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 l actation curve in the first three lactations of Holstein cows. Wood's funct ion was reparameterized to include the logarithm of persistency as a parame ter. The data consisted of 65 677 test-day records of 2875 cows. All cows w ere required to have first lactation test-day milk yield records. A three-s tage 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 ge netic 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, seco nd 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 decr ease with parity, mainly due to an increase in residual variance. Heritabil ities of persistency were 0.17, 0.16 and 0.14 for first, second and third l actations, 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, b ut tended to be larger in absolute value than genetic correlations. (C) 200 1 Elsevier Science B.V. All rights reserved.