Yearly production trends and genetic and environmental influences in a genetically closed Brahman herd. 2. Phenotypic, genetic and environmental trends of three weights

Abstract

In order to genetically improve the growth of Brahman animals registered in a genetically closed herd located in the north central zone of Venezuela, the bulls were selected for 12 years for their phenotypic value (at the beginning) and estimated genetic value ( after) at 18 months. The variance of the 1,405 weights at birth (PN), 1 371 at weaning (PD), and 1 177 to 18 months (P18) of calves born to 45, 48, and 44 parents, respectively, was analyzed with two models (I and II) by least squares, in order to evaluate phenotypic, genetic, and environmental trends. Both models included the effects month and year of birth, sex, age of the cow, the interaction year × month and the regression of weight on age (PD and P18). Model I also included effect of sire. The phenotypic trend was estimated by the regression of the anus constants of model II on anus; the genetic tendency for the regression on anus, twice the difference of the anus constants between model II and I; while the environmental trend by regression of the differences between the phenotypic and genetic constants. The phenotypic, genetic and environmental changes per year were for PN: .35 kg (P <.001), .62 kg (P <.001), -.27 kg (P <.05); for PD: .95 kg (P <.01). 1.74 kg (P <.001), -.79 (ns); for P18: 2.5 kg (P <.01), 3.5 kg (P <.001), -1.0 kg (ns). In recent years, the greatest genetic progress and the greatest negative environmental trend were evidenced.