Genetic evaluation of lactation persistency in the Gyr breed by using a two-trait random regression model.

Context: Lactation persistency (LP) is an economically important characteristic to include in the selection objectives of the Gyr breed. Two persistency measures were tested to establish their contribution to the genetic evaluation of milk production in this genotype. The second measure of persistency studied would be the more appropriate measure to use in the genetic evaluation of lactation persistency. Aim: The aim of this work was to study LP in Gyr cows by using a random regression model (RRM) in two-trait analysis. Methods: Test-day milk yields (TDMY) of the first two lactations of Gyr cows were analysed. RRM was performed by Bayesian inference using the GIBBS3F90 program. Fourth-order Legendre polynomials were used to describe the random additive genetic and permanent environmental effects of the animal. The fixed effects included in the model were contemporary group and, as covariates, age of cow at calving and the regression function according to the TDMY class of lactation as the average trajectories of lactation curve, distinguished by calving order. Two persistency measures (PS i , i.e. PS1 and PS2) were used in the analyses. Rank correlations were calculated between the predicted breeding values for PS i , 305-day lactation milk yield (MY305), and the regression coefficients a 0 and a 1, to determine the percentage of matching animals between rankings when different selection intensities are applied on the basis of the ranking for MY305. Key results: The heritability estimates for PS1, PS2 and for MY305 were 0.19, 0.12 and 0.41 respectively, in the first lactation, and 0.43, 0.27 and 0.38 in the second lactation. The genetic correlation between MY305 and PS1 was negative and of low magnitude. The rank correlation between breeding values for PS i , obtained for bulls with at least five daughters with production records, was higher than 0.80 in the two lactation periods. Conclusions: The results indicated that PS2 is the more suitable of the two persistency measures used in this work, for inclusion in genetic evaluations of lactation persistency in Brazilian Gyr cows. Implications: PS2 must be included as a selection criteria in a breeding program for the Gyr breed. The Gyr breed has been selected in Brazil for 40 years, mainly for total milk yield. The lactation persistency is closely related to the profitability of a milk production system. More persistent cows need less inputs, they get less sick, have more stayability and behave better reproductively. The results indicated that it is possible to make a selection for lactation persistency and that the second measure considered is better suited to the behavior of milk yield in Gyr breed. [ABSTRACT FROM AUTHOR]