Pregnancy rate in zebu cows with two different postpartum intervals exposed to a two-bull rotational system

In order to evaluate the reproductive effect of a rotational breeding system, the pregnancy rate of 69 Zebu cows was assessed, 32 late postpartum (211 +/- 93 days postpartum, LP) and 37 early postpartum (averaging 63 +/- 8 days after calving, EP). In the LP group, 60% (19/32) were cycling before exposure to the bulls but only 38% (14/37) in the EP group. The two groups were dissimilar from the start (p < 0.05). The cows were exposed in a rotational breeding system, with six Brahman bulls with previous sexual experience paired off in three groups (A-B, C-D and E-F). The cows were exposed to each pair of bulls for a period of 3 weeks, with one week in between each period, when the females were without the presence of a male. In the LP group, the pair of bulls A B obtained a pregnancy rate of 72% (23/32), whereas for bulls C-D the pregnancy rate was 33% (3/9) and for bulls E-F the pregnancy rate was 67% (4/6). In contrast, for the EP cows, bulls A-B obtained a pregnancy rate of 32% (12/37), bulls C-D a pregnancy rate of 67% (12/18) and for bulls E-F a pregnancy rate of 55% (6/11). The percentage pregnancy obtained using the pair of bulls A-B differed (p < 0.05) between LP and EP, whereas for bulls C-D and E-F it was similar. The pregnancy rate was different (p < 0.05) in the first 3 weeks of mating for the LP group, the highest number of cows becoming pregnant during the first 3 weeks of the study. In contrast, the highest number of pregnancies in EP occurred evenly during the last 6 weeks. This study suggests that the reproductive performance of pairs of bulls in a 9-week rotational programme with the overall pregnancy rate is similar (94% in the LP and 81% in the EP). However, the time taken for the females to become pregnant, and hence the performance of the bulls, is related to the average number of days postpartum for the cows.