GENOTYPE-ENVIRONMENT INTERACTION FOR REPRODUCTIVE TRAITS OF NELORE FEMALES USING REACTION NORMS

Abstract

Reducing age at first birth is directly linked to efficiency and profitability of beef production. Moreover, it is known that herds with greater sexual precocity have greater availability of animals, both for sale and for selection, allowing more intensive selective and therefore higher genetic progress. There are several aspects to improve the Zebu and among the most important, it can be is the low reproductive precocity. Despite its importance, the traits of early sexual activity in females, have been widely used in breeding programs for beef cattle in Brazil, as a result of its low heritability and the difficulty of measurement and interpretation. With respect to females, is difficult to identify easily measurable characteristics that are genetically related to fertility. Currently in Brazil, published summaries of several bulls to breed bulls, with several economically important traits, including all large data sets of animals distributed in various regions of the country. These evaluations, the assumption is commonly assumed the absence of genotype-environment interaction. Recently, models of norms of reaction are being used to verify the effect of genotype-environment interaction on different characteristics of economic importance. In the standard model of reaction, the expression of a genotype in different environments is described as a linear function of a value or environmental gradient. In cattle, yet there are few studies of genotype-environment interaction for the traits of early sexual activity in females. The objective of this project is to verify the occurrence of genotype-environment interaction for characteristics associated with early sexual maturity, age at first calving and days to first calving in Nellore females using reaction norms. The data set contains information on age at first calving and days to calving in Nellore females of the properties participating in a large breeding program with farms located in different regions of Brazil. Different groups will be set according to the environmental information of the farm of origin of the data, climatological and geographical location in order to identify factors that may help identify possible environmental factors that cause genotype-environment interaction. For the estimation of genetic parameters will be applied to a sire model using random regression analysis. For those traits will be considered in the model as fixed effects of contemporary group, the curve down the population and, as a covariate, and age of the heifer at the beginning of the breeding season (only for days for delivery). The contemporary group will be formed by animals born variables farm, year and season of birth, group management of birth, weaning and yearling and service type (natural mating, breeding or AI controlled) as random effect is considered the genetic effect additive bull. The (co) variances and breeding values are estimated by the intercept and the coefficient of the reaction norm of each bull. Legendre polynomials of the environmental groups will be used to model the genetic effect of the bull, the corner down the population. Estimates of (co) variances and genetic parameters are obtained by the method of restricted maximum likelihood derived from using the computer program Wombat.