Cows treated with Synchromate B may cluster their sexual behaviour independent of follicular growth at the time of oestrus.

In order to study the relative influence of social versus hormonal influences on sexual behaviour, three groups were formed from a herd of 16 Brahman cows, two of which groups were synchronized, implanting the second group the day the first had the Synchromate B (SMB) withdrawn. The third group was the untreated control. Cows in the sexually active group (SAG) were subjected to an intensive ultrasound (US) schedule to monitor ovarian activity. Alternatively, US examinations and progesterone (P4) samples were obtained twice weekly starting one week before the SMB implant until one week after the last implant was removed. Twenty mounting series involving 13 cows were observed. All cows in Groups 1 and 2 and 50% of controls displayed mounting behaviour, grouped on two periods. Seven cows showed mounting activity on both periods (8 days apart). Three cows in Group 2 (60%) displayed riding behaviour when the SMB implant was in place. No differences (P=.39) were found between the average size of the major follicle from animals displaying oestrus at the expected time (6.95+/-2.6 mm), and those displaying oestrus outside this period in harmony with induced animals (7.79+/-1.5 mm), nor (P=.71) between treated (7.72+/-2.0 mm) and untreated (7.28+/-2.1 mm) cows. It is concluded that treated and untreated animals, and even cows with an implant in place, may display mounting activity concomitantly with oestrous-induced cows and this behaviour is not driven by follicular size at the time of the expected oestrus. Furthermore, the follicular development suggests that SMB treatment does not impede and may even enhance oestrous response in certain cows when applied together with exogenous estrogens.