Heat Stress and in utero Heat Stress: Effects on Boar Reproduction and the Efficacy of Nutritional Mitigation
With the majority of commercial swine farms utilizing artificial insemination, reproductive efficiency is a primary focus of producers and researchers. Boar reproduction specifically, can have a major impact on the overall reproductive efficiency of the swine industry. Heat stress has a negative effect on the swine industry. In growing pigs it causes reduced growth, in sows reduced fertility, and in boars it causes reduced sperm production and quality. In addition, more recent studies have implied that gestational (in utero ) heat stress (IUHS) can add insult to injury, resulting in further reductions in animal growth and reproduction. However, very little emphasis has been placed on the effects of IUHS on postnatal boar reproduction. If IUHS further reduces boar reproduction, the consequences of heat stress are far more damaging than initially estimated. With this in mind, researchers and producers have been investigating methods to mitigate heat stress on animals. Some of these methods include sprinklers and cooling pads, but others focus on mitigating heat stress with nutritional modifications. Based on these findings, experiments were conducted to further investigate the effects of heat stress with an emphasis on the effects of in utero heat stress on boar reproduction and the efficacy of nutritional strategies to mitigate the negative effects of heat stress on boar reproduction. Boars that were heat stressed in utero were reared and studied through postnatal development and into adulthood. These boars produced less sperm than their in utero thermoneutral (IUTN) conspecifics. Additionally, they may be more adapted to maintain sperm quality during postnatal thermal challenge than IUTN boars. Betaine has been studied as a stress mitigant in many species and situations. Phytase on the other hand is generally added to diets to improve nutrient digestibility but may also play a role in heat stress mitigation. A study was conducted to determine the effect of betaine and a combination of betaine and phytase on boar reproduction during and after a heat stress event. While sperm production, motility and percentage of morphologically normal sperm were not impacted by betaine or betaine with phytase, these diets appeared to better prevent sperm morphological abnormalities during and immediately after heat stress compared to control boars. This study utilized a mild heat stress, which may have concealed the impacts of these supplements on semen quality. Additionally, the study was conducted in the fall and it is possible that the boars were acclimatized to the heat stress. Vitamins have also been long studied for their antioxidant capabilities, specifically vitamins C and E. Vitamin D plays a major role in metabolism and may also have a role as a heat stress mitigant. A study was conducted on a commercial boar stud during the summer months to determine the impacts of these vitamins on semen quality. This study did not observe any major effects of diet on semen quality in boars. The barn temperatures in this experiment were extremely mild due to the cool-cell system used. However, reductions in semen quality were still observed overall throughout the duration of the study, suggesting a seasonal effect. We have hypothesized two potential causes of this effect. One possible explanation is that the boars have a reduced upper critical temperature, which may have been a result of intensive genetic selection. Alternatively, it has been suggested that pigs are seasonal breeders and the effects seen in this study may have been a result of this seasonality that was affected by something other than temperature. Overall, these studies have resulted in the need for further research in the areas of IUHS in boars and further investigation into the upper critical temperature in modern day boars.
Stewart, Purdue University.
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