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Joint Influence of Month and Age of Calving on Milk Yield of Holstein Cows in the Northeastern United States

Department of Animal Science, Cornell University, Ithaca, New York 14850

ABSTRACT

Month-age constants were estimated from milk records on 242,425 purebred Holstein cows from the northeastern United States by the maximum likelihood method with elimination of biases due to environmental trend, herd differences, and selection based on previous performance. These constants revealed month of calving by age interactions of sufficient magnitude that comparisons of individual records and bull proofs could be badly biased if no adjustments were made for interaction. It was concluded that monthly mature-equivalent factors could not be adequately grouped into a few seasons, that all records should be adjusted for season of calving, and that the most appropriate method of adjusting for both season and age of calving was by multiplicative factors which simultaneously adjust records to the expected yield, had all other conditions remained the same.