Extending Lactation Records in Progress to 305-Day Equivalent

Abstract

The data consisted of 204,558 complete Holstein lactations in 2,100 herds compiled by the New York Dairy Records Processing Laboratory from 1959 to 1969. All lactation records had to have a 9th or 10th test day to be complete. The generalized least squares analysis used a model including mean, herd-year, and season-age-stage as fixed effects. The error term included random effects of cow and residual variation. Means for specified stages of lactation for milk and fat were estimated for lactations 1, 2, 3, and 4 or greater which were grouped by age of freshening, two age groups for the first three lactations and cne for fourth and later lactations. Extension factors differed from current United States Department of Agriculture factors. The new factors emphasize the need to consider lactation number, season, and age at freshening in extending lactation records in progress. Extension factors differ for each lactation, especially between the first two and later lactations. To consider season of freshening is important for early stages of lactation. Age is important for factors for earlv stages of lactation but less so as length of lactation increases. Differences due to age also become less important as lactation number increases. Factors for third and later lactations are only slightly affected by age of cow. © 1973, American Dairy Science Association. All rights reserved.