Phenotypic assessment of F0 generation lactating sows genomically selected for heat stress tolerance

Genomic selection for greater HS tolerance (TOL) may enhance swine welfare under heat stress (HS) conditions. However, genomic selection for TOL based on variability in performance traits across environmental gradients tends to be genetically associated with reduced productivity. Therefore, the study objective was to biologically characterize genomic selection for TOL based on the rate of vaginal temperature (TV) change with rising environmental temperatures. Lactating sows (N = 26; Large White × Landrace; parity = 4.85 ± 0.75) divergently genomically selected for TOL (n = 14) or HS sensitivity (SEN; n = 12) in the F0 generation were subjected to cycling temperatures ranging from 28.90 to 30.23 °C from days 2.5 ± 1.0 post-farrowing to weaning (days 21.3 ± 1.1) across 4 repetitions. Indirect calorimetry was used to estimate total metabolic heat production (THP) on days 4, 8, 14, and 18 of lactation on an individual sow basis. Implanted data loggers measured TV daily in 15 min intervals. Respiration rate, skin temperature (ear, shoulder, rump, tail), sow posture, and waterer interactions were recorded daily at 0800, 1200, 1600, and 2000 hours. Sow feed intake (FI), waterer use, and non-nutritive drinker manipulation behaviors were measured daily. Litter weights were measured at birth, days 4, 8, 14, 18, and weaning. Data were analyzed with sow, litter, or sow + litter as the experimental units. Overall, TOL sows had greater THP/kg BW0.75 (P = 0.04; 5.68 ± 0.24 kcal/h/kg BW0.75) versus SEN sows (4.74 ± 0.18 kcal/h/kg BW0.75), that was driven by an increase in THP/kg BW0.75 on day 4 for TOL (P < 0.01; 6.65 ± 1.11 kcal/h/kg BW0.75) versus SEN (3.54 ± 0.59 kcal/h/kg BW0.75) sows. Despite the increase in THP, no TV differences were observed (P = 0.80; 40.11 ± 0.15 °C) between TOL and SEN sows. Respiration rate was greater in TOL (P < 0.01; 5.21%) versus SEN sows. Shoulder, rump, and tail skin temperatures were greater in SEN (P < 0.01; 37.44 ± 0.06, 37.65 ± 0.60, and 36.94 ± 0.10 °C, respectively) versus TOL (37.22 ± 0.06, 37.43 ± 0.60, and 36.69 ± 0.10 °C, respectively) sows. Waterer use and non-nutritive water manipulation frequencies were increased for TOL (P ≤ 0.05; 65.55% and 99.58%, respectively) versus SEN sows. In conclusion, while TV was similar between TOL and SEN sows under HS conditions, TOL sows appeared to maintain TV by relying on behavioral thermoregulation (e.g., waterer use) and latent heat loss (e.g., respiration rate) whereas SEN sows had reduced THP.