Environmental and genetic effects on reproduction traits of Nellore females under fixed breeding season

The objectives of this study were the evaluation of generic (sire) and environmental effects on age at first calving (n=1582), first (n=1582) and second (n=644) calving dares and first calving interval (n=644) in Nellore cattle. Data were analyzed using least squares methodology. Observed means and standard deviations were 1170.8 +/- 138.0 days; 62.1 +/- 32.4 days; 71.9 +/- 31.3 days; and 383.2 +/- 29.8 days, respectively, for age at first calving, first and second calving dates, and calving interval. The effects of year of birth were important (P<.001)for all traits. Month of birth was important (P<.001) only for age at first calving, younger females calving from August to December. For age at first calving and first calving date birth weight of the calf was an important (P<.05) source of variation. Larger (P<.05) second calving dates and first calving intervals were observed for females nursing male and, or, heavier calves. Month of the first calving affected (P<.001)first calving interval and second calving date. Among all traits, sire was important (P<.05) only for first calving date. The phenotypic association between first and second calving dates was favorable. For 10 days of delay in the date of the first calving there was a 3.7-day delay in the second calving dare. However, the phenotypic association between first calving date and first calving interval was not favorable. For each 10 days of delay in the first calving date there was a reduction of 6.2 days in the first calving interval. Results suggested that selection for reduction bl the first calving interval would not be effective when trying to improve female reproduction under fixed breeding seasons.