130. Genetic Parameters for Traits Related to Heat Tolerance in a Crossbreed Beef Cattle Population.

Abstract High environmental temperatures and humidity during the summer period can impair thermoregulation in beef cattle, leading to increased core body temperatures, reduced production efficiency, and compromised animal welfare. Therefore, identifying and selecting animals capable of regulating body temperature more efficiently under heat stress conditions is essential for improving both animal welfare and profitability in beef production systems. Vaginal temperature (TV) is a reliable proxy for core body temperature, and TV traits derived from longitudinal measurements may be valuable for genetic improvement of heat tolerance in beef cattle. In this context, this study aimed to 1) estimate genetic parameters for daily average TV, TV at the maximum temperature–humidity index (THI) within a day, TV at the minimum THI within a day, and ΔTV (TV at maximum THI within a day − TV at minimum THI within a day), body condition score (BCS; scale from 1 to 9), skin thickness (ST, mm), and coat score (CS; 2 to 7) in a crossbred beef cattle (Angus × Simmental × Brahman) population. Vaginal temperature was recorded in 227 females (58 heifers and 169 cows) using iButton temperature loggers (model DS1922L; Maxim Integrated, San Jose, CA, USA) inserted into a blank Controlled Internal Drug Release device. Weather data were obtained from NASA POWER, and hourly THI was calculated based on these records. Variance components for individual traits were estimated using single-trait single-step GBLUP models using the software blupf90+. Genetic correlations were estimated using two-trait single-step GBLUP models through Bayesian inference. Heritability estimates were 0.20 ± 0.08, 0.17 ± 0.05, 0.10 ± 0.05, and 0.04 ± 0.04 for average daily TV, TV at the maximum THI within a day, TV at the minimum THI within a day, and ΔTV, respectively. Corresponding repeatability estimates were 0.48 ± 0.03, 0.30 ±0.03, 0.33 ± 0.03, and 0.14 ± 0.04. Genetic correlations among average daily TV, TV at the maximum THI within a day, and TV at the minimum THI within a day were all greater than 0.98. Genetic correlations between ΔTV and all other TV-derived traits ranged from 0.69 to 0.79, suggesting that ΔTV may capture a partially distinct genetic component from other TV-derived traits, such as the animal’s sensitivity to daily thermal load. BCS, CS, and ST had heritability estimates of 0.50 ± 0.14, 0.02 ± 0.02, and 0.13 ± 0.014, respectively. The genetic correlations between TV traits and BCS, CS and ST were not reliable (high PSD) due to the limited sample size in this preliminary study. Overall, TV-derived traits are heritable, supporting their potential use as indicator traits for heat tolerance in beef cattle breeding programs. As a next step, additional phenotypic records will be added to this study.