Abstract
The development of effective genetic evaluations and selection of sires requires accurate estimates of genetic parameters for all economically important traits in the breeding goal. The main objective of this study was to assess the relative performance of the traditional lactation average model (LAM) against the random regression test-day model (RRM) in the estimation of genetic parameters and prediction of breeding values for Holstein Friesian herds in Ethiopia. The data used consisted of 6,500 test-day (TD) records from 800 first-lactation Holstein Friesian cows that calved between 1997 and 2013. Co-variance components were estimated using the average information restricted maximum likelihood method under single trait animal model. The estimate of heritability for first-lactation milk yield was 0.30 from LAM whilst estimates from the RRM model ranged from 0.17 to 0.29 for the different stages of lactation. Genetic correlations between different TDs in first-lactation Holstein Friesian ranged from 0.37 to 0.99. The observed genetic correlation was less than unity between milk yields at different TDs, which indicated that the assumption of LAM may not be optimal for accurate evaluation of the genetic merit of animals. A close look at estimated breeding values from both models showed that RRM had higher standard deviation compared to LAM indicating that the TD model makes efficient utilization of TD information. Correlations of breeding values between models ranged from 0.90 to 0.96 for different group of sires and cows and marked re-rankings were observed in top sires and cows in moving from the traditional LAM to RRM evaluations.
Highlights
In dairy production, genetic change and improvements in milk production performance are realized when the parents of the generation of animals are accurately chosen
The results from this study showed slightly lower correlations between estimated breeding value (EBV) from the Lactation average model (LAM) and regression test-day model (RRM) indicating a possible re-ranking and rank changes for sires and cows
The result from the present study indicates that the ranking of cows was much more affected in case of moving from LAM to RRM
Summary
Genetic change and improvements in milk production performance are realized when the parents of the generation of animals are accurately chosen. Accurate estimation of the genetic merit of dairy animals has long been the subject in many studies and great advances have been made in the last decades. In dairy cattle, the selection for milk yield in most countries is based on the use of the traditional 305-d lactation records (Hammoud and Salem, 2013; Goshu et al, 2014). In this method, incomplete lactations or part lactations are extended to 305-d leaving a room for introduction of some errors. The arbitrary standardization of lactation yields to 305-d and the simple compilation of test-day (TD) records into 305-d lactation records, as practiced in most countries, suffers from lack of correction for short term
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