Effects of previous days open and days dry as well as current days open on
milk production during the first and later lactations were determined for 451 Friesian
cows belonging to Animal Production Research Institute, Ministry of Agriculture. Two
models were used for analysis of variance. Model 1. for first lactations included sire,
current days open, age at calving and season of calving. Model 2. for later lactations,
. included the previous effects plus previous days open, previous milk yield. The milk production traits were total milk yield (TMY), milk yield 305-days (MY-305 d), and
lactation period (LP), current days open had highly significant effects on all studied
traits for both first and later lactations. Differences due to previous days open were insi~nificant for all traits under
investigation. TMY and MY-305d tended to increase with increasing days dry till the
forth period (91-120 days) then slightly decreased. Previous milk yield affected
significantly (P<0.01) both TMY and MY-305d.