Effects of in Utero Heat Stress on Boar Growth and Reproduction Prior to, during, and after Puberty.

The objective of this study was to determine the effects of in utero heat stress (IUHS) on reproductive performance in boars. Confirmed pregnant gilts at the University of Missouri were subjected to either thermoneutral conditions (15 to 20ºC; RH 60 to 50%; TN) during gestation (IUTN), or were subjected to TN from d 1-30, heat stress conditions (27 to 37 ºC; ~55% relative humidity) from d 30-60, and TN from d 60 until farrowing. Intact males were weaned at 3 weeks of age (WOA), and transported to Purdue University (n = 5 per treatment). Anogenital distance was recorded at birth and testicular measurements (digital calipers) and body weights (BW) were taken on a monthly basis (3-42 WOA). Animals were trained for semen collection at 24 weeks of age and libido scores recorded. Semen was collected one time per week and evaluated for total sperm production per ejaculate (volume*concentration), morphological abnormalities, and motility and mobility estimations using computer assisted semen analysis (Ceros II, IMV Technologies). Statistical ANOVA was performed using the mixed procedure of SAS 9.4. Anogenital distance at birth tended to be greater in IUHS compared to IUTN boars (P = 0.065). Testicular growth rate was less during puberty (11-24 WOA; 4.61 cm²/wk vs. 7.04 cm²/wk ± 0.58; P = 0.015), greater post puberty (24-42 WOA; 3.51 cm²/wk vs. 2.37 cm²/wk ± 0.34; P = 0.031), and tended to be greater prior to puberty (3-11 WOA; 1.79 cm²/wk vs. 1.47 cm²/wk ± 0.11; P = 0.060) for IUHS compared to IUTN. Average weekly libido score and BW did not differ (P = 0.841 and P = 0.938, respectively). Total sperm production per ejaculate was lower in IUHS (41.4x10⁹ vs. 33.1 x10⁹ ± 1.3, P < 0.001) and tail abnormalities were higher in IUHS (5.47% vs. 3.45% ± 0.64, P = 0.010) compared with IUTN. There tended to be an increased percentage of sperm with distal mid-piece reflexes in IUHS compared to IUTN (2.12% vs. 1.27% ± 0.43, P = 0.098). Amplitude of lateral head displacement was less for IUHS (3.9 μm vs. 4.9 μm ± 0.3, P = 0.018), and curvilinear velocity was slower for IUHS (84 μms^-1 vs. 103 μms^-1 ± 5.7, P = 0.045) compared to IUTN. Total motility did not differ (P = 0.200). In summary, IUHS had decreased sperm production, diminished sperm quality, and delayed testicular growth during puberty, indicating the negative repercussions on boar reproduction.