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Effectiveness of an Age Herd-Level Adjustment Procedure for Milk and Fat Yield¹

Animal Research Institute, Central Experimental Farm, Ottawa, Ontario, Canada

ABSTRACT

The method of age and herd-level adjustment proposed by Hickman and Gravir adjusted for differences in age at freshening better than the currently used BCA² method. Expression of the records as percentages 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 freshening 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 differences did not exist for fat yield. Heterogeneity 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 freshening.

¹ Contribution no. 364.