Dynamics in sexually active groups of Zebu cattle (Bos indicus) comparing two procedures for estrus induction

Two schemes for estrous synchronization with a controlled internal drug releasing device (CIDR) were compared in a herd of 17 zebu cows. In the first stage, 3 subgroups were formed at random (n = 6, 6, and 5, respectively) and synchronized at 2-day intervals. All 3 subgroups received the same treatment. In the second stage, 25 days after CIDR withdrawal of the last subgroup, the treatment each subgroup received was applied to the combined herd (n = 17) simultaneously. Sexual activity was monitored by continuous observation from first subgroup CIDR withdrawal to 100 hours after the last implant from the third subgroup was removed, and for 100 hours after CIDR withdrawal following whole herd treatment. When subgroups were compared with group synchronization, fewer cows displayed estrus in subgroup treatments than in group synchronization (64% [66%, 16%, and 20% for subgroups 1, 2, and 3, respectively] vs. 35%; P=0.08), there were longer periods from CIDR withdrawal to estrus onset in subgroup treatments (141.6 ± 6 vs. 27 ± 3 h; P<0.01), and estrus was shorter in subgroup treatments (19.0 ± 6.1 vs. 34.1 ± 4.5 h; P<0.01). Estrous cows from the subgroup treatments were mounted by fewer herdmates (1.67 ± 0.33 vs. 4.55 ± 1.0 cows; P<0.01), and most mounts in this group were merely riders (84% vs. 14%; P<0.01). Estrous signs were more intense (P<0.05) when the herd was treated as a group rather than in subgroups, displaying more mounts and attempts to mount (1.10 ± 0.30 vs. 0.27 ± 0.05 and 0.20 ± 0.70 vs. 0.05 ± 0.29 frequency/cow/hour in estrus, respectively). No relationship was found between the number of sexual partners for each individual cow and estrous intensity or length (r<0.4; P>0.05). It was concluded that (1) the social facilitation effect does not depend on the number of sexual participants with a given cow, and (2) cows that are not in estrus are more reluctant to interact sexually, and when they do, they execute a reduced number of mounts.