Evaluating phenotypes associated with heat tolerance and identifying moderate and severe heat stress thresholds in lactating sows housed in mechanically or naturally ventilated barns during the summer under commercial conditions

Lay Summary Climate change and the associated increase in global temperatures have a well-described negative impact on swine production. Therefore, improving swine heat stress resilience is of utmost importance to reduce the deleterious effects of heat stress on swine health, performance, and welfare. Genomic selection for heat stress resilience may be a viable strategy to improve swine productivity in a changing climate. However, identifying environmental conditions that constitute heat stress and deriving novel traits that can be easily collected on farm and provide accurate and precise predictions of heat stress tolerance is a necessary step. The present study demonstrated that housing conditions had a limited influence on heat stress tolerance phenotypes, several anatomical and thermoregulatory measures were correlated, and housing conditions impacted heat stress threshold temperatures. Results from this study may be applied to large-scale phenotyping initiatives to develop or refine genomic selection indexes for heat stress resilience in pigs. Housing conditions influenced heat stress threshold temperatures but had a limited impact on phenotypes that predict heat stress tolerance in lactating sows housed under commercial conditions. An accurate understanding of heat stress (HS) temperatures and phenotypes that indicate HS tolerance is necessary to improve swine HS resilience. Therefore, the study objectives were 1) to identify phenotypes indicative of HS tolerance, and 2) to determine moderate and severe HS threshold temperatures in lactating sows. Multiparous (4.10 +/- 1.48) lactating sows and their litters (11.10 +/- 2.33 piglets/litter) were housed in naturally ventilated (n = 1,015) or mechanically ventilated (n = 630) barns at a commercial sow farm in Maple Hill, NC, USA between June 9 and July 24, 2021. In-barn dry bulb temperatures (T-DB) and relative humidity were continuously recorded for naturally ventilated (26.38 +/- 1.21 degrees C and 83.38 +/- 5.40%, respectively) and mechanically ventilated (26.91 +/- 1.80 degrees C and 77.13 +/- 7.06%, respectively) barns using data recorders. Sows were phenotyped between lactation days 11.28 +/- 3.08 and 14.25 +/- 3.26. Thermoregulatory measures were obtained daily at 0800, 1200, 1600, and 2000 h and included respiration rate, and ear, shoulder, rump, and tail skin temperatures. Vaginal temperatures (T-V) were recorded in 10 min intervals using data recorders. Anatomical characteristics were recorded, including ear area and length, visual and caliper-assessed body condition scores, and a visually assessed and subjective hair density score. Data were analyzed using PROC MIXED to evaluate the temporal pattern of thermoregulatory responses, phenotype correlations were based on mixed model analyses, and moderate and severe HS inflection points were established by fitting T-V as the dependent variable in a cubic function against T-DB. Statistical analyses were conducted separately for sows housed in mechanically or naturally ventilated barns because the sow groups were not housed in each facility type simultaneously. The temporal pattern of thermoregulatory responses was similar for naturally and mechanically ventilated barns and several thermoregulatory and anatomical measures were significantly correlated with one another (P < 0.05), including all anatomical measures as well as skin temperatures, respiration rates, and T-V. For sows housed in naturally and mechanically ventilated facilities, moderate HS threshold T-DB were 27.36 and 26.69 degrees C, respectively, and severe HS threshold T-DB were 29.45 and 30.60 degrees C, respectively. In summary, this study provides new information on the variability of HS tolerance phenotypes and environmental conditions that constitute HS in commercially housed lactating sows.

Automated summary

Climate change and global warming have a negative impact on swine production, so improving swine heat stress resilience is crucial. Genomic selection for heat stress resilience could be an effective strategy. This study identified environmental conditions for heat stress and determined phenotypes that predict heat stress tolerance in lactating sows. The findings can be applied to develop or refine genomic selection indexes for heat stress resilience in pigs.