Four random regression models using alternative definitions of herd effects were compared. Models included a fixed or random herd-test day (HTD) only or a HTD effect and an additional herd-year (HY) effect with fixed and random regression curves. Six random samples including milk yield data from the first three lactations were used to obtain genetic parameters. Two criteria, the log of the marginal density of the data and a cross validation predictive statistics (D) were also obtained for the three largest samples. Estimates of the permanent environmental variances were hardly affected by the model. The model fitting a random HTD and a fixed HY curve provided substantially smaller genetic variance estimates at the end of the lactation. Small differences among models were observed for both comparison statistics. The models fitting a fixed HTD effect and an additional random HY curve showed the worst results.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume , , 25.09, 2006
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