The objective of this work was to verify if it is necessary to adjust the 305-day milk yield for yield in the early lactation in determining the effect of days open on milk production. The yields in early lactation were represented by the cumulative yield up to 80, 100 or 120 days and summit production. The analyses were conducted for the first and following lactation using models that accounted for present and previous days open, respectively. An animal model for 305-day milk yield included the fixed effects of herd-year-calving season, genetic group, age of cow and days open in classes, early lactation yield and the random effects of animal, permanent environmental and error. When early lactation yield was included for the first lactation cows, the relative differences of each days open class, for example, 160 days, remained practically unalterable. For example, as the initial yield were up to 80 to 100 days, the relative differences among cows with 40 to 160 days of the present days open were 675 and 677 kg, respectively. When early lactation yield was ignored this difference was of 672 kg. For the following lactation of the cows, there was no effect of the variation of early lactation time on the relation between milk production and previous days open. The estimated heritability value for milk production was 0.24, when there was no adjustment and, decreased to 0.15, when summit production was considered. It was concluded that it is not necessary to account for early lactation yield on adjusting milk production for days open.
days open; Holstein; milk production
