Multiplicative Age-Season Adjustment Factors by Maximum Likelihood, Gross Comparisons, and Paired Comparisons

¹ Animal Research Centre, Agricultural Canada, Ottawa, ON, Canada K1A 0C6

Age-season adjustment factors for milk yield were estimated by three procedures (maximum likelihood, gross comparisons, and paired comparisons) from 24,881 lactations of 7753 Holstein cows that calved between 1975 and 1987 in the south region of Brazil. Multiplicative age-season factors were almost identical for maximum likelihood and paired comparison and were about .04 smaller than the gross adjustment factors for cows calving in the dry season (April to September) and up to 45 mo of age. At younger ages, there were no differences between age adjustment factors for the two seasons, but at mature ages (85 to 96), these differences varied from 6 to 8%. Age adjustment factors obtained in this study were very similar to those reported for Holstein cows in Canada and the United States, despite large differences in milk production between cows in Brazil and North America. Furthermore, the maximum likelihood methodology proved to be robust with respect to the repeatability estimates chosen.

Key Words: age-season • milk • Brazil

Submitted on November 29, 1988
Accepted on September 18, 1989