Estimation of genetic parameters for test-day milk yield in Girolando cows using a random regression model

ABSTRACT The objective of this study was to estimate the components of variance and genetic parameters of test-day milk yield in first lactation Girolando cows, using a random regression model. A total of 126,892 test-day milk yield (TDMY) records of 15,351 first-parity Holstein, Gyr, and Girolando breed cows were used, obtained from the Associação Brasileira dos Criadores de Girolando. To estimate the components of (co) variance, the additive genetic functions and permanent environmental covariance were estimated by random regression in three functions: Wilmink, Legendre Polynomials (third order) and Linear spline Polynomials (three knots). The Legendre polynomial function showed better fit quality. The genetic and permanent environment variances for TDMY ranged from 2.67 to 5.14 and from 9.31 to 12.04, respectively. Heritability estimates gradually increased from the beginning (0.13) to mid-lactation (0.19). The genetic correlations between the days of the control ranged from 0.37 to 1.00. The correlations of permanent environment followed the same trend as genetic correlations. The use of Legendre polynomials via random regression model can be considered as a good tool for estimating genetic parameters for test-day milk yield records.

摘要 本研究旨在采用随机回归模型，估算首胎吉罗兰多（Girolando）奶牛泌乳期单日产奶量（test-day milk yield, TDMY）的方差组分与遗传参数。本研究使用来自巴西吉罗兰多养殖者协会（Associação Brasileira dos Criadores de Girolando）的15351头初产荷斯坦（Holstein）、吉尔（Gyr）及吉罗兰多奶牛的126892条单日产奶量记录。为估算（协）方差组分，研究通过三种函数形式的随机回归模型，分别估计加性遗传效应与永久环境效应的协方差：威尔明克（Wilmink）函数、三阶勒让德多项式（Legendre Polynomials）以及带三个结点的线性样条多项式（Linear spline Polynomials）。结果表明，勒让德多项式模型的拟合质量最优。单日产奶量的遗传方差与永久环境方差分别介于2.67至5.14、9.31至12.04之间。遗传力估计值从泌乳初期的0.13逐步升高至泌乳中期的0.19。各测定日之间的遗传相关系数范围为0.37至1.00；永久环境相关系数的变化趋势与遗传相关系数一致。综上，基于随机回归模型的勒让德多项式方法可作为估算单日产奶量记录遗传参数的优良工具。