Effect of Age at which Semen Collection Regimens are Initiated on Production of Spermatozoa in Boars
North Carolina State University Swine Reproduction Research from 2001. Sixty boars were randomly assigned to a factorial arrangement of treatments involving age at first collection, 160 or 190 days of age, and collection frequency, once or twice per week (n=15 per treatment combination). The total number of spermatozoa per ejaculate was monitored until the boars were 24 months of age. Total number of spermatozoa per ejaculate increased then appeared to plateau for each treatment combination. This plateau was considered to be an estimate of the adult level of spermatozoa production for boars. Semen production reached adult levels between 10 and 11 months of age for boars whose collection regimen was initiated at 190 days of age compared with 13 to 14 months of age for boars whose collection regimen began at 160 days of age. However, once the adult level of semen production was reached, no differences in the total number of spermatozoa per ejaculate were observed between boars collected for the first time at 160 and 190 days of age. These data indicate that initiation of collection regimens at 160 days of age does not have a significant, long-term effect on semen production in boars.
Recently, it has been suggested that if boars are placed on a semen collection regimen at young ages, then their lifetime sperm production will be compromised. The rationale for this assumption is not clear. However, it may be based, in part, upon the commonly accepted relationship between age at first breeding and subsequent litter size in gilts. In general, most studies examining this relationship indicate that an age threshold for acceptable reproduction exists for gilts. In general, gilts mated at ages below this threshold have lower farrowing rates and litter sizes compared to their counterparts who are older than this critical age at breeding (Kirkwood, 1980). However, few studies have examined the relationship between age at first collection and sperm production for boars. Consequently, the objective of this study was to examine the influence of the age at which semen collection regimens were initiated on production of spermatozoa in boars.
Materials and Methods
Sixty boars (Hampshire x Duroc x Yorkshire x Large White) were randomly assigned to a factorial arrangement of treatments involving age at first collection, 160 or 190 days of age, and collection frequency, once or twice per week. The total number of spermatozoa per ejaculate was monitored until the boars were 24 months of age.
The total number of spermatozoa per ejaculate was analyzed with analysis of variance procedures for repeated measures in a factorial arrangement of treatments (Gill and Hafs, 1971). The model consisted of age at first collection, collection frequency, time (week of study) and appropriate interactions. Boar nested within age at first collection and collection frequency was the error term used to test the main effects of age at first collection, collection frequency, and their interaction. Significant interactions between time, collection frequency and age of first collection were present (p=0.03). Consequently, changes over time within each combination of age at first collection and collection frequency were evaluated. In order to simplify presentation of data, total number of sperm per ejaculate are presented on a monthly basis.
Results and Discussion
In general, initiation of semen collection at 160 days of age delayed the age at which production of spermatozoa appeared to reach a fairly consistent level (Figure 1). This is evident in that the adult plateau in total spermatozoa per ejaculate occurred between 10 and 11 months of age for those first collected at 190 days of age and between 13 and 14 months of age for those first collected at 160 days of age. Frequency of collection did not appear to alter when production of spermatozoa stabilized, except possibly for boars collected twice per week beginning at 160 days of age. They appeared to reach their adult level of production between 14 and 15 months of age. It appears from the data in Figure 1 that a collection frequency of 2 times per week decreases production of spermatozoa. When considered on a per ejaculate basis, this is correct. However, it is important to remember that boars are producing 2 ejaculates per week. As a result, the total spermatozoa produced per boar per week is actually twice the value shown in Figure 1 for these treatments. Therefore, total production of spermatozoa per boar on a weekly basis is actually higher for boars collected twice than once per week.
Data presented in Figure 1 illustrate some important relationships between age at initiation of semen collection, collection frequency and sperm production for boars. These data indicate that neither collection frequency nor age at first collection had a significant, longterm effect on total numbers of spermatozoa collected per boar. This statement is based on the observation that total sperm production per month eventually leveled off at the same plateau for each treatment. However, it does appear that twice per week collections in conjunction with a young age at first collection delays the age at which boars reached their adult plateau.
In boars, a new group of type A spermatogonia begins to develop every 6 to 8 days and their transition to mature spermatozoa takes between 5 and 6 weeks (Flowers, 1997). After maturation, spermatozoa remain stored in the cauda epididymi until they are ejaculated or reabsorbed. If age at first collection influenced adult sperm production, then it is reasonable to speculate that it should affect either the number or rate at which spermatozoa mature. There is limited physiological evidence supporting this speculation. Instead, what probably happens is that younger boars have fewer spermatozoa stored in their epididymi compared with older boars. In the present study, a difference of 30 days in age at the time of first collection would be equivalent to 5 additional groups of sperm cells that had time to mature prior to their removal. Consequently, ejaculates from boars collected for the first time at young ages would be expected to have reduced numbers of spermatozoa initially. However, under a constant collection frequency, removal of spermatozoa should reach an equilibrium with sperm maturation and numbers of spermatozoa per ejaculate should stabilize.
These data do not support the speculation that age at first collection has lasting, negative influence on subsequent production of spermatozoa in boars. However, they do demonstrate that total sperm per ejaculate requires a longer period of time to plateau in boars collected for the first time at young ages compared with their older counterparts. As a result, the age at which semen collection is initiated in boars is an important consideration for boar studs in terms of meeting daily production demands. For operations that routinely bring AI boars into production at younger ages, it probably is more critical for the introduction of new boars to be evenly distributed throughout the year, if possible. This should minimize any decreases in the daily production of AI doses. If this is not possible, then consideration should be given to delaying the removal of older boars until the semen production from young boars gets closer to its mature level.
Flowers, W.L. 1997. Management of boars for efficient semen production. J. Reprod. Fert. Suppl. 52, 67-78.
Gill, J.L. and Hafs, H.D. 1971. Analysis of repeated measurements. J. Anim. Sci. 33, 331-340.
Kirkwood, R.N. 1980. Puberty in the gilt. Ph.D. thesis. University of Leeds.