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.