Effects of days dry, previous days open, and current days open on milk yields of cows in Zimbabwe and North Carolina.

Effects of days dry and previous and current days open on milk yields during the first three lactations were determined for Holsteins from Zimbabwe and North Carolina. The animal models used included animal; permanent environmental effects of cows for herd-year, month of calving, age, current days open, and DIM; and the inverse of additive numerator genetic relationships. Model 1 for first lactations included only these effects, but Model 2, the animal model for later lactations, included these effects plus previous days open, previous days dry, and previous milk yield. The dependent variable in both models was unadjusted milk yield for the lactation. Current and previous days open, previous days dry, and DIM were also fitted as dependent variables with Models 1 and 2, except for previous milk yield, when appropriate. As current days open increased, milk yield rose, regardless of milk yield during prior lactations. Milk yields were reduced for lactations following < 60 d dry and showed little advantage for longer dry periods. The effects of previous days open are real but are overestimated unless permanent environmental effects of cows are considered simultaneously. The heritabilities and repeatabilities of current and previous days open and previous days dry were higher for cows in Zimbabwe than for those in North Carolina. Results suggest that, to obtain unbiased estimates of breeding values of sires and cows, yield data should be adjusted for the environmental effects of days dry as well as those of previous and current days open.