Effect of the month of calving on subsequent pregnancy rate and weaning weight of calves from Nelore cows.

Background: The concentration of births in the most appropriate time (months) of the year results in uniform lots of calves and allows the adoption of different management practices, aimed at reducing mortality and increasing the weaning weight. The aim of this study was to evaluate the effect of the month of calving on subsequent pregnancy rate and weaning weight of calves from Nelore cows during the birth season. Materials, Methods & Results: Were used 91 Nellore cows between three and ten years, belonging from Experimental Farm Capim Branco, Uberlandia, Minas Gerias, Brazil, from August 2008 to May 2009. The birth season lasted 110 days and occurred from 09/17/2008 to 01/05/2009 (110 days), that period being divided into four months: 1 (9/17 to 16/10), 2 (10/17 to 11/16), 3 (11/17 to 12/16) and 4 (12/17 to 01/05). On the day of birth, calves (males and females) were weighed in analog scale, and then, between the sixth and seventh months of life (months to weaning) were weighed again. The next breeding season was held from 12/05/2008 to 03/05/2009, lasted 90 days. Eleven days before the beginning of the breeding season, the cows that had more than 30 days postpartum (DPP) and body condition score equal to or greater than 2.50 were submitted through a time artificial insemination protocol (TAI). Pregnancy diagnosis was performed in cows with 28 to 32 days of insemination. No effect of month of calving on pregnancy rate (P 0.05). But there was effect of the birth month (P < 0.001) in weaning weight of the males calves. Males born in the first months were heavier when compared to those born in the months 3 and 4. The animals born in month 2 showed no significant differences when compared to those born in the month or 1 month 3 and 4. The weaning weight was probably influenced by environmental variation occurring during the birth season. Discussion: Cows that calved in the early birth season had a higher pregnancy rate at the end of breeding season and subsequent greater number of DPP in the conception rate than cows that have calved at the end of breeding season. These results show the importance of holding a short breading season to increase the reproductive efficiency of beef cows. If all cows had calved within the first two months of birth season the result of subsequent breeding season would be more satisfactory. The effect of month of birth on weaning weight of male calves born early in the calving season has the least adverse environmental conditions, because they were born in the late dry season when the disease incidence is lower. The nutritional requirements of males may not have been satisfied enough with the lowest average in the month, resulting in lighter calves. The weaned heavier born in the early birth season indicating that environmental conditions have favored the results. In this study, we can conclude that cows that calved at the beginning of breeding season had a higher rate of subsequent pregnancy in the next breeding season, yet the month of birth affected the weaning weight of males, but does not affected females.