The use of multilevel models to evaluate sources of variation in reproductive performance in dairy cattle in Reunion Island

Sources of variation in measures of reproductive performance in dairy cattl e were evaluated using data collected from 3207 lactations in 1570 cows in 50 herds from five geographic regions of Reunion Island (located off the ca st coast of Madagascar). Three continuously distributed reproductive parame ters (intervals from calving-to-conception, calving-to-first-service and fi rst-service-to-conception) were considered, along with one Binomial outcome (first-service-conception risk). Multilevel models which take into account the hierarchical nature of the data were used to fit all models. For the o verall measure of calving-to-conception interval, 86% of the variation resi ded at the lactation level with only 7, 6 and 2% at the cow, herd and regio nal levels, respectively. The proportion of variance at the herd and cow le vels were slightly higher for the calving-to-first-service interval (12 and 9%, respectively) - but for the other two parameters (first-service-concep tion risk and first-service-to-conception interval), >90% of the variation resided at the lactation level. For the three continuous dependent variable s, comparison of results between models based on log-transformed data and B ox-Cox-transformed data suggested that minor departures from the assumption of normality did not have a substantial effect on the variance estimates. For the Binomial dependent variable, five different estimation procedures ( penalised quasi-likelihood, Markov-Chain Monte Carlo, parametric and non-pa rametric bootstrap estimates and maximum-likelihood) yielded substantially different results for the estimate of the cow-level variance. (C) 2001 Else vier Science B.V. All rights reserved.