Removal of within-cow differences between morning and evening milk yields

Methods were explored for removing within-cow differences between a.m. and p.m. milk yields to allow direct comparison of consecutive milk yields. Daily a.m. to p.m. ratios of milk yield were studied for 504 lactations of 310 Holstein cows. Ratios varied within lactations and among cows. When the incomplete gamma function was used to characterize lactation curves, 89% of the variation in individual milk yields was explained when an a.m.-p.m. term was included. The a.m. to p.m. ratios increased over the course of lactations. Within-cow adjustment factors for a.m.-p.m. milking, based on weighted and unweighted averages of previous a.m. to p.m. ratios, changed as lactation length increased. A weighted moving average technique, which weighted the last a.m. to p.m. ratio by .15, compared with .85 for previous ratios, was considered to be the optimal method of calculating a factor for removing a.m.-p.m. effects.