title: Random regression analysis of test-day milk yields in the first and second lactations of Brazilian Gyr cows.
abstract: The objective of the present study was to estimate the genetic parameters for test-day milk yields (TDMY) in the first and second lactations using random regression models (RRM) in order to contribute to the application of these models in genetic evaluation of milk yield in Gyr cattle. A total of 53,328 TDMY records from 7118 lactations of 5853 Gyr cows were analyzed. The model included the direct additive, permanent environmental, and residual random effects. In addition, contemporary group and linear and quadratic effects of the age of cows at calving were included as fixed effects. A random regression model fitting fourth-order Legendre polynomials for additive genetic and permanent environmental effects, with five classes of residual variance, was applied. In the first lactation, the heritabilities increased from early lactation (0.26) until TDMY3 (0.38), followed by a decrease until the end of lactation. In the second lactation, the estimates increased from the first (0.29) to the fifth test day (0.36), with a slight decrease thereafter, and again increased on the last two test days (0.34 and 0.41). There were positive and high genetic correlations estimated between first-lactation TDMY and the remaining TDMY of the two lactations. The moderate heritability estimates, as well as the high genetic correlations between half the first-lactation TDMY and all TDMY of the two lactations, suggest that the selection based only on first lactation TDMY is the best selection strategy to increase milk production across first and second lactations of Gyr cows.
authors.affiliation: Departamento de Zootecnia, Faculdade de Ciências Agrárias e Veterinárias, Universidade Estadual Paulista, Jaboticabal, SP, Brasil.
authors.forename: L G
authors.initials: LG
authors.lastname: Gonzalez-Herrera
authors.affiliation: Centro de Pesquisas de Bovinos de Corte, Instituto de Zootecnia, Sertãozinho, SP, Brasil.
authors.forename: L
authors.initials: L
authors.lastname: El Faro
authors.affiliation: Grupo de Melhoramento Animal de Mato Grosso, Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Mato Grosso, Campus Universitário de Rondonópolis, Rondonópolis, MT, Brasil.
authors.forename: A B
authors.initials: AB
authors.lastname: Bignardi
authors.affiliation: Grupo de Melhoramento Animal de Mato Grosso, Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Mato Grosso, Campus Universitário de Rondonópolis, Rondonópolis, MT, Brasil.
authors.forename: R J
authors.initials: RJ
authors.lastname: Pereira
authors.affiliation: Associação Brasileira dos Criadores de Zebu, Superintendente Técnico-Adjunto de Melhoramento Genético, Uberaba, MG, Brasil.
authors.forename: C H C
authors.initials: CH
authors.lastname: Machado
authors.affiliation: Departamento de Zootecnia, Faculdade de Ciências Agrárias e Veterinárias, Universidade Estadual Paulista, Jaboticabal, SP, Brasil.
authors.forename: L G
authors.initials: LG
authors.lastname: Albuquerque
mesh_terms: D000818:Animals; D001938:Brazil; D002417:Cattle; D004777:Environment; D005260:Female; D059647:Gene-Environment Interaction; D056726:Genetic Association Studies; D007774:Lactation / Q000235:genetics; D008892:Milk; D019655:Quantitative Trait, Heritable; D012044:Regression Analysis