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Variation in Ratio Factors for Age-Adjusting Part-Lactation Records

Animal Husbandry Research Division, USDA, Beltsville, Maryland

ABSTRACT

Age factors for placing cumulative and noncumulative part-lactation yields on a mature-equivalent part-lactation basis were constructed from 39,328 lactations representing 27,538 Iowa Holstein cows. Variation in the age-production curves associated with season, test day, and yield trait (milk or fat) was investigated.

Different sets of factors are required for each combination of test day, season, and yield trait. The later stages of the lactation exhibited a smaller increase in production with age than the earlier stages. Cows that calved in the summer months showed a smaller rate of increase with age than cows calving at other seasons of the year. The rate of increase in fat yield to maturity was less than that for milk production.

The results provided further evidence that the extrapolation of cumulative part records to a 305-day basis should consider age at calving.