Improving Boar Fertility
Box 7621, North Carolina State University, Raleigh, NC, USA 27695-7621
Introduction
Sows and boars are generally considered to be equal partners in the outcome of a mating. The boar's contribution is placement of adequate numbers of fertile spermatozoa in the female reproductive tract, while the sow's involvement consists of production of fertile eggs and maintenance of a uterine environment conducive to embryonic and fetal development. However, a single boar is vastly more important than an individual sow in terms of affecting the overall fertility of a herd. This simply is due to the fact that boars normally breed between 50 and 1000 sows in a year depending on the type of mating that is used, whereas sows routinely farrow between two and three litters during the same time period. Consequently, decisions regarding boar management and its effect on male fertility could be viewed as being exponential in terms of their effects on farrowing rate and litter size, compared to those made for individual or even groups of sows. As a result, boar fertility has a major impact on the reproductive efficiency of swine operations.
Fertile boars are expected to produce large numbers of spermatozoa that are capable of fertilization. Therefore, improvements in boar fertility can result from increasing numbers of sperm produced and/or improving their ability to fertilize eggs. This is true regardless of whether matings are natural or artificial. Consequently, the primary intent of this presentation is to examine recent developments in terms of biological and environmental factors that influence qualitative and quantitative aspects of sperm production in boars.
Measurement of Boar Fertility
One could argue, very effectively, that before improvements can be made in boar fertility, an accurate method of measuring it needs to be available. The best measure of boar fertility is actually the production of live offspring, i.e., farrowing rate and numbers of pigs born alive. The ideal situation for measuring these would be to use semen from boars to breed large numbers of females on many different operations and monitor their subsequent reproductive performance. Breeding large numbers of females on different operations is important because it safeguards against the possibility that erroneous conclusions are made due to a sampling error. Sampling errors occur when factors other than, or in addition to, male fertility are present and not equally represented in each boar's matings. For example, one boar, by chance, may breed a group of females that are exceptionally fertile while another boar, also by chance, could be mated to sows that are below average in terms of their reproductive potential. Thus, differences in fertility could be due to the boars, sows, or both. In theory, as data from the number of females and locations increases in fertility evaluations, then the probability of making sampling errors decreases because males are likely to encounter sows that are both above and below average fertility (or other factors) with an equal frequency.
Use of Individual Boar Records
The dairy industry has an excellent system, based on individual bull records, for evaluation of male fertility. Frozen semen from bulls is routinely purchased by different farms and used to breed large numbers of cows over time. Thus, it is not uncommon to have several thousand calving records on each bull for use in assessing fertility. Consequently, estimations of fertility for dairy bulls are very accurate. Based on the success of the dairy industry, it is often suggested that individual records could also be used to evaluate boar fertility. While the recent increase in the implementation of AI has increased the feasibility of this approach, there are still several reasons that, at present, limit its usefulness for the swine industry.
In contrast to dairy cattle, pigs have a relatively short generation interval due to their short gestation length and young age at sexual maturity. As a result, in order to optimize genetic improvement, boars within herds are replaced at a faster rate than dairy bulls. It is not uncommon within progressive segments of the swine industry for boars to be used only 12 to 18 months before they are culled. Although this practice is advantageous for genetic improvement, it actually makes the use of individual records for assessing boar fertility very difficult and, perhaps, impractical. It is generally accepted that at least 100 litters per boar are necessary to make sure that differences in litter size are actually due to boar fertility and not sampling errors. Based on the assumption that a young boar could be used to breed 2 sows per week with natural service (4 matings per week) and 8 sows via AI (16 doses per week), then it would take approximately 16 and 9 months, respectively, just to obtain enough data to use for assessment of his fertility. For an operation using natural service, it is conceivable that adequate reproductive data would not be available until just before the boar is replaced. The situation is only slightly improved for AI boars.
Moreover, a breeding regimen that uses homospermic matings is required for use of individual boar records for measuring male reproductive performance. If sows are bred to different boars during natural matings or pooled semen is used for AI, then it is impossible to determine which boar was responsible for the success or failure of the mating without the use of very sophisticated tests. A recent summary of breeding practices in North America estimated that over 70% of sows receive heterospermic inseminations (Flowers, 1997). Consequently, use of individual records for assessing boar fertility would require that the industry implement homospermic mating regimens instead of continuing their current practice of heterospermic inseminations.
Despite its shortcomings, use of individual records for evaluation of male fertility could be implemented within the swine industry. However, it would require a major change in the manner in which boars normally move from the nucleus segment to the commercial sector of the industry. Boars from genetic suppliers are sold to commercial operations at 5 to 6 months of age, and after an isolation period of approximately 30 days are used to breed sows. With this current system, there is virtually no information concerning boar fertility. As mentioned previously, an evaluation period of at least 9 months would be necessary to obtain adequate records if individual farrowing rates and litter size were to be used as an index of fertility. Consequently, boars subjected to this 9 month test period would be 14 to 15 months of age when they enter commercial production. The additional time required for fertility evaluations would most likely result in increased costs and add to the purchase price of boars. Whether or not fertility information obtained in this way would offset the added costs is difficult to predict. As a result, it is doubtful that use of individual boar records will be adopted by the swine industry to assess or improve boar fertility.
Microscopic Semen Tests
An alternative approach for assessment of boar fertility is to evaluate semen quality and hope that the various parameters measured are positively correlated with male reproductive performance. The three most common qualitative measures performed on semen are motility, morphology, and acrosome integrity. While it is generally accepted that ejaculates with high percentages of motile and morphologically normal spermatozoa are of higher quality than their counterparts with reduced values, the exact relationship between these qualitative observations and semen fertility is less clear.
In a recent study, Flowers (1997) attempted to determine the relationship between motility, morphology, acrosome integrity, and fertility. Twelve boars were collected once per week for one year. Ejaculates were evaluated for the percentage of spermatozoa exhibiting forward motility and the percentage of spermatozoa with normal tail and acrosome morphology; they were then extended and used to breed at least 8 sows per week. Estimates of semen quality were divided into 10 categories: 0 to 10%; 11 to 20%; etc. for each measurement that was taken. Results from this study for motility are presented in Figure 1. These data indicate that there appears to be a biphasic relationship between estimates of motility and fertility. As the percentage of motile spermatozoa in an ejaculate increases from very low values, farrowing rate and numbers of pigs born alive also increase. However, at a certain point, motility estimates continue to increase, but fertility estimates do not. Thus, the critical point for motility in this data set, was 60%. The same relationships apply for morphology and acrosome integrity with the critical points being 70% and 50%, respectively. These data demonstrate that common microscopic tests performed on semen are qualitative rather than quantitative with regard to their relationship with fertility. In other words, they can be used to group ejaculates into two categories, fertile and subfertile. However, for ejaculates within the fertile category, they cannot be used to further distinguish differences in fertility.
Figure 1. Relationship among farrowing rate, number of pigs born alive and the percentage of spermatozoa exhibiting forward motility
These results should not be surprising. The forward motion of spermatozoa and the enzymes within their acrosomes provide the mechanical and biochemical means to penetrate the cells and membranes around the egg during fertilization. However, once penetration has occurred, the sperm cell still must fuse with the egg and deposit its genetic information. Therefore, it is entirely possible, and probable, that spermatozoa which appear to be motile and morphologically normal could actually differ considerably in their ability to fertilize eggs. In spite of this, microscopic evaluation of semen still is an important method of improving boar fertility. The critical points for motility, normal morphology and acrosome integrity can serve as acceptance criteria for use of boars or ejaculates in operations using natural matings and AI, respectively. On the average, boars or ejaculates that do not meet these criteria would be expected to have reduced fertility and should not be used.
In an attempt to determine which microscopic parameter was the most repeatable indicator of fertility, various statistical procedures were performed to compare predictions of fertility with the actual values obtained. These results revealed that acrosome morphology was a more sensitive predictor of fertility than either motility or morphology. This was primarily due to the fact that during the experiment the majority of the ejaculates evaluated either had a very high (> 90) or low (<50) percentage of spermatozoa with normal acrosomes. In contrast, for both motility and normal morphology, ejaculates were relatively evenly distributed among the ten categories. Consequently, motility evaluations, even though they are easy to perform under commercial conditions, probably receive more attention than they merit in terms of being estimates of semen fertility. In contrast, evaluation of normal acrosome morphology requires additional time, technical expertise and equipment compared to the assessment of motility. However, it is a more reliable parameter upon which to qualitatively estimate semen fertility.
Other Fertility Tests
A number of other procedures that measure various aspects of interactions between spermatozoa and eggs during fertilization have been implicated as being able to improve the accuracy and precision of fertility tests. Some of these include assessment of the nuclear structure of DNA from sperm cells, egg penetration, sperm membrane properties, and the presence of specific proteins in seminal plasma. Of these, characterization of the protein profile in seminal plasma appears to be the most practical to implement in commercial situations. This is due to the fact that a relatively simple test could be developed for seminal plasma for the identification of specific proteins in much the same way the milk progesterone kit was developed for detection of pregnancy in dairy cattle. The other procedures mentioned, at the present time, require either sophisticated equipment or long incubation periods for accurate results. In summary, based on the lack of a truly predictive fertility test for boar semen, it is difficult to definitively establish cause and effect relationships between boar fertility and things such as nutritional regimens, housing conditions, collection frequencies, photoperiod and other aspects of the production environment.
Improving Production of Number of Spermatozoa
As mentioned previously, increasing the number of spermatozoa produced per ejaculate is an important aspect of boar fertility. A boar can possess fertile sperm cells, but if adequate numbers are not ejaculated in a consistent manner, then fertility can be compromised. Because numbers of spermatozoa are quantified with AI, changes in sperm output are known and, thus, can be dealt with by adjusting the number of doses per ejaculate. However, significant and unexpected reductions in sperm numbers can create problems in that the collection frequency of another boar (or boars) may have to be increased and this, indirectly, could result in alterations in their semen output and, possibly, fertility. In contrast, with natural matings, the number of spermatozoa placed into the female reproductive tract is never known. Therefore, maintaining a consistently high production of sperm cells is critically important aspect for boars used for natural service.
Production of Numbers of Spermatozoa
Before the influence of various management factors on sperm production are discussed, consideration of the biological basis for spermatogenesis is important because it can be used, to some extent, to prioritize management needs for boars. Figure 2 contains a schematic that illustrates the process of spermatogenesis. The production of sperm cells requires between 50 and 60 days in the boar. Every two to three days, a new group of sperm cells leave the resting pool and begin to develop. The majority of the developmental process takes place in the testicles. However, the final two weeks occurs in the epididymi. After spermatogenesis is complete, mature sperm cells are stored in the epididymi where they reside until they are ejaculated. If they are not ejaculated, then they will eventually be broken down and absorbed as new ones are produced and enter storage.
Based on the schematic, there are two basic ways that numbers of sperm produced can be increased - either by increasing the number of spermatozoa that leave the resting pool with each group that develops or by decreasing the two to three day interval between consecutive developmental groups. The mechanisms controlling these processes are still under investigation and as more information about them becomes available new strategies for increasing sperm production may become available. From a boar management perspective, factors affecting spermatogenesis and, ultimately, boar fertility can be viewed as having either a positive or negative effect. Consequently, managing boars for production of large numbers of fertile spermatozoa involves accentuating the positive effects while, simultaneously, minimizing the negative ones. Additionally, the relative magnitude of these factors can serve as a way to prioritize management practices for improving or maintaining sperm production.
Schematic representation of spermatogenesis in boars.
Positive Influences on Production of Sperm Numbers
Recently, the effect of photoperiod on sperm production in boars has received renewed interest, especially with the advent of AI and large, centrally-located boar studs. Studies investigating the influence of photoperiod on sperm production have produced inconsistent results - some have seen marginal improvements, while others have seen no effect. From a physiological perspective, researchers have not been able to demonstrate a consistent endocrine response to alterations of the photoperiod in boars. This is an important observation since in most animals whose reproduction is linked to photoperiod, changes in testosterone and other hormones respond to changes in the light/dark ratio. Consequently, changes in photoperiod may marginally affect spermatogenesis in boars, but it is not a major avenue through which sperm production in boars can be consistently improved.
In contrast to the influence of photoperiod, there appears to be a high positive correlation between testis size and sperm production in both prepubertal and mature boars. This is probably due to the fact that testicular size is positively correlated to the number of sertoli cells. Sertoli cells are the cells which nurture spermatozoa during development. Thus, a boar with increased numbers of sertoli cells would be expected to have more spermatozoa and, consequently, enhanced sperm output. Finally, the relationship between testis size and sperm production is highly repeatable. Collectively, these data indicate that selection or use of boars with large testicles can be used to enhance the reproductive capacity of boars in terms of numbers of sperm produced.
Negative Influences on Production of Sperm Numbers
The general consensus from studies investigating the influence of various nutritional regimens on sperm production is that prolonged periods of undernutrition reduce the total number produced. In most of the situations where nutrition limited spermatogenesis, the magnitude of the protein and energy deficiencies were about 20 to 25% and the period over which the restriction occurred was at least 6 to 8 weeks. While these situations probably are never likely to occur in a well-managed herd, it is important to examine current management strategies to determine if analogous situations could possibly exist. A common practice in boar management is to limit-feed young, sexually mature boars in order to limit their weight gain. This increases the usefulness of the boar in that he can be used to breed gilts and small sows for a longer period of time. This practice may continue throughout the 12 to 18 month period that the boar is used for breeding or longer. Based on the results of nutritional restriction studies, this practice could be viewed as a mild form of long-term, underfeeding of a young growing animal. Whether or not it is sufficient to influence sperm production has not been investigated extensively, especially in modern, lean genotypes that tend to mature, physiologically, at advanced ages. However, this practice may need to be re-examined, especially, in light of recent work that demonstrates clearly that similar nutritional regimens applied to young, maturing gilts can limit their reproductive performance during their first several parities.
Perhaps, a more relevant point of discussion concerning boar nutrition is the early identification of suboptimal nutritional regimens. Because spermatogenesis requires between 50 to 60 days in boars, sperm numbers and morphology would not be expected to immediately reflect nutritional deficiencies. Consequently, a problem could remain undetected for several months. In most of the nutritional restriction studies, reductions in boar libido and mating behaviours occurred earlier than any observed changes in sperm production. This is not surprising since the physical act of mating requires a significant expenditure of energy. Therefore, changes in normal mating behaviours probably could be used as an early indicator of subsequent problems in sperm production due to nutritional deficiencies in boars.
In contrast to the effects of nutrition, exposure to elevated ambient temperatures has immediate, severe and consistent effects on spermatogenesis. These include ejaculation of a large proportion of spermatozoa with morphological abnormalities, decreased sperm numbers and reduced fertility. The minimum exposure time and critical air temperature above which sperm production is impaired probably vary from location to location. However, boars exposed to at least 3 days with temperatures greater than 30C should be considered as being "at risk" of experiencing the negative consequences of heat stress. Another important point to remember is that a boar's ability to dissipate body heat is influenced by both the ambient temperature and humidity. Consequently, a heat index probably is a more accurate way to monitor the climatic environment for boars. For example, even though the temperature may only be 25C within a barn, if the humidity is greater than 90%, then the boar's body may actually react like it is being exposed to temperatures greater than 30C. The best management strategy to minimize the negative influence of heat stress on spermatogenesis is to have supplemental cooling systems in locations where boars are housed and have them set to activate at 2 to 5C below the critical temperature for heat stress. Furthermore, the activation temperature may need to be adjusted daily to adjust for daily changes in humidity.
Obviously, an important aspect of efficient semen production is to prevent boars from being exposed to acute periods of heat stress. However, few studies have examined alterations in sperm output from boars exposed for extended time periods to temperatures in the upper range of their thermal comfort zone, 26 to 29C. Chronic exposure to temperatures within this range probably occur routinely during the summer months in mechanically ventilated facilities located in temperate and semi-tropical climates. Figure 3 represents field data from seven commercial boar studs in southeastern North Carolina from June through September. Each stud houses an average of 200 boars and has a weekly production goal of 2000 insemination doses. The average weekly high temperature in these facilities never increased above 29C during the recording period. However, a significant increase in the number of ejaculates that were rejected due to poor quality and a corresponding decrease in the number of insemination doses per ejaculate were observed. It is interesting to note that the downward trend in the number of doses per ejaculate began five to six weeks after the weekly high temperatures stabilized at 27.5C. More controlled studies are needed to verify that a chronic exposure to temperatures near the upper limit of a boars thermoneutral zone can reduce sperm cell numbers and quality. However, these field data demonstrate the importance of a conservative policy for establishing guidelines for setting the activation temperatures of cooling systems for boars.
Figure 3. Average changes in weekly high ambient temperatures, number of ejaculates rejected and total number of doses per ejaculate during the summer months from seven commercial boar studs in North Carolina (with permission, J. Reprod. Fert. Suppl. 52 in press)
Summary
In summary, improving boar fertility may not be as easy as it first appears. A major limiting factor is the absence of a quick, accurate and proactive fertility test for boar semen. Development of such a test would greatly facilitate management of boar fertility. In spite of the lack of such a test, there are still some things that should be done on every operation to optimize the fertility of the boars that are present.
References
Flowers, W.L.
(1997) Management of boars for efficient semen production. Journal of Reproduction and Fertility Supplement 52 (in press).