Animal and Dairy Science Department, University of Georgia, Athens, GA 30602, USA.
J Dairy Sci. 2011 Jan;94(1):487-92. doi: 10.3168/jds.2010-3421.
The threshold for heat stress on milk yield of Holstein crossbreds under climatic conditions in Thailand was investigated, and genetic effects of heat stress on milk yield were estimated. Data included 400,738 test-day milk yield records for the first 3 parities from 25,609 Thai crossbred Holsteins between 1990 and 2008. Mean test-day milk yield ranged from 12.6 kg for cows with <87.5% Holstein genetics to 14.4 kg for cows with ≥93.7% Holstein genetics. Daily temperature and humidity data from 26 provincial weather stations were used to calculate a temperature-humidity index (THI). Test-day milk yield varied little with THI for first parity except above a THI of 82 for cows with ≥93.7% Holstein genetics. For third parity, test-day milk yield started to decline after a THI of 74 for cows with ≥87.5% Holstein genetics and declined more rapidly after a THI of 82. A repeatability test-day model with parities as correlated traits was used to estimate heat stress parameters; fixed effects included herd-test month-test year and breed groups, days in milk, calving age, and parity; random effects included 2 additive genetic effects, regular and heat stress, and 2 permanent environment, regular and heat stress. The threshold for effect of heat stress on test-day milk yield was set to a THI of 80. All variance component estimates increased with parity; the largest increases were found for effects associated with heat stress. In particular, genetic variance associated with heat stress quadrupled from first to third parity, whereas permanent environmental variance only doubled. However, permanent environmental variance for heat stress was at least 10 times larger than genetic variance. Genetic correlations among parities for additive effects without heat stress considered ranged from 0.88 to 0.96. Genetic correlations among parities for additive effects of heat stress ranged from 0.08 to 0.22, and genetic correlations between effects regular and heat stress effects ranged from -0.21 to -0.33 for individual parities. Effect of heat stress on Thai Holstein crossbreds increased greatly with parity and was especially large after a THI of 80 for cows with a high percentage of Holstein genetics (≥93.7%). Individual sensitivity to heat stress was more environmental than genetic for Thai Holstein crossbreds.
本研究旨在调查泰国气候条件下荷斯坦杂交奶牛产奶量的热应激阈值，并估计热应激对产奶量的遗传效应。数据包括 1990 年至 2008 年间 25609 头泰国荷斯坦杂交奶牛的前 3 胎 400738 个泌乳日的产奶记录。平均泌乳日产量从遗传荷斯坦奶牛比例<87.5%的奶牛的 12.6kg 到遗传荷斯坦奶牛比例≥93.7%的奶牛的 14.4kg。使用 26 个省级气象站的日温度和湿度数据计算温度湿度指数（THI）。除了遗传荷斯坦奶牛比例≥93.7%的奶牛的 THI 高于 82 外，第一胎产奶量的泌乳日变化与 THI 关系不大。对于第三胎，遗传荷斯坦奶牛比例≥87.5%的奶牛的 THI 高于 74 时，泌乳日产奶量开始下降，THI 高于 82 时下降速度加快。采用具有相关性状的重复泌乳日模型估计热应激参数；固定效应包括畜群-泌乳月-泌乳年和品种组、泌乳天数、产犊年龄和胎次；随机效应包括 2 个加性遗传效应、常规和热应激以及 2 个永久环境效应、常规和热应激。将热应激对泌乳日产奶量影响的阈值设定为 THI 80。所有方差分量估计值均随胎次增加而增加；与热应激相关的效应的增加最大。特别是，与热应激相关的遗传方差从第一胎增加到第三胎增加了四倍，而永久环境方差仅增加了一倍。然而，热应激的永久环境方差至少是遗传方差的 10 倍。不考虑热应激的加性效应的各胎次间的遗传相关系数范围为 0.88 至 0.96。热应激加性效应各胎次间的遗传相关系数范围为 0.08 至 0.22，单个胎次常规效应和热应激效应间的遗传相关系数范围为-0.21 至-0.33。泰国荷斯坦杂交奶牛的热应激效应随胎次增加而显著增加，对于高比例荷斯坦遗传（≥93.7%）的奶牛，THI 高于 80 时热应激效应尤其显著。泰国荷斯坦杂交奶牛对热应激的个体敏感性比遗传更受环境影响。
