Breeding Boar Nutrient Recommendations and Feeding Management
Boars have a profound influence on the swine breeding program. Not only do they provide a source of genetic improvement, but they also affect farrowing rate and litter size. Although nutrition of the boar is often given the least amount of attention compared to pigs in other phases of production, it serves as an important factor in determining reproductive performance of the breeding herd and overall animal well-being. Nutrient status influences libido (sex drive), structural soundness and longevity, sperm production, and semen quality. Factors affecting nutrient requirements include age and stage of maturity, body condition, environmental conditions, and ejaculation frequency. Many farms feed boars the sow gestation diet(s), but this feeding program may neglect some of the unique nutrient requirements that must be accounted for to optimize breeding herd performance in working boars.
Scientific research regarding the nutrition of boars is lacking. Therefore, feeding strategies used in the pork industry and those presented in this paper are based on a minimal amount of research.
The objectives of this fact sheet are to review and increase understanding of the following areas related to nutrition of the breeding boar:
- Feeding management goals for different production priorities and situations
- Nutritional influences on boar reproductive performance
- Factors affecting nutrient needs of the boar
- Relation of nutrition to structural soundness and longevity
- Nutrient recommendations for formulating boar diets
Feeding during isolation
Incoming boars should be quarantined from the rest of the breeding herd at least 45 – 60 days to allow time to check health status, acclimation to their new environment, and evaluation for reproductive soundness. A 60-day isolation period that provides excellent management also allows time for sperm production to recover if boar(s) were heat-stressed or sick prior to arrival at the isolation unit. A diet similar to that provided by the source herd should be fed initially to boars during the isolation period. Diets can then be gradually changed to the new herd diet, minimizing stress during this transition period.
Feeding management goals for natural service boars
Feeding management goals differ between younger (300 – 400 lbs body weight) and more mature (400 – 650 lbs body weight) breeding boars and if boars are being utilized for natural service (pen or hand mating) or are collected for artificial insemination. If boars are used in natural mating systems, a primary goal is to minimize mature body weight in order to allow boars to mate with smaller sows and gilts. Overfeeding boars can impair libido and could lead to reproductive problems and decreased length of service in the herd; therefore, limit feeding is required. Boars used for pen-mating can be limit-fed by never allowing them to eat with the sows. This can be accomplished by training the boar(s) to exit the sow pen prior to eating. If the boar(s) have to be returned to the sow pen immediately after eating, this procedure does require the establishment of a boar eating and drinking area.
Young boars are still growing, and therefore still need to be fed to allow for moderate weight gain, approximately 0.40 to 0.55 lb/day (145 to 200 lb/year when they are between about one to two years old). Providing 5 to 5.5 pounds of a balanced 14% crude protein corn-soybean based diet per day is generally sufficient for young boars under normal conditions. The goal is to restrict energy intake to achieve a gradual growth rate while maintaining high amino acid, vitamin and mineral intakes to preserve fertility and libido. As boars become older, their growth rates decrease (see Table 1) and proportion of nutrients provided for body maintenance increases.
Table 1. Estimated Growth Rate of Working Boars Based on Body Weight 
|Initial BW, lb||Final BW, lb||ADG, lb/d|
Mature boars should be fed at a level to meet body maintenance needs while still allowing for optimal reproductive performance. Because body maintenance nutrient requirements are directly related to body weight, current body weight and condition of the boar needs to be accounted for. Feeding 5 to 6 pounds to mature boars of a diet similar to that used for the young working boar provides a sufficient baseline level. Increases in feed allowances should be made when boars are subjected to cold weather or when boars exhibit exceptionally high libido. To account for temperatures below the boar’s thermoneutral zone, provide an additional 0.1 pounds of feed per day for each degree below 68°F. Boars that provide greater than two matings per week should be provided an additional 0.2 lbs of feed per day . For practical purposes, smaller sized pork enterprises will often limit-feed a sow gestation diet in lieu of providing a boar-specific diet. A well-balanced sow gestation diet containing 65 grams of crude protein, 3.0 grams lysine and 2.0 grams methionine and cystine per pound should be adequate for breeding boars . Nutritional information for the gestating sow is presented in PIG Factsheet #07-01-11 (Gestating Swine Nutrient Recommendations and Feeding Management).
Feeding management goals for artificial insemination (AI) boars
The nutrition program for stud boars should be managed in order to optimize sperm production and quality of semen, while ensuring overall well-being of the boar. Welfare issues such as lameness are important because this impacts the boar’s desire and ability to mount the dummy sows for collection. Reduced emphasis is placed on minimizing mature body size compared to natural service boars, since the ability of smaller sows to handle the weight of larger boars is not an issue. However, there is a worker handling and safety factor that must be considered when determining acceptable weight of the boar, and should be considered along with the temperament of the animal when setting maximum weight criteria. Most boars in an AI stud are maintained for less than 2 years – cull and removal rate is generally greater than 50 – 60% annually to maintain high genetic progress. Therefore, feeding for body size of the boar is not nearly as important as feeding for the body condition of the boar. Feeding levels should allow for a minimum level of growth to occur throughout the lifetime of the boar in order to optimize sperm output. Table 2 provides guidelines for feeding levels at different bodyweight levels in order to maximize sperm production . These guidelines may need to be adjusted due to genotypes of the boars.
Table 2. Feeding guidelines for breeding boars 
|Initial BW, lb||Final BW, lb||ADG, lb/d|
aBased on a corn/soybean meal diet with 1.48 Mcal/lb bAssumes 80% efficiency cAdd 0.25 lb feed/d/ejaculate collected over a typical breeding week dAdd 0.1 lb feed for each degree temperature below 68°F
Influence of nutrition on reproductive performance
Nutrition affects boar libido, sperm output, and semen quality. Therefore, reproductive performance can be significantly decreased if a proper nutritional program is not implemented. Although all of these items profoundly affect overall breeding herd performance, relatively little research has been conducted to more precisely ascertain nutrient requirements for specific reproductive functions.
Libido is the sexual interest, or prowess, of a boar, and in combination with appropriate mounting and production of ejaculate, lends towards successful production of viable semen for insemination, albeit natural or assisted. Severe and prolonged restrictions in feeding level, resulting in significant losses in body weight, have been shown to cause boars to refuse service . Feeding low protein diets, especially when energy intake is also reduced, has been shown to significantly reduce boar interest in mounting a dummy sow and producing an ejaculate. Breeding performance is reduced due to decreased blood levels of the hormone estradiol-17 beta [6, 7]. It appears, however, that short-term restrictions in feeding level or nutrient intake have minimal effects on libido [8, 9, 10]. Some boar studs will feed higher protein levels than required (5 to 10% above recommended levels) in order to enhance aggressiveness in boars and minimize dummy mount time. Potentially more significant is the detrimental effect of over-conditioning boars on subsequent libido. Excessive weight gain reduces the activity level in pigs by making them fatter and more lethargic, and also may decrease the physical ability of a boar to mount a sow by reducing locomotive soundness and balance .
The process of producing sperm in the testis and allowing them to mature in the epididymis takes approximately 6 weeks; therefore, nutritional effects cannot be observed when examining sperm characteristics for at least 42 days after implementing a nutritional change. Providing a lower plane of nutrition (50 – 70% of requirements) has resulted in reduced semen volume and total sperm production [10, 12], while increasing the nutrient levels back to required levels returned semen volume and sperm production to normal.
Historical data, however, indicates that boars can tolerate widely varying levels of nutrition without detrimental effects on quality of the sperm produced, including motility, abnormality, viability and fertilization capacity . Further research in needed in this area to determine if modern boars, which are leaner and are not housed in large pens to allow for increased exercise, are more sensitive to feeding adjustments, particularly if being underfed.
Mineral and vitamin needs
Boars require the same minerals and vitamins in similar amounts as gestating sows. Increased calcium and phosphorus levels above those normally provided in grow-finish diets are necessary to enhance and maintain structural soundness in adult breeding animals, including boars, and play a key part along with weight management in maintaining locomotion and enhancing longevity. Zinc and selenium are minerals directly involved in sperm production. The levels of these and other minerals and vitamins are sufficient at the same levels as generally provided in gestating sow diets.
Feed containing mycotoxins should be avoided in breeding herd rations. Mycotoxins, such as zearalenone and aflatoxin B1, can severely affect the ability of the boar to properly utilize dietary nutrients. Mycotoxins can also severely affect animal health and reproductive performance.
Zearalenone resembles many characteristics of steroids, and competes effectively with 17B-estradiol for the specific binding sites for estrogen receptors (13). Mimicking estrogen, it initiates a sequence of events resulting in a wide variety of changes to hormonal levels and patterns, especially involving testosterone. Zearalenone exposure interferes with spermatogenesis, reducing testosterone synthesis. Fertilization ability of boar sperm is reduced because of negative affects on viability, motility, and acrosome reaction. Additionally, depression in synthesis and circulation of testosterone induces feminization and suppresses libido (14).
Other mycotoxins also have been shown to negatively impact reproductive performance of the boar. Fusarium has been shown to decrease testes weight by 30% and reduce sperm quality and viability, resulting in decreased fertility of the boar (15). Additional information regarding mycotoxins, their characteristics, and feeding management is available in PIG Factsheets #07-01-16 (Diet and Health Interactions in Swine) and #07-06-07 (Utilization of Weather-Stressed Feedstuffs in Swine Diets).
Nutrient recommendations for breeding boars
Nutrient recommendations for energy, amino acids, calcium, and phosphorus for breeding boars are provided in Table 3. These recommendations have been based on evaluating a combination of published university and industry research and recommendations. The recommended nutrient levels take into account base nutrient requirements plus a safety margin. This safety margin is necessary to account for variability in individual boar performance levels and nutrient needs, along with inherent ingredient nutrient variability. These baseline recommendations assume normal conditions and do not account for extremes in environment and activity, which must be accounted for as previously described.
Protein needs are expressed in terms of amino acid recommendations, reflecting that pigs require amino acids, not protein, in their diets to meet their needs. Lysine is the first limiting amino acid in grain-soybean meal-based diets. Amino acid recommendations are provided on a total basis and a standardized ileal digestible (SID) basis. Formulating diets on a SID basis allows one to account for differences in the useable amino acids present in the diet and more closely meets the pig’s amino acid needs while minimizing excess nitrogen excretion. Amino acid recommendations were determined utilizing amino acid ratio guidelines relative to lysine (presented in PIG Factsheet #07-02-03 (Understanding Swine Nutrient Recommendations).
For ease of use, amino acid levels as well as calcium and phosphorus recommendations are presented in terms of concentration in the diet (%) as well as level relative to energy (g/Mcal ME). Use of higher or lower energy diets compared to the recommendations provided in Table 3 may preclude the need to evaluate and formulate diets based on amount of nutrient/unit of energy rather than strictly on an overall diet concentration basis, as has been traditionally done. Use of this approach allows users to more accurately supply nutrient levels when utilizing alternative ingredients.
Table 3. Amino acid, calcium and phosphorus recommendations for breeding boars (as-fed basis)a
|Body weight, lb||300 to 400||400 to 650|
|Assumed daily feed intake, lbb||5.4||6.0|
|Dietary metabolizable energy, Mcal/lb||1.50||1.50|
|% of diet|
|Lysine, total, %||0.75||0.75|
|Standardized ileal digestible, %|
|Methionine + cysteine||0.45||0.45|
|Phenylalanine + tyrosine||0.64||0.64|
|Standardized ileal digestible|
|Methionine + cysteine||1.35||1.35|
|Phenylalanine + tyrosine||1.94||1.94|
aAll diets fed under thermoneutral conditions. bAdjust to achieve a desired body condition or weight gain. cTotal phosphorus recommendations should be used as a guideline only; those recommendations may not be obtained when formulating practical diets on an available or digestible phosphorus basis which is recommended. Also, total phosphorus recommendations will not be achieved when phytase is included in the diet. d Recommended amount relative to dietary metabolizable energy (ME) density; energy values of ingredients from PIG Factsheet #07-07-09 (Composition and Usage Rate of Feed Ingredients for Swine Diets) were used in the calculations.
Vitamin, trace mineral, and salt recommendations were determined from the scientific literature and are presented in Tables 4 and 5. Due to the wide range in values that have been published and lack of current research defining precise requirements, ranges for recommended additions of vitamins, trace minerals and salt to breeding boar diets have been provided in Table 4. Except for salt, the minimum values generally represent the total amount required in the diet according to the NRC (1998). Upper values do not represent safe or tolerance levels, but instead a reference point above which further additions will not likely improve performance.
Note that the recommendations provided for vitamins, trace minerals, and salt indicate levels of dietary addition and do not account for nutrient levels that may be present in other ingredients. Therefore, if producers are considering ingredients that may contain large levels of available nutrients, or especially salt, their contribution to the overall level of that nutrient in the diet should be considered. Specific recommendations for dietary trace mineral and vitamin additions are shown in Table 5. The values represent our best estimate of trace mineral and vitamin needs of working boars in practical situations. These values are based on NRC requirements to which a safety margin has been added. Those seeking nutritional information for manufacturing base mixes and premixes for swine diets can consult PIG Factsheet #07-02-06 (Trace Minerals and Vitamins for Swine Diets).
Table 4. Ranges for dietary additions of salt, trace minerals and vitamins from concentrates, base mixes or premixes for breeding boarsa
|Salt, %||0.4 to 0.6|
|Sodium, %||0.15 to 0.25|
|Chloride, %||0.12 to 0.3|
|Copper, ppm||5 to 20|
|Iodine, ppm||0.15 to 0.5|
|Iron, ppm||80 to 200|
|Manganese, ppm||20 to 45|
|Selenium, ppmb||0.15 to 0.3|
|Zinc, ppm||50 to 200|
|Vitamin A, IU/lb||1800 to 7000|
|Vitamin D3, IU/lb||90 to 700|
|Vitamin E, IU/lb||20 to 40|
|Vitamin K, mg/lbc||0.25 to 3|
|Riboflavin, mg/lb||2 to 8|
|Niacin, mg/lb||5 to 35|
|Pantothenic acid, mg/lb||5 to 20|
|Choline, mg/lbd||250 to 500|
|Biotin, mg/lb||0.1 to 0.3|
|Vitamin B12, mg/lb||0.007 to 0.02|
|Folic acid, mg/lb||0.6 to 1.8|
|Vitamin B6, mg/lb||0 to 2.25|
aAll diets are limit-fed under thermoneutral conditions. bMaximum legal addition is 0.3 ppm. cMenadione activity. dNRC requirement is 570 mg/lb for choline.
Table 5. Specific recommended dietary additions of trace minerals and vitamins from concentrates, base mixes or premixes for breeding boarsa
|Vitamin A, IU/lb||4000|
|Vitamin D3, IU/lb||300|
|Vitamin E, IU/lb||30|
|Vitamin K, mg/lbe||2|
|Pantothenic acid, mg/lb||10|
|Vitamin B12, mg/lb||0.01|
|Folic acid, mg/lb||0.75|
|Vitamin B6, mg/lb||0|
aDiets are limit-fed under thermoneutral conditions. bSalt is usually added at the rate of 10 lb/ton in breeding boar diets to help provide a signifi- cant portion of the total dietary sodium and chloride recommendations. cRecommendations for sodium and chloride represent total dietary amounts, not additions. dMaximum legal addition is 0.3 ppm. eMenadione activity.
Omega-3 Fatty Acids
An emerging area of interest for enhancing or ensuring optimal boar fertility includes the dietary addition or formulation of diets for fatty acid composition, in particular omega-3 fatty acids. Fats and oils have traditionally been included in swine diets as an excellent source of energy, but with little understanding or consideration for other properties. Omega-3 fatty acids, polyunsaturated fatty acid components of some fat and oil sources, appear to positively influence boar fertility. These fatty acids include linolenic, eicosapentaenoic (EPA), and docosahexaenoic (DHA) fatty acids.
Pig spermatozoa contain large amount of DHA, suggesting that DHA is most likely an essential component for optimal boar fertility (16). Studies examining the inclusion of omega-3 fatty acids via fish oil in the diet of boars for 6 – 12 weeks indicate that DHA concentration in spermatozoa can be increased with dietary fish oil supplementation (17, 18). Additionally, sperm viability and proportion of sperm with progressive motility and normal acrosome score were also improved. Dietary supplementation of a proprietary mixture of DHA and antioxidants to boars (19) and subsequent mating with gilts resulted in an increased concentration of gilts being successfully bred and greater number of pigs born alive along with a similar increase in proportion of DHA in boar spermatozoa.
Traditional corn-soybean meal diets contain very little omega-3 fatty acids. Linseed (flax) and canola are excellent sources of linolenic acid, while fish oils are the only dietary sources of EPA and DHA. It appears that sufficient quantities of DHA and/or EPA can be metabolized from linolenic acid in the boar. However, it should be noted that all omega-3 fatty acids are highly unstable, and thus need to be protected by antioxidants in the diet, such as vitamin E or ethoxyquin, in order to maintain biological activity. More information on inclusion of dietary fatty acids in swine diets is provided in PIG Factsheet #07-03-03 (Feed Additives for Swine).
Nutrition of the boar is often given little attention, but deficiencies or excessive allowances for various nutrients can result in suboptimal reproductive performance, including reduced boar libido, lower sperm production, and poor semen quality. Boars should be transitioned onto new diets when entering the breeding herd from a separate unit. Goals of the boar feeding program differ based on whether the boar is used for natural or artificial insemination. Proper feeding program management can significantly improve leg and structural soundness, and improve overall longevity. Taking into consideration the unique nutrient needs of the adult working boar can provide improvements in efficiencies and economic dividends to the entire breeding herd.
1. Sulabo RC, Quackenbush J, Goodband RD,Tokach MD, Dritz SS, DeRouchey JM, and Nelssen JL. Predicting growth rates of adult working boars in a commercial boar stud. Kansas State Swine Day 2006; pp 13-18.
2. Singleton WL, Flowers WL. Management of the boar for natural service. Pork Information Gateway Factsheet PIG 08-02-02; 2006.
3. Life Cycle Swine Nutrition, PM-489. Iowa State University, 1996. p. 18.
4. Tri-State Swine Nutrition Guide, Bulletin 869. The Ohio State University; 1998.
5. Stevermer EJ, Kovacs MF, Hoekstra WC, and Self HL. Effect of feed intake on semen characteristics and reproductive performance of mature boars. J. Anim. Sci. 1961; 20:858-865.
6. Louis GF, Lewis AJ, Weldon WL, Miller PS, Kittok RJ, and Stroup WW. Calcium levels for boars and gilts. J. Anim. Sci. 1994; 72:2038-2050.
7. Louis GF, Lewis AJ, Weldon WL, Miller PS, Kittok RJ, and Stroup WW. The effect of protein intake on boar libido, semen characteristics and plasma hormone concentrations. J. Anim. Sci. 1994; 72:2051-2060.
8. Dutt RH, Barnhart CF. Effect of plane of nutrition upon reproductive performance of boars. J. Anim. Sci. 1959; 18:3-13.
9. Ju JC, Cheng SP, andYen HT. Effects of amino acid additions in diets on semen characteristics of boars. J. Chinese Soc. Anim. Prod. 1985; 14:27-35.
10. Kemp B, Verstegen MWA, den Hartog LA, and Grooten HJG. The effect of environmental temperature on metabolic rate and partitioning of energy intake in breeding boars. Livestock Prod. Sci. 1989; 22:87-98.
11. Westendorf P., Richter L. Nutrition of the boar. Ubersicht furTierernahrung 1977; 5:161-184.
12. Beeson WM, Crampton EW, CunhaTJ, Ellis NR, and Leucke RW. Nutrient Requirements of Swine. NRC, 1953.
13. Minervini F and Dell’Aquila ME. Zearalenone and reproductive function in farm animals. Int. J. Mol. Sci. 2008; 9:2570-2584.
14. D’Mello JPF, Placinta CM, and MacDonald AMC. Fusarium mycotoxins: A review of global implications for animal health, welfare, and productivity. Anim. Feed Sci. Technol. 1999; 80:183-205.
15. Mirocha CJ, Pathre SV, and Christensen CM. Zearalenone. In Mycotoxins in Human and Animal Health, Rodricks JV, Hesseltine CW, Mehlman MA Eds; Pathotox Publisher Inc. Park Forest South, IL, USA; 1977, pp 346-364.
16. Reese D. Omega-3 fatty acids and swine reproduction – A review. In: Nebraska Swine Report, Univ. of Nebraska – Lincoln. 2003, pp 30-33.
17. Paulenz H, Taugbol O, Kommisrud E, and Grevle I. Effect of dietary supplementation with cod liver oil on cold shock and freez Effect of dietary supplementation with cod liver oil on cold shock and freezability of boar semen. Reprod. In Domestic Animals. 1999; 34(5):431-435.
18. Rooke JA and Speake BK. Effects of feeding tuna oil on the lipid composition of pig spermatozoa and in vitro characteristics of semen. Reproduction. 2001; 121:315-322.
19. Strzezek J, Fraser LO, Kuklinska M, Dziekonska A, and Lecewicz M. Reprod. Biol. 2004; 4(3):271-287.
20. NRC Nutrient Requirements for Swine. 10th ed. Natl. Acad. Press, Washington, DC. 1998.
Frequently asked questions
Can fibrous ingredients be included in boar diets?
Yes. Plant-based feed ingredients contain fiber which cannot be broken down by the pig’s own digestive system, but is fermented by microorganisms in the large intestine. Mature pigs are more capable of utilizing high fiber ingredients than growing pigs due to increased microbial activity in the hindgut. Including ingredients that are high in fiber content (i.e. wheat middlings, soybean hulls, alfalfa meal, oats, sugar beet pulp) decreases the energy density of the diet, requiring that increased feeding levels be provided to ensure equivalent energy intake. Feeding diets higher in fiber results in a boar that is more “satisfied” after consuming a meal. Considerations should be made for increases in manure handling due to reduced diet digestibility and larger volume of solids produced.
Will higher dietary levels of calcium, phosphorus, and other nutrients above recommendations further improve feet and leg soundness?
Although proper nutrition is clearly important in maintaining feet and leg soundness, feeding excessive levels probably will not improve soundness. Many research studies have investigated the influence of nutrition on feet and leg soundness in sows and boars. If diets contain nutrient densities similar to the recommendations in this publication, no further improvements in feet and leg soundness would be expected. Problems encountered would likely then be caused by genetic or environmental factors other than nutrition. Also of concern is the ratio of calcium to phosphorus – if calcium levels are excessive in relationship to phosphorus, negative effects on diet palatability and mineral utilization may occur.
Should boars be fed prior to or after being used for breeding?
We suggest boars be fed prior to breeding or semen collection. In pen mating situations it is easier to rotate boars for sexual rest if they are fed after they are moved.
How can weight gain of boars in pen-mating systems be controlled?
Minimizing the time the boars are together with females will help control boar weight gain. Otherwise, boars are likely to consume more feed than they need because they are the dominate animal in the pen. In addition, feeding higher rates to increase body condition of sows during pen mating can result in excessive weight gain in boars. Establish specific breeding periods during which the boars are together with the females for mating purposes. Otherwise, house the boars separately from the females so they can be fed specifically to their needs.