Abstract
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.
Highlights
Crossbreeding has been adopted for many years in Brazilian dairy farming as a strategy for genetic improvement to combine the high milk production capacity and adaptability to tropical conditions
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
Lower values of likelihood function, AIC and BIC were calculated for the model that considered the parametric function of Legendre polynomials, indicating it is the best model to be considered in the genetic evaluation of test-day milk yield (TDMY)
Summary
Crossbreeding has been adopted for many years in Brazilian dairy farming as a strategy for genetic improvement to combine the high milk production capacity and adaptability to tropical conditions. This strategy led to creation of Girolando breed (5/8 Holstein and 3/8 Gyr). Notable for their excellent productivity, Girolando and crossbred animals stand out for their high fertility rates and good vigor. These traits made the breed conquer space and become predominant in dairy farms (Stafuzza et al, 2017), representing 80% of milk production in Brazil (Nanzer, 2011). Associated with different mathematical functions in the most accurate search to infer genetically about the breed, the RRM methodology allows estimating genetic parameters, predicting breeding values, and identifying the most efficient and profitable genotypes (Daltro et al, 2020), allowing higher genetic gains
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