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Parity in Age Adjustment for Milk and Fat Yield

University of Guelph, Guelph, Ontario, Canada

ABSTRACT

Canadian Record of Performance data on the first two consecutive lactations of 56,959 Holstein cows from 1959 to 1969 were used to examine the joint effects of parity and age prior to 47 mo on 305-day yield. This provided 24,479 records in the overlapping period of ages for first and second lactation cows from 31 to 37 mo of age to examine the need for joint adjustment of age and lactation number or age adjustment within lactation. The model had fixed elements of parity, age, parity by age interaction, and herd-year-season. Interaction between parity and age after absorption of herd-year-seasons was nonsignificant for 305-day milk yield but significant at 5% for 305-day milk fat yield. Least squares means for first lactation milk and fat yield were larger than second Jactation yield for calving ages of 31 to 33 and 34 to 35 mo but lower for 36 to 37 mo.

¹ Present address: Department of Dairy Science, Michigan State University, East Lansing 48823.