Influence of Neonatal Environment
on Sperm Production of Mature Boars

W.L. Flowers

Summary

Littermate pairs of boars were raised in either groups of 12 or 6 animals from birth until 90 days of age. Nutrition and space requirements were similar between the two groups of animals from weaning through 90 days of age. Boars raised in groups of 6 were heavier at maturity than their counterpart maintained in groups of 12. No differences were present between treatments in testicular size at maturity. Total number of spermatozoa per ejaculate and daily sperm production were greater in boars raised in groups of 6 than 12. These data demonstrate that the first 3 months of life, a period largely ignored in boar development programs, has important ramifications for adult sperm production.

Introduction

It has been well established that there is a positive relationship between testicular size and sperm production in boars (Huang and Johnson, 1996). Males with large testes produce ejaculates with higher numbers of spermatozoa compared to their counterparts with small testicles. This phenomenon primarily is due to the fact that sertoli cells constitute the majority of tissue found in the testicle. As a result, boars with large testicles have increased numbers of sertoli cells and, thus, a greater capacity for sperm production. These relationships have important implications for management of boars during sexual maturation, since the total number of sertoli cells in adult boars actually is determined during the first three months of life. It is believed that sertoli cells undergo two active periods of mitosis. One begins shortly after birth and continues for 30 days, while the second is thought to occur between 60 and 100 days of age (Colenbrander et al., 1982). In essence, the sperm production potential that a boar has as an adult actually is established during these two periods of active sertoli cell divisions.

Few controlled studies in boars have examined the influence of various management factors during adolescence on adult levels of sperm production. Those that have been conducted have failed to show dramatic effects of feeding or housing on sperm concentrations and semen quality. However, most of these experiments did not begin until boars were at least 35 days of age. Consequently, it is possible that treatments were applied after the framework for adult reproductive function was already established. Thus, the objective of this experiment was to determine the effect of lactation litter size (neonatal environment) on spermatogenesis in adult boars.

Materials and Methods

Fifteen pairs of littermate boars (Hampshire x Duroc x Large White x Yorkshire) were housed in mixed-sex groups of 12 or 6 animals from birth until 90 days of age. At birth, littermate boars were randomly assigned to litters of 6 or 12 animals. Each litter consisted of equal numbers of male and female pigs. After weaning (21 days of age) until 90 days of age, lactation litters remained intact and space requirements for pigs in both treatments exceeded published recommendations (3 square feet per pig). At 90 days of age, boars were separated from gilts and housed in groups of 3. Space allocations per boar were 9 square feet. At 150 days of age, all boars were placed in individual crates (24 square feet) and trained for collection from a dummy sow. From weaning through 150 days of age, boars were fed a complete corn, soybean meal diet ad libitum that exceeded NRC recommendations for growing swine. From 150 days of age through the end of the experiment, boars were fed 7 pounds per day. Water was also provided ad libitum.

Body weights and testicular size were measured at 21, 90 and 120 days of age. After all boars had been successfully trained to mount a dummy sow, semen was collected once per week for 1 year. The only exception to this collection regimen was that during 1 week every four months daily production of spermatozoa was estimated. This was done by collecting boars for 4 consecutive days; resting them for 3 days; and then collecting them once immediately after the rest period. Four consecutive days of semen collection presumably depletes the majority of the spermatozoa stored in the cauda epididymi. During the rest period, new spermatozoa are transported to and stored in the cauda epididymi. Consequently, the total number of spermatozoa in the ejaculate after the rest period divided by the number of days in the rest period is an estimate of daily sperm production.

Body weights and testicular size were analyzed with analysis of variance procedures for repeated measures (Gill and Hafs, 1971). The model included birth litter, treatment (groups of 6 versus groups of 12), age (21, 90 and 120 days) and appropriate interactions. Boar nested within treatment was the error term used to test for treatment effects. When significant interactions between treatment and age were present, differences between treatments within each age group were determined with analysis of variance procedures (Snedecor and Cochran, 1989). Total number of spermatozoa per ejaculate and daily production of spermatozoa were also analyzed with analysis of variance procedures for repeated measures. The model included birth litter, treatment, time (week) and appropriate interactions. Boar nested within treatment was the error term used to test for treatment effects. No significant interaction between time and treatment was present (p=0.61). Consequently, only data concerning the main effect of treatment is presented.

Results and Discussion

In general, boars housed in groups of 12 weighed less (p=0.05) and had smaller testicles (p=0.05) than their counterparts housed in groups of 6 (Table 1). The difference in body weight present at weaning was maintained through 120 days of age. Differences in testicle size, estimated by determining testicular volume, were first observed at weaning and were still present at 90 days of age. However, by 120 days of age, testicle size was similar (p=0.41) between treatments. No differences were observed between the two treatments in the percentage (p=0.51) of, or age (p=0.67) at which boars were successfully trained for semen collection (data not shown). However, total spermatozoa per ejaculate and estimates of daily production of sperm cells, were greater (p=0.05) for boars housed in groups of 6 compared with their counterparts reared in groups of 12. Farrowing rate and litter size data from sows bred to boars from each treatment is currently being collected.

The experiment was designed such that boars were provided the same amount of space, identical nutritional regimens, and equivalent opportunities for feed consumption during the experiment. The only exception to this would be during lactation, where competition for access to the sow’s underline during nursing obviously was greater for animals raised in litters of 12 than 6. In this situation, it is reasonable to speculate that boars raised in litters of 12 consumed less milk than those in litters of 6. This statement is based on the 0.8 kg difference in piglet weight at weaning that was observed between the two groups. However, from weaning through 90 days of age, the primary difference between the two treatments was the social environment (11 versus 5 pen mates) in which the boars resided.

Table 1. Effect of Group Size during Sexual Maturation on Sperm Production in Boars.

Parameter	Group Size of 6	Group Size of 12
Weaning Body weight (kg) Testicular size (cm^2)	7.7 + 0.2 5.5 + 0.5	6.9 + 0.3* 4.1 + 0.4*
90 Days of Age Body weight (kg) Testicular size (cm^2)	54.2 + 2.1 53.6 + 3.2	47.1 + 2.3* 45.7 + 3.3*
120 Days of Age Body weight (kg) Testicular size (cm^2)	83.4 + 2.9 106.2 + 5.3	75.2 + 2.8* 105.1 + 5.8
Sperm Characteristics (> 180 days) Spermatozoa per ejaculate (x 10^9) Daily sperm production (x 10^9)	40.5 + 0.2 0.5 + 0.03	32.1 + 0.1* 0.4 + 0.05*
*different from Group Size of 6 (p < .05)

These data indicate that the social environment (through 90 days of age) and, perhaps, the level of nutrition that boars receive during the first several months after birth can have a significant effect on sperm output as an adult. Neither the physiological mechanisms, nor the causative agents within the production environment responsible for this effect are known at the present time. However, as mentioned earlier, it is possible that less competition during these periods may allow for increased sertoli cell mitosis and, thus, more potential for production of spermatozoa as an adult.

Implications

These data demonstrate that the first 3 months of life, a period largely ignored in boar development programs, has important ramifications for adult sperm production. Consequently, until more information is available, a conservative approach of reducing the number of pigs per litter and per pen in which potential A.I. boars are raised probably is advisable. It is interesting to note that this practice is common among programs for development of replacement gilts due to published reports of an inverse relationship between the number of pigs per litter during lactation and subsequent reproduction as an adult (Nelson and Robison, 1976).

Literature Cited

Colenbrander, B., Frankenhuis, M. and Wensing, C.J.G. 1982. Male sexual development. In: Control of Pig Reproduction. D.J.A. Cole and G.R. Foxcroft (eds.), Butterworths Scientific, London, pp. 3-24.

Gill, J.L. and Hafs, H.D. 1971. Analysis of respeated measurements. J. Anim. Sci. 31, 331-340.

Huang, Y.-T. and Johnson, R.K. 1996. Effect of selection for size of testis in boars on semen and testis traits. J. Anim. Sci. 74, 750-760.

Nelson, R.E. and Robison, O.W. 1976. Effects of postnatal maternal environment on reproduction in gilts. J. Anim. Sci. 43, 71-77.

Snedecor, G.W. and W.G. Cochran. 1989. Statistical Methods (8^th Ed.). Iowa State University Press, Ames, Iowa.