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

Abstract

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 (TDB) and relative humidity were continuously recorded for naturally ventilated (26.38 ± 1.21°C and 83.38 ± 5.40%, respectively) and mechanically ventilated (26.91 ± 1.80°C and 77.13 ± 7.06%, respectively) barns using data recorders. Sows were phenotyped between lactation d 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 (TV) 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 TV as the dependent variable in a cubic function against TDB. 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 TV. For sows housed in naturally and mechanically ventilated facilities, moderate HS threshold TDB were 27.36 and 26.69°C, respectively, and severe HS threshold TDB were 29.45 and 30.60°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.

Genomic regions, candidate genes, and pleiotropic variants associated with physiological and anatomical indicators of heat stress response in lactating sows

“… 0.57 ± 0.12 Ear length EL The ear length was calculated using a photo taken with a digital camera, where a 10.2 × 15.2 cm grid card containing 1 cm × 1 cm squares was placed next to the sows’ ear. 0.53 ± 0.12 a Data collection protocols have been described by Johnson et al [ 10 ] and Freitas et al [ 11 ] b These heritability estimates were previously reported by Freitas et al [ 11 ] …”

“… a Data collection protocols have been described by Johnson et al [ 10 ] and Freitas et al [ 11 ] b These heritability estimates were previously reported by Freitas et al [ 11 ] …”

“…Estimated breeding values for heat tolerance (slope of the reaction norms) based on this approach tend to be unfavorably correlated with the level of production (e.g [ 7 ]). Therefore, identifying physiological, behavioral, and anatomical indicators of HS response that could be incorporated in selection indexes to breed for improved climatic resilience is promising in modern pig breeding programs [ 10 ]. Breeding for indicators of HS response could capture additional biological mechanisms involved in HS response and enable improvements in climatic resilience without compromising productivity [ 11 ].…”

“…Breeding for indicators of HS response could capture additional biological mechanisms involved in HS response and enable improvements in climatic resilience without compromising productivity [ 11 ]. Johnson et al [ 10 ] described several traits as HS response indicators, such as skin and automatically-recorded vaginal temperatures, and respiration rate, and Freitas et al [ 11 ] showed that these traits are heritable, i.e., they can be improved through genetic selection. Understanding the genetic background of these novel traits is a key step before including them in a breeding scheme.…”

Genomic predictions and GWAS for heat tolerance in pigs based on reaction norm models with performance records and data from public weather stations considering alternative temperature thresholds

“…Once again, this pattern may be related to the presence of GxE interaction effects that the model is able to capture when the temperature is above 10°C and not when it is below 10°C. Nevertheless, this relationship should be evaluated in further studies, and the results should be compared with predictions made using in-barn climatic data, which remains the gold standard for the evaluation of HS conditions in pig farms (Freitas et al, 2023;Johnson et al, 2023).…”