Medidas de persistência da produção de leite em rebanhos Gir Leiteiro utilizando modelos de regressão aleatória / Persistency of milk yield measures in Gyr herds using random regression models

AUTOR(ES)

Rodrigo Junqueira Pereira

DATA DE PUBLICAÇÃO

2009

RESUMO

Data comprising 27,000 test-day milk yield (TDMY) records of 3,362 first lactations of Gyr cows calving between 1990 and 2007 were used to compare random regression models for biweekly TDMY and to evaluate persistency of lactation milk yield (PS) measures. Records were analyzed by random regression models (RRM). Random trajectories were fitted by Wilminks (W) or Ali &Schaeffers (AS) parametric functions. The models included the fixed effects of contemporary group (herd-year-month of test), age of cow at calving as covariate (linear and quadratic effects) and the average trend of population. Residual variances (RV) were fitted by 1, 4, 6, or 10 classes. Models were compared by Akaikes (AIC) and Schwarzs Bayesian (BIC) information criteria. The AS function used for modeling the additive genetic and permanent environmental effects with heterogeneous RV adjusted with four classes was the best fitted model. RV estimates for W were higher than RV estimates for AS. TDMY heritability estimates ranged from 0.12 to 0.32 for AS function and from 0.09 to 0.33 for W function and were larger in the first half of the lactation period. Genetic correlations decreased from near unity between adjacent TDMY to negative values between early and late lactation. The AS function used for modeling the additive genetic and permanent environmental effects with heterogeneous RV adjusted four classes (AS4) would be parsimonious to fit TDMY of Gyr cows in Brazil. The AS4 model was used to evaluate nine PS measures. Heritability estimates for persistency measures ranged from 0.09 to 0.40. Genetic correlations between persistency and 297-day milk yield (Y297) ranged from - 0.59 to -0.11. At high PS and Y297 selection intensities there were a few animals in common. A larger percentage of animals in common was selected, as the selection intensity for both traits decreased. The average predicted breeding values for Y297 by the year of birth of cows showed substantial annual increase in the Y297, supporting the effectiveness of Gyr cattle national breeding program. However, no improvement in the genetic average of persistence of lactation yield was observed. The results allow to conclude that the selection for total milk yield does not provide sires and cows that are genetically superior for persistency of lactation yield.

ASSUNTO(S)

persistência da produção genetica e melhoramento dos animais domesticos regressão aleatória persistency of milk yield random regression

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