Hierarchical nonlinear model for persistency of milk yield in the first three lactations of Holsteins

A nonlinear model, Wood's function, was used to describe the shape of the lactation curve in the first three lactations of Holstein cows. Wood's function was reparameterized to include the logarithm of persistency as a parameter. The data consisted of 65 677 test-day records of 2875 cows. All cows were required to have first lactation test-day milk yield records. A three-stage Bayesian hierarchical nonlinear model was implemented. The first stage described within-cow variation and the second stage accounted for between-animal variation. The third stage consisted of the priors used. Negative genetic correlations between the first (measure of yield) and second (related to the increasing yield phase of lactation) parameters of Wood's function were found for all three lactations: -0.59, -0.55 and -0.39 for first, second and third lactations, respectively. The genetic correlation between the first parameter of Wood's function and log-persistency was negative in each of the three lactations (-0.20, -0.31 and -0.31). The genetic correlation between the second parameter and log-persistency was low (0.06, 0.09, 0.03 for each of the lactation). Heritabilities of all parameters tended to decrease with parity, mainly due to an increase in residual variance. Heritabilities of persistency were 0.17, 0.16 and 0.14 for first, second and third lactations, respectively. The genetic correlation between persistency in the three lactations was 0.26 (first and second), 0.31 (second and third) and 0.23 (first and third). Residual correlations followed a similar pattern, but tended to be larger in absolute value than genetic correlations. (C) 2001 Elsevier Science B.V. All rights reserved.