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Cornell University, Ithaca, New York 14850
ABSTRACT
Purposes of Age Adjustment
Adjustment of lactation milk and fat records for age of the cow at calving has been done historically for the following reasons: (1) to remove biases from comparisons of cows (or groups of cows) of different ages, (2) to reduce sampling variations due to unequal ages, and (3) to estimate what a specific record most probably would have been had all conditions been the same except the age of the cow.
Speculation about what might have been is an interesting but sterile exercise.
Reductions in sampling variation are, at best, small because variations in age account for somewhat less than 10% of the variation in milk records within herds. Within first lactation, variation due to age is larger than that due to sires, but random sampling variations in estimates of means are not reduced greatly by adjustment for age.
The primary usefulness of age adjustment is to remove biases from comparisons of cows of different ages, particularly in sire evaluation where daughters of some bulls may be uniformly young but compared with older herdmates.
1 Current address: American Breeders Service, DeForest, Wisconsin 53532.
This article has been cited by other articles:
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  H. D. Norman, P. M. VanRaden, R. L. Powell, J. R. Wright, and W. R. VerBoort Effectiveness of National and Regional Sire Evaluations in Predicting Future-Daughter Milk Yield J Dairy Sci, February 1, 2005; 88(2): 812 - 826. [Abstract] [Full Text] [PDF]
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