Definition of environmental variables and critical periods to evaluate heat tolerance in large white pigs based on single-step genomic reaction norms

Authors: FREITAS, PEDRO - Purdue University; Johnson, Jay; CHEN, SHIYI - Purdue University; OLIVERIA, HINAYAH - Purdue University; TIEZZI, FRANCESCE - North Carolina State University; LAZARO, SIRLENE - Purdue University; HUANG, YIJIAN - Smithfield Foods, Inc; GU, YOUPING - Smithfield Foods, Inc; SCHINCKEL, ALLAN - Purdue University; BRITO, LUIZ - Purdue University

Submitted to: Frontiers in Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2021
Publication Date: 11/23/2021
Citation: Freitas, P., Johnson, J.S., Chen, S., Oliveria, H., Tiezzi, F., Lazaro, S., Huang, Y., Gu, Y., Schinckel, A., Brito, L. 2021. Definition of environmental variables and critical periods to evaluate heat tolerance in large white pigs based on single-step genomic reaction norms. Frontiers in Genetics. 12. Article 717409. https://doi.org/10.3389/fgene.2021.717409.
DOI: https://doi.org/10.3389/fgene.2021.717409

Interpretive Summary: Climate change is increasing the incidence and severity of heat stress events, which has the potential to negatively impact livestock production. Specifically, climate change induced heat stress is considered a major welfare and production issue in the swine industry. One strategy to improve pig productivity and welfare under heat stress conditions is through breeding more heat tolerant pigs. However, the best environmental metrics and critical periods to be used when evaluating pig heat tolerance needs to be defined. Therefore, the study objectives were to evaluate the best environmental measures and time periods to select for heat tolerance traits in pigs. It was determined that measures of relative humidity and maximum daily temperature are the recommended environmental measures to be considered and that taking these measures between 20 and 30 days prior to heat tolerance trait measures is the recommended time frame. These data provide relevant recommendations of critical periods and climactic gradients for several heritable heat tolerance traits and also demonstrate that heat tolerance is heritable and genetic progress can be achieved through genetic or genomic selection.

Technical Abstract: Heat stress (HS) is a major welfare and economic issue in the swine industry, especially as adverse climatic events become more common. Therefore, there is an urgent need to genetically improve animal climatic resilience. However, properly quantifying environmental HS is still a major challenge. The definition of critical periods and climatic variables to be used as the environmental gradient in the reaction norm models is a key step for genetically evaluating heat tolerance (HT). Therefore, the main objectives of this study were to define the best critical periods and environmental descriptors (based on public weather station information) to evaluate HT, and estimate variance components for HT in Large White pigs. The traits included in this study are: ultrasound backfat thickness (BFT; mm), ultrasound muscle depth (MDP; mm), piglet weaning weight (WW; kg), off-test weight (OTW; kg; measured at approximately 5.5 months of age), interval between farrowing (IBF; days), total number of piglets born (TNB), number of piglets born alive (NBA), number of piglets born dead (NBD), number of piglets weaned (WN), and weaning to estrus interval (IWE; days). The number of phenotypic records ranged from 6,059 (WN) to 172,984 (TNB), and genotypes were available for 8,686 animals. Seven climatic variables (maximum, minimum, and average temperatures; average relative humidity; dew point; discomfort index; and temperature-humidity index) were compared based on three criteria including: 1) the strongest GxE estimate as measured by the slope term, 2) the variable yielding the highest theoretical accuracy of the genomic estimated breeding values (GEBV), and 3) the variable yielding the highest distribution of GEBV per environmental value. Relative humidity (for BFT, MDP, NBD, WN, and WW) and maximum temperature (for OTW, TNB, NBA, IBF, and IWE) are the recommended environmental gradients based on the three analyzed criteria. The acute HS (average of 30 days before measurement date) is the critical period recommended for OTW, BFT, and MDP. For WN, WW, IBF, and IWE, a period ranging from 34 days prior to farrowing up to weaning is recommended. For TNB, NBA, and NBD, the critical period from 20 days prior to breeding up to 30 days into gestation is recommended. The average heritability estimates for HT (based on reaction norm slope) ranged from 0.04 (IBF) to 0.42 (BFT). The lowest genetic correlations between divergent environmental gradients were observed for WN (-0.27) and WW (-0.11). The correlation values for WN, WW, IBF, and IWE indicate that these traits are largely affected by genotype-by-environment (GxE) interactions, while the remaining traits had moderate (OTW, TNB, and NBA) or weak (MDP, BFT, and NBD) GxE interactions. Re-ranking of individuals were observed for all traits, except MDP, BFT, and NBD. Additionally, a small re-ranking was observed for TNB. A largely comprehensive landscape of GxE interactions is provided in this study, and relevant recommendations of critical periods and climatic gradients were made for several traits in order to evaluate HS in maternal-line pigs. These findings demonstrate that HT in maternal-line pigs is heritable and genetic progress can be achieved through genetic or genomic selection.