In utero heat stress causes reduced testicular area at puberty, reduced total sperm production, and increased sperm abnormalities in boars.

In utero stress has been shown to negatively affect intact male rats and mice, though very little research has been conducted in boars. The objectives of the present studies were to determine the effects of in utero heat stress (IUHS) on postnatal development and the response to postnatal heat stress of boars. Ten boars were selected at weaning from litters subjected to IUHS or in utero thermoneutral (IUTN) during 30-60 days of gestation. The boars were evaluated for reproductive performance from birth through 57 weeks of age (WOA). Testicular area tended to be smaller for IUHS boars compared to IUTN boars at 24 WOA (P = 0.080). Libido did not differ for IUHS or IUTN (P = 0.818). Total sperm production was reduced in IUHS boars compared to IUTN boars (P ≤ 0.038). Semen volume and semen concentration did not differ (P ≥ 0.469 and P ≥ 0.664, respectively). Total motility and progressive motility did not differ for IUHS and IUTN boars (P ≥ 0.430 and P ≥ 0.652, respectively). In utero heat stressed boars had a greater incidence of sperm with tail abnormalities than IUTN (P ≤ 0.042). In utero heat stressed boars had a lower incidence of sperm with proximal droplets following mild, postnatal heat stress compared with IUTN (P = 0.005). In utero heat stress resulted in significant reductions in sperm production and increased sperm abnormalities in boars. The IUHS boars may be slightly more tolerant to postnatal heat stress, though more research is needed.