Effectiveness of an Age Herd-Level Adjustment Procedure for Milk and Fat Yield

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Abstract
The method of age and herd-level adjust- ment proposed by Hickman and Gravir adjusted for differences in age at freshen- ing better than the currently used BCA 2 method. Expression of the records as per- centages of predicted levels rather than differences from predicted produced more homogeneity of variances across herds and seasons. Separation of between- and within-herd effects of age at first freshen- ing would improve the adjustment for herd differences in first-lactation milk and fat yield. Different within herd-year-season regressions of milk yield on age in spring and fall seasons persisted even when herd- level was taken into account. These differ- ences did not exist for fat yield. Hetero- geneity of within-herd regressions of yield measures on age across herds was detected only for second- and later-lactation BCA milk and fat records. Differences between herds and seasons in heifer feeding and management affect first-lactation milk yield independently of feeding and management during lactation, and are partly identified by herd-season average age at first fresh- ening.