Measurements of milk yield (MY), somatic cell score (SCS), percentage of fat (FP), protein (PP), lactose (LP), casein (CP) and percentage of palmitic (C16:0), stearic (C18:0), oleic (C18:1), total saturated (SFA), unsaturated (UFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids in milk from 5,224 Holstein cows were evaluated as a function of a temperature and humidity index (THI). Legendre orthogonal polynomials from second to seventh order were tested. The best fit order for MY, PP and C18:0 was the third, whereas the second for all other traits. The heritability estimates decreased for MY (0.31 to 0.14), FP (0.28 to 0.16), LP (0.43 to 0.30), SCS (0.14 to 0.09), SFA (0.33 to 0.22) and C16:0 (0.31 to 0.26), whereas increased for CP (0.32 to 0.42), MUFA (0.08 to 0.13), UFA (0.07 to 0.11) and C18:1 (0.07 to 0.11) as the THI level increased. For PP, heritabilities (0.26 to 0.39) presented larger values in intermediate THI. For PUFA and C18:0, heritabilities were approximately constant (0.13 to 0.14 and 0.15, respectively). However, the greatest variations may have been the result of the limitations of Legendre polynomials at the extreme points of the curve, and the pattern of heritabilities curves was approximately constant for the evaluated traits. Spearman's rank correlations between breeding values in extreme THI levels were greater than 0.80 for all traits considering all animals, only cows and only bulls. When considering the top 1% and the top 50% animals (only cows, only bulls and all), Spearman correlations smaller than 0.70 were found, suggesting reranking of the animals. Although there was little variation in the variance components over THI, it is possible that there is no heat stress in the animals studied, because, on average, there was no great impact of the thermal load on the traits. One possible explanation is the use of herds with little climatic difference among herds, as well as the use of fans and sprinklers into the barns. However, the THI levels may be important factors in the selection process, as reranking of animals was verified.

中文翻译：

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作为温度和湿度指数函数的巴西荷斯坦奶牛产奶量和质量性状的遗传参数

牛奶产量 (MY)、体细胞评分 (SCS)、脂肪百分比 (FP)、蛋白质 (PP)、乳糖 (LP)、酪蛋白 (CP) 和棕榈酸 (C16:0)、硬脂酸 (C18: 0)、来自 5,224 头荷斯坦奶牛的牛奶中的油酸 (C18:1)、总饱和 (SFA)、不饱和 (UFA)、单不饱和 (MUFA) 和多不饱和 (PUFA) 脂肪酸被评估为温度和湿度指数的函数（ ））。测试了二阶到七阶的勒让德正交多项式。MY、PP 和 C18:0 的最佳拟合顺序是第三个，而所有其他特征的最佳拟合顺序是第二个。MY（0.31 至 0.14）、FP（0.28 至 0.16）、LP（0.43 至 0.30）、SCS（0.14 至 0.09）、SFA（0.33 至 0.22）和 C16:0（0.31 至 0.22）的遗传力估计值下降，而CP（0.32 至 0.42）、MUFA（0.08 至 0.13）、UFA（0.07 至 0.11）和 C18:1（0.07 至 0. 11) 随着 THI 水平的增加。对于 PP，遗传力（0.26 到 0.39）在中间 THI 中呈现出更大的值。对于 PUFA 和 C18:0，遗传力近似恒定（分别为 0.13 到 0.14 和 0.15）。然而，最大的变化可能是勒让德多项式在曲线极值点的局限性的结果，并且遗传曲线的模式对于评估的性状几乎是恒定的。考虑到所有动物，只有奶牛和只有公牛，极端 THI 水平的育种值之间的 Spearman 等级相关性大于 0.80。当考虑前 1% 和前 50% 的动物（只有奶牛、只有公牛和所有）时，发现 Spearman 相关性小于 0.70，表明对动物进行了重新排序。尽管 THI 的方差分量几乎没有变化，但研究的动物中可能没有热应激，因为平均而言，热负荷对性状没有太大影响。一种可能的解释是，使用畜群之间气候差异很小的畜群，以及在谷仓中使用风扇和洒水器。然而，THI 水平可能是选择过程中的重要因素，因为动物的重新排序得到了验证。