Heat stress-induced sperm DNA damage has recently been demonstrated in boars during tropical summer; which could negatively impact early embryo survival and litter size in sows. Given the boar’s inefficient capacity to sweat, non-pendulous scrotum and low antioxidant activity in seminal plasma, elevated endogenous levels of antioxidants are needed to combat reactive oxygen species induced during periods of heat stress. This should prevent the build-up of pathological levels of DNA damage in boar spermatozoa. Our aim was to investigate whether a combined antioxidant supplement could mitigate sperm DNA damage in boars exposed to tropical summer conditions. Terminal deoxynucleotidyl transferase dUTP nick end labelling and flow cytometry of 20,000 spermatozoa/boar/treatment revealed that boar diets supplemented with 100 g/day custom-mixed antioxidant during peak wet summer effectively reduced sperm DNA damage by as much as 55% after 42 and 84 days treatment respectively (16.1 ± 4.9 peak wet control vs . 9.9 ± 4.5 42 day vs . 7.2 ± 1.6% 84 day treatments; P ≤ 0.05). Supplementation did not improve sperm concentration beyond control levels for either season ( P > 0.05); nor alter total motility, progressive motility or several other motion parameters measured by computer assisted sperm analysis of 20 x 10 6 sperm/mL at 38°C ( P > 0.05). Antioxidant supplementation during tropical summer appears to mitigate the negative impact of heat stress on DNA integrity but not concentration nor motility of boar spermatozoa; which may provide one solution to the problem of summer infertility in the pig.

Summer infertility continues to undermine pig productivity, costing the pig industry millions in annual losses. The boar’s inefficient capacity to sweat, non-pendulous scrotum and the extensive use of European breeds in tropical conditions, can make the boar particularly vulnerable to the effects of heat stress; however, the link between summer heat stress and boar sperm DNA damage has not yet been demonstrated. Semen from five Large White boars was collected and evaluated during the early dry, late dry and peak wet seasons to determine the effect of seasonal heat stress on the quality and DNA integrity of boar spermatozoa. DNA damage in spermatozoa during the peak wet was 16-fold greater than during the early dry and nearly 9-fold greater than during the late dry season. Sperm concentration was 1.6-fold lower in the peak wet than early dry whereas no difference was found across several motility parameters as determined by computer-assisted sperm analysis. These results demonstrate that tropical summer (peak wet season) induces DNA damage and reduces concentration without depressing motility in boar spermatozoa, suggesting that traditional methods of evaluating sperm motility may not detect inherently compromised spermatozoa. Boar management strategies (such as antioxidant supplementation) need to be developed to specifically mitigate this problem.

Abstract.Temperature is a crucial factor in mammalian spermatogenesis. The scrotum, pampiniform plexus, and cremaster and dartos muscles in mammals are specific adaptations to ensure sperm production in a regulated environment 4À6 C below internal body temperature. However, the limited endogenous antioxidant systems inherent in mammalian spermatozoa compounded by the loss of cytosolic repair mechanisms during spermatogenesis, make the DNA in these cells particularly vulnerable to oxidative damage. Boar sperm is likely to be more susceptible to the effects of heat stress and thus oxidative damage due to the relatively high unsaturated fatty acids in the plasma membrane, low antioxidant capacity in boar seminal plasma, and the boar's non-pendulous scrotum. Heat stress has a significant negative impact on reproductive performance in piggeries, which manifests as summer infertility and results in productivity losses that amount to millions of dollars. This problem is particularly prevalent in tropical and subtropical regions where ambient temperatures rise beyond the animal's zone of thermal comfort. Based on preliminary studies in the pig and other species, this article discusses whether heat stress could induce sufficient DNA damage in boar sperm to significantly contribute to the high rates of embryo loss and pregnancy failure observed in the sow during summer infertility. Heat stress-induced damage to sperm DNA can lead to disrupted expression of key developmental genes essential for the differentiation of early cell lineages, such as the trophectoderm, and can distort the timely formation of the blastocyst; resulting in a failure of implantation and ultimately pregnancy loss. Confirming such a link would prompt greater emphasis on boar management and strategies to mitigate summer infertility during periods of heat stress.

Abstract.The sperm reservoir is formed when spermatozoa bind to the epithelium of the utero-tubal junction and caudal isthmus of the oviduct. It is an important mechanism that helps synchronize the meeting of gametes by regulating untimely capacitation and polyspermic fertilisation. This study investigated the influence of epididymal maturation and caudal fluid on the ability of spermatozoa to bind to oviduct epithelium using a model porcine oviduct explant assay. Spermatozoa from the rete testis, middle caput (E2-E3), middle corpus (E6) and cauda (E8) of Large White or Large White x Landrace boars at 10-14 months of age were diluted in modified Androhep solution and incubated with porcine oviduct explants. Results reported in this study support our hypothesis that testicular spermatozoa need to pass through the regions of the epididymis in order to acquire the ability to bind to the oviduct. There was a sequential increase in the number of spermatozoa that bound to oviduct explants from the rete testis to caudal epididymis. Binding of caudal spermatozoa to isthmic explants was the highest (15.0 ± 1.2 spermatozoa per 1.25 mm 2 ; mean ± standard error of the mean; P ≤ 0.05) and lowest by spermatozoa from the rete testis (2.0 ± 0.3 per 1.25 mm 2 ), and higher to isthmus from sows compared to gilts (35.8 ± 6.7 per 1.25 mm 2 vs. 14.8 ± 3.0 per 1.25 mm 2 ; P ≤ 0.05). Binding of ejaculated spermatozoa to porcine isthmus was higher than for caudal spermatozoa (26.3 ± 1.4 per 1.25 mm 2 vs. 15.0 ± 0.8 per 1.25 mm 2 ; P ≤ 0.05), and higher to porcine than to bovine isthmus (26.3 ± 2.3 per 1.25 mm 2 vs. 18.8 ± 1.9 per 1.25 mm 2 ; P ≤ 0.05). Incubation of spermatozoa from the caput and corpus in caudal fluid 2 increased the ability of spermatozoa to bind to oviduct epithelium (P ≤ 0.05). In conclusion, the capacity of testicular spermatozoa to bind to oviduct epithelium increases during their maturation in the epididymis, and can be advanced by components of the caudal fluid.