By Dr. Tasha Gruhot and Dr. Benny Mote.
As many as half of the young boars entering the boar stud are reported to fail quality thresholds to be used in production. Poor semen quality is also the top reason for boar culling regardless of age, only led by removal due to genetic merit. Semen quality thresholds are put in place as poor semen quality used in matings results in lower conception rates and litter size. Failure of a boar to meet quality thresholds or meeting thresholds by a minimum can have large economic impacts on production as this reduces the number of matings genetically superior boars contribute to and reduces response of important production traits in offspring.
Summer heat has been shown to exacerbate already existing semen quality issues, causing genetically inferior boars to be utilized as well to meet semen production needs, further reducing response in offspring. Heavy selection pressure and management focus has been put on female reproductive success, but little to no emphasis has been placed on selecting boars for increased semen quality, though semen quality is half of the equation to a successful pregnancy. Several studies were conducted at University of Nebraska- Lincoln (UNL) to help understand the genetic components of semen quality and to lay the groundwork for potential selection of boar reproductive factors and are described in this article.
Proper spermatogenesis cannot occur at body temperature, which is why the testicles are located outside of the main body cavity and arterial blood is cooled prior to entering the testicles. The pampiniform plexus is the venous network that functions to cool arterial blood prior to it entering the testicles. When dilation or damage to the pampiniform plexus occurs, this cooling system does not work properly and has been shown to have detrimental effects on semen quality in other species. Dilation of the pampiniform plexus veins is referred to as varicocele. Pampiniform plexus vessel size and varicocele was determined via ultrasonography in 327 maternal line boars, across three generations, at the UNL research farm. Boars with varicocele had significantly lower sperm concentration (p <0.05), increased distal droplet percentage (p<0.05), and tended to have decreased sperm head area (p=0.06) compared to boars without varicocele.
Through a heat stress trail that was conducted on both boars with and without varicocele, results suggest that boars with varicocele may be more susceptible to heat-stress related semen quality issues as an interaction between collection time point post heat stress and varicocele presence existed for distal droplet percentage. The heritability of varicocele and pampiniform plexus vessel size was also assessed. Varicocele had a heritability of approximately 0.25 and average vessel size heritability was 0.49. These heritability estimates suggest that selecting against varicocele would be possible. Varicocele was diagnosed in approximately 23 percent of the boars ultrasounded in this study. This prevalence agrees with a past study that utilized palpation to diagnose varicocele in terminal line boars (Kleve-Fled et al., 2015). If this prevalence is confirmed in other populations, this could indicate that varicocele may be
influencing semen quality on a large portion of boars in production leading to significant economic consequences.
A second trial was conducted in which semen was processed on 363 Duroc boars from May through October 2017 to assess the heritability and repeatability of sperm quality traits over the summer season. The phenotypic and genetic correlations between sperm quality traits were also assessed along with the effect of these traits on total number born. The following are the sperm quality traits that were assessed: total percentage of motile cells, total percentage of progressively motile cells, percentage of cells with a distal droplet, percentage of cells with a proximal droplet, percentage of cells with a bent tail, percentage of cells with a distal midpiece reflex, mean sperm head elongation, and total sperm cells per ejaculate.
To the researcher’s knowledge, this is the first study utilizing boars to break out the morphological defect traits and analyze them individually versus using a total percent of abnormal cells. All traits showed to be lowly heritable (0.08 to 0.24) indicating selection would be possible to improve semen quality. Repeatability estimates ranged from 0.21 to 0.62, suggesting that during times of heat stress, semen quality is lowly to moderately repeatable between collections. The motility traits were negatively correlated with the morphological defect traits, which means if motility was selected on, less morphological defects should occur. Total sperm per ejaculate was also positively correlated with motility, indicating that selecting on motility would increase total sperm and vice versa.
This study also demonstrated that mean sperm head elongation was positively correlated with motility, meaning larger head size positively influences motility. In agreement with previous research, motility positively influenced total number born, while droplet percentage (both distal and proximal) negatively influenced total number born. To the researcher’s knowledge, this is the first study to describe a negative relationship between bent tail percentage and total number born, where an increase of 4 percent in bent tails could cause a decrease of one pig born per litter. Results from this study also demonstrate that improving semen quality above what is considered the minimal quality threshold at stud would have positive consequences on litter size. All litter data in this study came from collections that passed traditional quality thresholds, and still significant differences in litter size were seen to be associated with sperm quality traits.
By selecting on semen quality, the amount of boars rejected at stud should decrease and the genetically superior animals would be able to be more highly utilized causing an increase response in terminal offspring leading to more productive and efficient market pigs. Better semen quality would also positively influence female reproductive traits such as total number born. As post cervical artificial insemination and other techniques to more highly utilize superior boars gain in popularity, semen quality becomes even more important and it is promising that results indicate selection could be successful for increased sperm quality.