FACTORS FOR EXTENDING IN-PROGRESS LACTATION RECORDS TO A 305-DAY EQUIVALENT

Paper for Presentation at the 64th Annual Meeting of the American Society of Animal Science. 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 present to be regarded as complete. The generalized least squares analysis used a model including effects due to the mean, herd-year and season-age-stage of freshening which were assumed to be fixed. The error term included the random effects due to the cow and residual. Generalized least squares estimates for specified stages of lactation for milk and fat were found for lactations 1, 2, 3 and ≥ 4 which were grouped by age of freshening, two age groups for the first three lactations and one for ≥ four lactations. Extension factors will be presented which differ from current U.S.D.A. factors. The new factors emphasize the need for considering lactation number, season and age at freshening when extending in-progress lactation records. The extension factors differ for each lactation. The major differences occur between the first two and later lactations. The importance of considering season of freshening is apparent for the early stages of the lactation. The age effect is significant for the factors for early stages of lactation but decreases as lactation length increases. Differences due to age become less important as lactation number increases. The factors for third and later lactations are only slightly affected by age of the cow