The evaluated animals were raised in grazing system without supplementation in the dry season, from 1988 to 1996. A total of 587 observations of days open (DO) was taken from the archives of the regional technical office of ABCZ (Escritório Técnico Regional da ABCZ) in Fortaleza - CE and from zootechical registers of the Farm Entre Rios. The GLM procedure of the SAS Program was used to analyze the data. The model included the sire of the cow random effect, the fixed effects of genetic group of last calf, of the year and the month of the last calving and the interactions between last genetic group and month of the last calving, genetic group and year of the last calving, and the age of the cow at calving as covariate. DO was influenced only by the effect of the sire of the cow. The average estimated for minimum square and its respective standard error (SE) and coefficient of variation (CV) were 146.51 ± 43.05 days and 97.63%, respectively. The post-partum interval of almost five months is long, considering the herd submitted to the artificial insemination, even in the grazing system. The high results of SE and CV can be attributed to the great variability of the studied trait, that is affected by variations of the environment.
bovines; Nellore; post-partum interval; reproduction
