D. A. 1982. Factors Influencing Semen Production and Quality in Boars Reared in a Subtropi­cal Environment. Doctorial Thesis, Department of Veterinary Medicine, University of Queensland, Brisbane, Queensland, Australia.
 
Cameron, R. D. A. 1985a. Mea­surement of semen production rates of boars. Australian Veterinary Journal 62:301-304.
 
Cameron, R. D. A. 1985b. Factors influencing semen characteristics in boars. Australian Veterinary Journal 62:293-297.
 
Cameron, R. D. A., and A. W. Blackshaw. 1980. The effect of elevated ambient temperature on spermatogen­esis in the boar. Journal of Reproduc­ tion and Fertility 59:173-179.
 
Cassar, G., R. N. Kirkwood, J. J. Seguin, T. M. Widowski, A. Farzan, A.J. Zanella, and R. M. Friendship. 2008. Influence of stage of gestation at grouping and presence of boars on
farrowing rate and litter size of group­ housed sows. Journal of Swine Health and Production 16:81-85.
 
Christenson, R. K., ) S. Teague, A. 1 Grifo, Jr., and 8 L. Roller. 1972. The effect of high environmental tempera­ ture on the boar. Ohio Swine Research and Information Report. Research Summary 61. Ohio Agricultural Research and Development Center, Wooster, Ohio. Pages 19-23.
 
Correa, M. N., T. Lucia Jr., J. A.B. Afonso and J. C. Deschamps. 2002. Reproductive performance of early­ weaned female swine according to their estrus profile and frequency of artificial insemination. Theriogenology 58:103-112.
 
Dawson, A., R. Pitt and A. R. Peters. 1998. Seasonality and reproduction. In: J. Wiseman, M. A. Varley and J. P. Chadwick (Eds.). Progress in Pig Science. Nothingham University Press. Loughborough, Leicestershire, United Kingdom. Pages 327-342.
 
Docking, C. M., R. M. Kay, J. E. L. Day, and H. L. Chamberlain. 2001. The effect of stocking density, group size and boar presence on the behaviour, aggression and skin damage of sows mixed in a specialized mixing pen at weaning. Proceedings British Society of Animal Science. Scarborough. United Kingdom. Page 46 (abstract).
 
Einarsson, S., Y. Brandt, N. Lunde­ helm, and A. Madej. 2008. Stress and its influence on reproduction in pigs: a review. Acta Veterinaria Scandinavica 50 (Supplement 1):48-55.
 
Einarsson S., Y. Brandt, H. Rodriguez-Martinez, A. Madej. 2008. Conference Lecture: Influence of stress on estrus, gametes and early embryo development in the sow. Theriogenol­ ogy 70:1197-1201.
 
Einarsson S., A. Ljung, : Brandt, M. Hager, and A. Madej. 2007. Impact of exogenous ACTH during pro-oestrus on endocrine profile and oestrous cycle characteristics in sows. Reproduction of Domestic Animals 42:100- 104.
 
Einarsson, S., A. Madej, and V Tsuma. 1996. The influence of stress on early pregnancy in the pig. Animal Reproduction Science 42:165-172.
 
English, P., W. Smith, and A MacLean. 1982. The Sow – Improv­ ing Her Efficiency. 2nd Edition. Farm­ ing Press Limited, Wharfedale Road, Ipswich, Suffolk (England).
 
Fahmy, M. H. 1981. Factors influ­encing the weaning to oestrus interval in swine: A review. World Review of Animal Production 17:15-28.
 
Fahmy, M. H., W. B. Holtmann, and R. D. Baker. 1979. Failure to recycle after weaning, and weaning
to oestrus interval in crossbred sows. Animal Production 29:193-202.
 
Frangez, R., T. Gider, and M. Kosec. 2005. Frequency of boar collection and its influence on semen qual ity, pregnancy rate and litter size. Acta Veterinaria Brno 74:265-273.
 
Grigoriadis, D. F., S. A. Edwards, P. R. English, and F. M. Davidson. 2000. The reproductive behaviour of pigs in a dynamic service system for gilts. Applied Animal Behaviour Science 66:203-216.
 
Grigoriadis, D. F., S. A. Edwards, P. R. English, and F. M. Davidson. 2001. The effect of oestrous cycle number, at constant age, on gilt reproduction in a dynamic service system. Animal Sci­ ence 72:11-17.
 
Gonyou, H. W. 2003. Group hous­ing: Alternative systems, alternative management. Advances in Pork Pro­ duction 14:101-107.
 
Heitman, H. and J. R. Cockrell. 1984. Cycling ambient temperature effect on boar semen. Animal Produc­ tion 38:129-132.
 
Hemsworth, P.H. and J. Til­ brook. 2007. Sexual behavior of male pigs. Hormones and Behavior 52:39-44.
Hoy, St., and J. Bauer. 2005. Dominance relationships between sows dependent on the time interval between separation and reunion. Applied Ani mal Behaviour Science 90:21-30.
 
Huang, W-Y, J. Behan, MacPherson and P. English. 1991. A study of the timing and duration of oestrus in sows post-weaning and of problems at mating. Proceedings. Aberdeen University Agriculture Grad­ uates Association. 3:44-45 (abstract).
 
Huang, W-Y and P. R. English. 1992. A study of the timing and dura­tion of oestrus in sows post weaning and of problems at mating. Animal Production 54:481-482 (abstract).
 
Hurtgen, J. P. and A. D. Leman. 1980. Seasonal influence on the fertil­ity of sows and gilts. Journal of Ameri­ can Veterinary Medical Association 177:631-635.
 
Johnson, L. A., R. J. Gerrits, and E. P. Young. 1969. Quantitative analysis of porcine spermatozoa and seminal plasma phospholipids as affected by frequency of ejaculation. Journal of Reproduction and Fertility 19:95-102.
 
Jones, D. D., L. B. Driggers, D. B. Gerber, K. A. Law, and R. Plain. 1987. Calculating swine schedules. Factsheet PIH-113. Pork Industry Handbook (2001 CD-ROM Edition). Purdue Uni­versity Cooperative Extension Service, West Lafayette, Indiana.
 
Kemp, B., and N. M. Soede. 1996. Relationship of weaning-to-estrus interval to timing of ovulation and fertilization in sows. Journal of Animal Science 74:944-949.
 
Kongsted, A. G. 2004. Stress and fear as possible mediators of reproduc­tion problems in group housed sows: A review. Acta Agriculturae Scandinavi­ca, Section A-Animal Science 54:58-66.
 
Kongsted, A. G., and J. E. Herman­ sen. 2008. The mating behavior and reproduction performance in a multi­ sire mating system for pigs. Therio­ genology 69:1139-1147.
 
Levis, D. G. 1984a. Evaluating replacement boars for sexual behavior. Agri-Practice 5:23-26.
 
Levis, D. G. 1984b. Pointers on pen mating. Hog Farm Management 16(9):23-24, 29.
 
Levis, D. G. 2001. What’s new with seasonal infertility? Proc. 42nd Annual George A. Young Swine Health and Management Conference. University of Nebraska Veterinary and Biomedi­cal Sciences. Pages 29-64.
 
Levis, D. G., J. J. Ford, and R. K. Christenson. 1997. An evaluation of three methods for assessing sexual behavior in boars. Journal of Animal Science 75:348-355.
 
Levis, D. G. and P. H. Hemsworth. 1995. How long does standing estrus last after initial boar exposure when heat checking? University of Nebraska Cooperative Extension. Nebraska Swine Report EC 94-219-A:3-4.
 
Love, R. J. 1978. Definition of a seasonal infertility problem in pigs. Veterinary Record 103:443-446.
 
Luescher, U. A., R. M. Friendship, and D. B. McKeown. 1990. Evaluation of methods to reduce fighting among regrouped gilts. Canadian Journal of Animal Science 70:363-370.
 
Luescher, 6 A., D. B. McKeown, and R. M. Friendship. 1987. Methods to decrease fighting in dry sows after grouping. Canadian Journal of Animal Science 67:1167-1168 (abstract).
 
Maurer, R. R., J. J. Ford, and R. Christenson. 1985. Interval to first postweaning estrus and causes for leav­ ing the breeding herd in Large White, Landrace, Yorkshire and Chester White females after three parities. Journal of Animal Science 61:1327-1334.
 
McNitt, J. * and N. L. First. 1970. Effects of 72-hour heat stress on semen quality in boars. International Journal of Biometeorology 14:373-380.
 
McNitt, C. B. Tanner, and N. L. First. 1972. Thermoregulation in the scrotal system of the boar. * Tempera­ture distribution. Journal of Animal Science 34:112-116.
 
McNitt, C. B. Tanner, and N. L. First. 1972. Thermoregulation in the scrotal system of the boar. II. Evapora­ tive heat exchange. Journal of Animal Science 34:117-121.
 
Moore, A. S., H. W. Gonyou, and A. W. Ghent. 1993. Integration of newly introduced and resident sows following grouping. Applied Animal Behaviour Science 38:257-267.
 
Mount, N. C. and M. F. Seabrook. 1993. A study of aggression when group housed sows are mixed. Applied
Animal Behaviour Science 36:377-383.
 
North Carolina State University. 1972, 1973, 1974, 1975, 1976. Produc­tion and Financial Summary. Swine Development Center, Rocky Mount, N.C. North Carolina Agricultural Extension Service. Raleigh, N.C.
Nowak, R., M. Paquignon, J. P. Signoret, J. Gautier, D. Col, and P. Despres. 1984. Production spermatique et fertilite de verrats soumis a un rythme intesifd’ejaculation (Sperm production and fertility in the boar under intensive ejaculation rhythm). Annales de Zootechnie 33:353-366.
 
Nowak, R., M. Pauignon, and J. P. Signoret. 1988. Possibilites et limites de l’utilisation du verrat en accouplement nature! (Possibilities and limitations of using boars in natural mating). INRA Production Animales 1(3):215-218. http:!Igranit.jouy. inra.fr!productions­ animales/ 1988/ProdAnim 1988 1309.
pdf. Accessed on March 25, 2010.
 
Paterson, A. M., J. Barker, and % 3 Lindsay. 1978. Summer infertility in pigs: its incidence and characteristics in an Australian commercial piggery. Austra­lian Journal of Experimental Agriculture and Animal Husbandry 18:698-701.
 
Paquignon, M., R. Nowak, Y. H. Kuo, and J. P. Signoret. 1984. Sperm production in the boar under inten­sive ejaculation rhythm. Proceedings 1Oth International Congress on Animal Reproduction and Artificial Insemina­ tion. University of Illinois, Urbana­ Champaign, IL. Volume 2:61-63.
 
Pedersen, L. J. 2007. Sexual behav­iour in female pigs. Hormones and Behavior 52:64-69.
 
Pedersen, L. J., T. Rojkittikhun, S. Einarsson, and L.-E. Edqvist. 1993. Postweaning grouped sows: effects of aggression on hormonal patterns and oestrous behaviour. Applied Animal Behaviour Science 38:25-39.
 
Plain, 3 L., J. 3 Foster, and K. A. Foster. 1995. Pork production systems with business analyses: Two groups of sows farrow-to-finish. Factsheet PIH-14. Pork Industry Handbook (2001 CD-ROM Edition). Purdue University Cooperative Extension Service, West Lafayette, Indiana.
 
Poleze, E., M. L. Bernardi, W. S. Amaral Filha, I Wentz, and ‘ P. Bor­ tolozzo. 2006. Consequences of varia­ tion in weaning-to-estrus interval on reproductive performance of swine females. Livestock Science 103:124-130.
 
Pruneda, A., E. Pinart, M. D. Briz, S. Sancho, N. Garcia-Gil, e. Badia, E. Kadar, J. Bassols, E. Bussalleu, M. Yeste, and S. Bonet. 2005. Effects of high semen collection frequency on the quality of sperm from ejaculates and from six epididymal regions in boars. Theriogenology 63:2219-2232.
 
Rasbech. N. 0. 1969. A review of the causes of reproductive failure in swine. British Veterinary Journal 125:599-616.
 
Razdan, P. 2003. Stress and early pregnancy in sows – Effect on endo­ crinology, ova transport and embryo development. Doctoral Thesis. Swedish University of Agricultural Sciences.
Uppsala, Sweden. http://diss-epsilon. slu.se:8080/ archive/00000284/011 kappafinall 11.pdf. Accessed: July 8, 2010.
 
Razdan, P., A.M. Mwanza, H. Kindahl, H. Rodriguez-Martinez, ‘ Hulten, S. Einarsson. 2002. Effect of repeated ACTH-stimulation on early embryonic development and hormon­al profiles in sows. Animal Reproduc­ tion Science 70:127- 137.
 
Seguin, M. J., D. Barney, and T.M. Widowski. 2006. Assessment of a group-housing system for gestating sows: Effects of space allowance and pen size on the incidence of superficial skin lesions, changes in body condi­tion, and farrowing performance. Journal of Swine Health and Produc­tion 14:89-96.
 
Seguin, M. J., R. M. Friendship, R.N. Kirkwood, A. J. Zanella, and T. M. Widowski. 2006. Effects of boar pres­ ence on agonistic behavior, shoulder scratches, and stress response of bred sows at mixing. Journal of Animal Sci­ence 84:1227-1237.
 
Shaw, J. M. and S. A. Edwards. 1995.A study of courtship and mating behaviour in pigs in an outdoor mutl­ tisire mating system. In: Rutter, S. M., J. Rushen, H. D. Randle, and H. D. Ed­dison, editors. Proceedings of the 29th International Congress of the Inter­ national Society for Applied Ethology. Pages 229-230.
 
Soede, N. M., J. B. Roelofs, R. J. E. Verheijen, W. P. G. Schouten, W. Ha­zeleger, and B. Kemp. 2007. Effect of repeated stress treatments during the follicular phase and early pregnancy on reproductive performance of gilts. Reproduction in Domestic Animals 42:135-142.
 
Soede, N. M., M. J. W.Van Sleuwen, R. Molenaar, ‘ W. Rietveld, W. P. G. Schouten, W. Hazeleger, and B. Kemp. 2006. Influence of repeated regrouping on reproduction in gilts. Animal Repro­duction Science 96:133-145.
 
Soede, N. M., C. C. H. Wetzels, W. Zondag, M. A. * de Koning, and B. Kemp. 1995. Effects of time of in­ semination relative to ovulation, as determined by ultrasonography, on fertilization rate and accessory sperm count in sows. Journal of Reproduc­ tion and Fertility 104:99-106.
 
Stanislaw, C. M. and K. D. Zer­ ing. 1984. Production and Financial Summary. Swine Development Center, Rocky Mount, North Carolina. North Carolina Agricultural Extension Ser­ vice. Raleigh, NC. AG-14.
 
Steverink, D. W. B., N. M. Soede, G. J. R. Groenland, ‘ W. van Schie, J. P. T. M. Noordhuizen and B. Kemp. 1999. Duration of estrus in relation to reproduction results in pigs on com­mercial farms. Journal of Animal Sci­ence 77:801-809.
 
Stone, B. A. 1981. Thermal charac teristics of the testis and epididymis of the boar. Journal of Reproduction and Fertility 63:551-557.
 
Stone, B. ” 1981/1982. Heat in­ duced infertility of boars: The inter­ relationship between depressed sperm output and fertility and an estimation of the critical air temperature above which sperm output is impaired. Animal Re­ production Science 4:283-299.
 
Strzezek, J., W. Kordan, J. Glogowski, P. Wysocki, and K. Borkowski. 1995. Influence of semen collection frequency on sperm qual­ity in boars, with special reference to biochemical markers. Reproduction in Domestic Animals 30:95-94.
 
Strzezek, J., W. Demianowicz, W. Kordan, J. Torska, P. Wysocki, and D. Holody. 1996. Biochemical status of boar spermatozoa and seminal plasma before and after a 10-day depletion test. Reproduction in Domestic Ani mals 31:245-246.
 
Suriyasomboon, A., A. Kunavong­ krit, N. Lundeheim, and S. Einarsson, S. 2004. Effect of temperature and hu­ midity on sperm production in Duroc boars under different housing systems in Thailand. Livestock Production Sci­ence 89:19-31.
 
Suriyasomboon, A., N. Lunde heim, A. Kunavongkrit, and S. Einars­son. 2005. Effect of temperature and humidity on sperm morphology in Dure boars under different housing systems in Thailand. Journal of Veteri nary Medical Science 67:777-785.
 
Suriyasomboon, A., N. Lunde heim, A. Kunavongkrit, and S. Einars­ son. 2006. Effect of temperature and humidity on reproductive perfor­ mance of crossbred sows in Thailand. Theriogenology 65: 606-628.
Tanida, H., Y. Murata, T. Tanaka, and T.Yoshimoto. 1989. Mounting efficiencies, courtship behavior and mate preference of boars under multi­ sire mating. Applied Animal Behaviour Science 22:245-253.
Tanida, H., N. Miyazaki, T. Tanaka, and T. Yoshimoto. 1990. Seasonal effect on sexual behavior of boars under multi-sire mating. Proceedings 5th Con­ gress Asian-Australasian Association of Animal Production Societies. Taipei, Taiwan. Volume 3:170 (abstract).
 
Tanida, H., H. Yusuke, T. Tanaka, and T. Yoshimoto. 1990. Comparisons of time spent on courtship behavior and number of mounts by boars in single and multi-sire mating. Japa­nese Journal of Zootechnical Science 61:283-288.
 
Tanida, H., N. Miyazaki, T. Tanaka, and T. Yoshimoto. 1991. Selection of mating partners in boars and sows under multi-sire mating. Applied Ani­ mal Behaviour Science 32:13-21.
 
Tarocco, C. 2001. The effect of estrus duration and number of artifi­ cial inseminations on fertility of gilts and multiparous sows having a four­ day wean-to-estrus interval. Journal of Swine Health and Production 9:117-120.
 
Thiengtham, J. 1992. Some rela tionships between sexual behavioural parameters and semen characteristics in the boar. Thai Journal of Veterinary Medicine 22:237-249.
 
Thiengham, J., P. H. Hemsworth, and D. B. Galloway. 1991. Sperm distri­ bution and the effects of sexual stimu­ lation on sperm number in the boar. In: Batterham, E. S. (Ed), Manipulating Pig Production III. Australasian Pig Science Association. Animal Research Institute, Attwood, Victoria, Australia.
 
Tubbs, R. C. 1990. Factors that in­ fluence the weaning-to-estrus interval in sows. Compendium (Food Animal) on Continuing Education for the Prac­ticing Veterinarian 12:105-115.
Turner, A. I., P. H. Hemsworth, and A. J. Tilbrook. 2002. Susceptibil ity of reproduction in female pigs to impairment by stress and the role of the hypothalamo-pituitary axis. Re­ production, Fertility and Development 14:377-391.
 
USDA. 2007. Swine 2006. Part 1: Reference of swine health and manage ment practices in the United States, 2006. USDA:APHIS:VS, CEAH, Fort Collins, Colorado. #N475.1007.
 
Van Wettere, 8 H. E. J., S. J. Pain, P. G. Stott, and P. E. Hughes. 2008. Mixing gilts in early pregnancy does not affect embryo survival. Animal Re­ production Science 104:382-388.
 
Vargas, A. J., M. – Bernardi, F. P. Bortolozzo, A. 1 G. Mellagi, and I. Wentz. 2009. Factors associated with return to estrus in first service swine females. Preventive Veterinary Medi cine 89:75-80.
 
Vargas, A. J., M. – Bernardi, T. F. Paranhos, M A. D. Goncalves, F. P. Bortolozzo, and I. Wentz. 2009. Repro­ ductive performance of swine females re-serviced after return to estrus or abortion. Animal Reproduction Sci­ ence 113:305-310.
 
Varley, M. A., and 3 Stedman. 1994. Stress and reproduction. In: Cole, D. J. A., Wiseman, J. and Varley, M. A. (Eds.). Principles in Pig Science. Nottingham University Press, Lough­ borough, Leicestershire, United King­ dom. Pages 277-296.
 
Walker, C. M. and G. Bilkei. 2002. Effect of mating frequency on sow reproductive performance under field conditions and natural mating in large outdoor production units. Reproduc­ tion in Domestic Animals 37:116-118.
 
Weitze, K. F., H. Wagner-Rietschel, D. Waberski, – Richter, and J. Krieter. 1994. The onset of heat after weaning, heat duration, and ovulation as major factors in AI timing in sows. Repro duction in Domestic Animals 29:433- 443.
 
Wettemann, 3 P. and F. W. Bazer. 1985. Influence of environmental tem­ perature on prolificacy of pigs. Journal of Reproduction and Fertility, Supplement 33:199-208.
 
Wettemann, R. P., M. E. Wells, and R. K. Johnson. 1979. Reproduc­tive characteristics of boars during and after exposure to increased ambient temperature. Journal of Animal Sci­ ence 49:1501-1505.
 
Willemse, A. H. and J. Boender. 1967. The relation between the time of insemination and fertility in gilts. Tijdschrift voor Diergeneeskunde 92:18-34.
 
Winfield, C. G., P. H. Hemsworth, D. B. Galloway, and A. 8 Makin. 1981. Sexual behaviour and semen
characteristics of boars: Effects of high temperature. Australian Journal of Experimental Agriculture and Animal Husbandry 21:39-45.