Institute of Veterinary Medicine and Animal Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, 51014 Tartu, Estonia.
J Dairy Sci. 2012 Nov;95(11):6760-9. doi: 10.3168/jds.2012-5495. Epub 2012 Sep 12.
The objective of this study was to estimate the effect of composite β-κ-CN genotypes on milk coagulation and composition traits, and on the additive genetic variation of these traits in Estonian Holstein dairy cattle. A total of 23,970 milk samples, repeated measurements from the first to third lactation from 2,859 Estonian Holstein cows from 78 herds across the country, were analyzed for milk yield, milk fat and protein percentages, somatic cell count, and milk coagulation properties (milk coagulation time and curd firmness). Each cow had at least 3 measurements per lactation. Two single-trait random regression animal models were fitted for the traits studied. The first model considered fixed effects of year-season of sampling and year-season of calving, calving age (nested within lactation), sample age (only for milk coagulation traits) and days in milk, and random herd, additive genetic, and permanent environmental effects. The animal and permanent environmental effects were modeled over the lactation period by using Legendre polynomials. The second model had the additional fixed β-κ-casein effect in the form of a third-order Legendre polynomial. The 2 most frequent β-κ-casein composite genotypes were A2A2AA and A1A2AA, both with prevalence greater than 20%. Percentages of the remaining 31 genotypes were less than 8%, including 20 genotypes with percentages less than 1%. The β-κ-casein genotype-specific lactation curves were significantly different for milk coagulation traits and milk protein percentage. The B variant of κ-casein showed a favorable effect on both milk coagulation traits, whereas the IB haplotype had an increasing effect on curd firmness and protein percentage. Inclusion of the β-κ-casein genotype effects in the model resulted in decreases in the mean additive genetic variations for milk coagulation time and curd firmness of 12.9 and 51.1%, respectively.
本研究旨在估计复合β-κ-CN 基因型对牛奶凝固和成分特性的影响，以及这些特性在爱沙尼亚荷斯坦奶牛中的加性遗传变异。分析了来自全国 78 个牛群的 2859 头爱沙尼亚荷斯坦奶牛的 23970 个牛奶样本，这些样本来自于第一至第三次泌乳期，包括牛奶产量、牛奶脂肪和蛋白质百分比、体细胞计数以及牛奶凝固特性（凝固时间和凝块强度）。每头奶牛的每个泌乳期至少有 3 次测量。对所研究的性状拟合了两个单性状随机回归动物模型。第一个模型考虑了采样年-季节和产犊年-季节、产犊年龄（嵌套在泌乳期内）、样本年龄（仅用于牛奶凝固特性）和泌乳天数的固定效应，以及随机牛群、加性遗传和永久性环境效应。通过使用勒让德多项式对动物和永久性环境效应进行了泌乳期内的建模。第二个模型在三阶勒让德多项式的形式中增加了β-κ-酪蛋白的固定效应。最常见的两种β-κ-酪蛋白复合基因型是 A2A2AA 和 A1A2AA，两者的流行率均大于 20%。其余 31 种基因型的比例均小于 8%，其中 20 种基因型的比例小于 1%。β-κ-酪蛋白基因型特异性泌乳曲线在牛奶凝固特性和牛奶蛋白质百分比方面存在显著差异。κ-酪蛋白的 B 变体对牛奶凝固特性均有有利影响，而 IB 单倍型对凝块强度和蛋白质百分比有增加作用。在模型中纳入β-κ-酪蛋白基因型效应会导致牛奶凝固时间和凝块强度的平均加性遗传变异分别减少 12.9%和 51.1%。
