Estimativas de parâmetros genéticos para produção de leite e persistência da lactação em vacas Gir, aplicando modelos de regressão aleatória

Abstract

RESUMO -Com o objetivo de estimar parâmetros genéticos para a produção de leite no dia do controle (PLDC), foram usadas as 2.440 primeiras lactações de vacas da raça Gir leiteira, com partos registrados entre 1990 e 2005. As PLDC foram consideradas em dez classes mensais e analisadas por meio de modelo de regressão aleatória (MRA) utilizando-se como efeitos aleatórios o genético-aditivo, o de ambiente permanente e o residual e, como efeitos fixos, o grupo de contemporâneos (GC), a co-variável idade da vaca ao parto (efeito linear e quadrático) e a curva média de lactação da população. Os efeitos genético-aditivos e de ambiente permanente as days between records increased. Predicted breeding values for total 305-day milk yield (MRA305) and specific periods of lactation (obtained by the mean of all breeding values in the periods) using the AS function were compared with that predicted by a standard model using accumulated 305-day milk yield (PTA305) by rank correlation. The magnitude of correlations suggested differences may be observed in ranking animals by using the different criteria which were compared in this study.

Comparison of random regression models to estimate genetic parameters for milk production in Guzerat (Bos indicus) cows

“…AS was thus better than the other parametric functions, according to the criteria evaluated, and was consistent with the results of most studies conducted with other breeds in Brazil (de Melo et al, 2007;Herrera et al, 2008;Pereira et al, 2010).…”

“…Freitas et al (2010), who also evaluated data on the Guzerat breed, found similar results but observed the highest heritability values at the start of lactation. Hereditability estimates similar to those in the present study, obtained using Wilmink and AS functions, were reported by Herrera et al (2008) and Pereira et al (2010), with values varying from 0.15 to 0.33 for TDMY of Gyr cows. Araújo et al (2006), who studied the Dutch breed, also found heritability estimates with this amplitude by using the same functions.…”

“…However, two of the models adjusted by the Ali and Schaeffer function (AS-1 and AS-6), which considered homogeneous variances, presented the best BIC values, and the second of these obtained better logL and AIC values than the first. These results thus indicate that the residual variance behaves differently during lactation, as also observed by Costa et al (2005), Herrera et al (2008), and Pereira et al (2010), studying parametric functions for milk production by the Zebu Gyr breed.…”

“…Negative estimates for genetic correlations by using RRMs adjusted by parametric functions were also found by Brotherstone et al (2000), López-Romero andCarabaño (2003), andde Melo et al (2007) for the Dutch breed; by Kettunen et al (2000) for the Ayrshire breed, and by Costa et al (2005) and Pereira et al (2010) for the Gyr breed. However, Cobuci et al (2005), who used the Wilmink function, and Herrera et al (2008), who used the Wilmink and AS functions to fit the additive genetic and permanent environmental effects, did not observe a negative correlation with data for the Dutch and Gyr breeds, respectively. The negative correlations obtained in this study were possibly attributable to the difficulty of using RRMs to model the start and end of lactation, since there are fewer records in these periods, particularly at the end of lactation, because of the high frequency of short lactation periods in the Guzerat breed.…”

“…Among the parametric functions used in RRMs to study test-day milk yield (TMDY), the exponential function of Wilmink and the logarithmic one of Ali and Schaeffer are more prominent (López-Romero and Carabaño, 2003;de Melo et al, 2007;Herrera et al, 2008;Pereira et al, 2010). Modifications of the Wilmink function have been proposed with the intention of improving the adjustment of curves to the data.…”