With global warming, there are growing challenges for raising taurine and composite beef cattle populations in tropical regions, including elevated temperatures, limited forage availability, parasite infestation, and infectious diseases. These environmental factors can trigger specific physiological responses in the developing fetus, which may have long-term implications on its performance. Therefore, the main objective of this study was to assess the influence of naturally-induced thermal stress during the gestation period on the subsequent performance of tropical composite beef cattle progeny. Furthermore, we aimed to investigate the impact of genotype-by-gestational thermal environment interaction (G×Eg) on traits under selection pressure in the breeding population. A total of 157,414 animals from 58 farms located in various Brazilian states were recorded for birth weight (BW), pre-weaning weight gain (PWG), yearling weight (YW), hip height (HH), scrotal circumference (SC), and days to first calving (DFC). We first applied a linear regression model to the BW data, which revealed that the last 40 days of gestation were suitable for calculating the mean temperature humidity index (THIg). Subsequent regression analyses revealed that for every 10-unit increase in THIg, detrimental effects of approximately 1.13% to 16.34% are expected for all traits evaluated. Genetic parameters were estimated through a reaction norm model using THIg as the environmental descriptor. The posterior means of heritability estimates (standard deviation) were 0.35 (0.07), 0.25 (0.03), 0.31 (0.03), 0.37 (0.01), 0.29 (0.07), and 0.20 (0.09) for the direct effect of BW, PWG, YW, HH, SC, and DFC, respectively. These estimates varied along the range of THIg values, suggesting a variable response to selection depending on the thermal environment during gestation. Genetic correlation estimates between more divergent THIg values were low or negative for YW, PWG, and DFC, indicating that the best-performing individuals at low THIg values may not perform as well at high THIg values and vice versa. Overall, thermal stress during gestation impacts the future performance of beef cattle offspring. Our results indicate the need for developing effective breeding strategies that take into account G×Eg effects and the re-ranking of breeding animals along the THIg scale, particularly for traits such as DFC that are highly sensitive to thermal stress.

中文翻译：

---

基因型与妊娠热环境的相互作用及其对热带复合肉牛后代未来性能的影响


随着全球变暖，热带地区饲养牛磺酸和复合肉牛种群面临着越来越大的挑战，包括气温升高、饲料供应有限、寄生虫感染和传染病。这些环境因素可以触发发育中胎儿的特定生理反应，这可能对其表现产生长期影响。因此，本研究的主要目的是评估妊娠期间自然引起的热应激对热带复合肉牛后代的后续性能的影响。此外，我们的目的是研究基因型与妊娠热环境相互作用（G×Eg）对育种群体选择压力下性状的影响。记录了来自巴西各州 58 个农场的总共 157,414 只动物的出生体重 (BW)、断奶前增重 (PWG)、一岁体重 (YW)、臀高 (HH)、阴囊周长 (SC) 和体重。首次产犊天数 (DFC)。我们首先对 BW 数据应用线性回归模型，结果表明妊娠最后 40 天适合计算平均温度湿度指数 (THIg)。随后的回归分析显示，THIg 每增加 10 个单位，预计对所有评估的性状都会产生约 1.13% 至 16.34% 的有害影响。使用 THIg 作为环境描述符，通过反应范数模型估计遗传参数。对于 BW、PWG、分别为 YW、HH、SC 和 DFC。 这些估计值随着 THIG 值的范围而变化，表明根据妊娠期间的热环境对选择的不同反应。对于 YW、PWG 和 DFC，更不同的 THIg 值之间的遗传相关性估计较低或为负，这表明在低 THIg 值时表现最好的个体可能在高 THIg 值时表现不佳，反之亦然。总体而言，妊娠期间的热应激会影响肉牛后代的未来表现。我们的结果表明需要制定有效的育种策略，考虑到 G×Eg 效应以及根据 THIg 等级对育种动物进行重新排名，特别是对于热应激高度敏感的 DFC 等性状。