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 y ields. Daily a.m. to p.m. ratios of milk yield were studied for 504 la ctations of 310 Holstein cows. Ratios varied within lactations and amo ng cows. When the incomplete gamma function was used to characterize l actation curves, 89% of the variation in individual milk yields was ex plained when an a.m.-p.m. term was included. The a.m. to p.m. ratios i ncreased over the course of lactations. Within-cow adjustment factors for a.m-.p.m. milking, based on weighted and unweighted averages of pr evious a.m. to p.m. ratios, changed as lactation length increased. A w eighted moving average technique, which weighted the last a.m. to p.m. ratio by .15, compared with .85 for previous ratios, was considered t o be the optimal method of calculating a factor for removing a.m.-p.m. effects.