Age-Month Adjustment Factors for Milk, Fat, and Protein Yields in Holstein Cattle

Age-month factors were developed for milk, fat, and protein yields. Records used in the analysis consisted of 769,410 lactation records of Holstein cows freshening in New York and New Jersey from July 1982 through August 1984. The mixed model used to describe the data contained fixed effects due to an overall mean, age-month of calving, herd-year, and random effects due to cow and error terms. There were 26 age groups by 12 mo of freshening for a total of 312 age-month estimates for each of the three yield traits. Results showed a definite age by month interaction with age being more important for younger cows while month effects assumed more importance as the cow matures. Multiplicative age-month correction factors were developed and are presented along with plots of the age-month factors for milk, fat, and protein yield. Factors differed for three yield traits. Fat and protein factors were higher than corresponding factors for milk.