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Herd-Level Methods for Age Adjustment of Milk Yields¹

Animal Research Institute, Canada Department of Agriculture, Ottawa

ABSTRACT

The herd-level method of evaluating records is customary in Scandinavia, New Zealand, and Australia with or without the type of age adjustment customary in the American system. An expectancy is calculated from relationships between production at all ages and herd production. The American system uses estimated gross yield-age relationships to adjust all records to an age standard and then expresses individually adjusted records as deviations from herd averages of adjusted records. In Britain no adjustment is used although simultaneously estimating sire values with least-squares equations that include age of daughters (13) is being considered. A general analysis of Canadian milk records (4) indicated a seasonal difference in yield-age relationships for first lactations, and such differences have been reported subsequently in U.S. data.

We undertook a series of analyses to determine whether the herd-level method provides a more general yield-age standard than do present methods and to discover why yield-age relationships differ under different classifications.

¹ Contribution 490 from the Animal Research Institute.