McWhorter T M, Sargolzaei M, Sattler C G, Utt M D, Tsuruta S, Misztal I, Lourenco D
Department of Animal and Dairy Science, University of Georgia, Athens, GA 30602.
Select Sires Inc., Plain City, OH 43064.
J Dairy Sci. 2023 Nov;106(11):7861-7879. doi: 10.3168/jds.2022-23144. Epub 2023 Aug 23.
The physiological stress caused by excessive heat affects dairy cattle health and production. This study sought to investigate the effect of heat stress on test-day yields in US Holstein and Jersey cows and develop single-step genomic predictions to identify heat tolerant animals. Data included 12.8 million and 2.1 million test-day records, respectively, for 923,026 Holstein and 153,710 Jersey cows in 27 US states. From 2015 through 2021, test-day records from the first 5 lactations included milk, fat, and protein yields (kg). Cow records were included if they had at least 5 test-day records per lactation. Heat stress was quantified by analyzing the effect of a 5-d hourly average temperature-humidity index (THI) on observed test-day yields. Using a multiple trait repeatability model, a heat threshold (THI threshold) was determined fowr each breed based on the point that the average adjusted yields started to decrease, which was 69 for Holsteins and 72 for Jerseys. An additive genetic component of general production and heat tolerance production were estimated using a multiple trait reaction norm model and single-step genomic BLUP methodology. Random effects were regressed on a function of 5-d hourly average (THI) and THI threshold. The proportion of test-day records that occurred on or above the respective heat thresholds was 15% for Holstein and 10% for Jersey. Heritability of milk, fat, and protein yields under heat stress for Holsteins increased, with a small standard error, indicating that the additive genetic component for heat tolerance of these traits was observed. This was not as evident in Jersey traits. For Jersey, the permanent environment explained the same or more of the variation in fat and protein yield under heat stress indicating that nongenetic factors may determine heat tolerance for these Jersey traits. Correlations between the general genetic merit of production (in the absence of heat stress) and heat tolerance genetic merit of production traits were moderate in strength and negative. This indicated that selecting for general genetic merit without consideration of heat tolerance genetic merit of production may result in less favorable performance in hot and humid climates. A general genomic estimated breeding value for genetic merit and a heat tolerance genomic estimated breeding value were calculated for each animal. This study contributes to the investigation of the impact of heat stress on US dairy cattle production yields and offers a basis for the implementation of genomic selection. The results indicate that genomic selection for heat tolerance of production yields is possible for US Holsteins and Jerseys, but a study to validate the genomic predictions should be explored.
过热引起的生理应激会影响奶牛的健康和生产性能。本研究旨在调查热应激对美国荷斯坦奶牛和泽西奶牛测定日产奶量的影响，并开发单步基因组预测方法以识别耐热动物。数据分别包括美国27个州的923,026头荷斯坦奶牛和153,710头泽西奶牛的1280万条和210万条测定日记录。从2015年到2021年，前5胎次的测定日记录包括牛奶、脂肪和蛋白质产量（千克）。如果每胎次至少有5条测定日记录，则纳入奶牛记录。通过分析5天每小时平均温湿度指数（THI）对观察到的测定日产奶量的影响来量化热应激。使用多性状重复性模型，根据平均校正产量开始下降的点，为每个品种确定一个热阈值（THI阈值），荷斯坦奶牛为69，泽西奶牛为72。使用多性状反应规范模型和单步基因组最佳线性无偏预测（BLUP）方法估计一般生产性能和耐热生产性能的加性遗传成分。将随机效应回归到5天每小时平均（THI）和THI阈值的函数上。荷斯坦奶牛和泽西奶牛分别有15%和10%的测定日记录出现在各自的热阈值及以上。热应激条件下，荷斯坦奶牛牛奶、脂肪和蛋白质产量的遗传力增加，且标准误差较小，表明观察到了这些性状耐热性的加性遗传成分。这在泽西奶牛的性状中不太明显。对于泽西奶牛，永久环境在热应激条件下解释了脂肪和蛋白质产量变异的相同或更多部分，这表明非遗传因素可能决定了这些泽西奶牛性状的耐热性。在无热应激情况下的一般生产遗传价值与生产性状的耐热遗传价值之间的相关性强度适中且为负。这表明在不考虑生产性状耐热遗传价值的情况下选择一般遗传价值，可能会导致在炎热潮湿气候下的生产性能较差。为每头动物计算了一般基因组估计育种值和耐热基因组估计育种值。本研究有助于调查热应激对美国奶牛生产性能的影响，并为实施基因组选择提供了依据。结果表明，对于美国荷斯坦奶牛和泽西奶牛，对生产性能的耐热性进行基因组选择是可行的，但应探索一项研究来验证基因组预测。
