Simplified Herd-Level Age-Correction Factors

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Abstract
Dairy records have to be corrected for age of cows, when records of cows of different ages are being compared. Multiplicative factors adjust the records by adding a percentage of each record to itself, the percentage being dependent on the age of the cow. Correction factors utilizing the level of herd production and the age of the cow were recently developed, but they involve a lengthy iterative calculation of regressions. The simplified herd-level factors presented in this paper are derived directly from multiplicative factors, and in application utilize both age of cow and herd average. The simplified factors for the Jersey breed in New Zealand are given, and an example of their use is shown. This method of correcting records on a within-herd basis can be used in situations other than correcting dairy records for age. Its application is given for correcting the weaning weights of beef calves for sex of calf and age of dam.