Male effect on the post-partum sexual activity of cows maintained on two nutritional levels

Forty cows were distributed in four homogeneous groups and fed on rations which provided 130% (HL, T1 and T3) and 70% (LL, T2 and T4) of their post-partum energy requirements. At 30 days post-partum, two vasectomised bulls were incorporated in Groups T1 and T2. Sexual activity was determined by oestrus detection, ovarian palpation, rectal examination and plasma progesterone level; body condition was determined by dividing liveweight by withers height (kg cm−1). Different energy intakes produced differences in body condition, which for HL and LL reached 3.71 kg cm−1 and 3.30 kg cm−1, respectively (P<0.01) at 15 weeks post-partum. A higher concentration of ovulations in Group HL with parturition-ovulation intervals of 57.9 days and 77.6 days (P<0.05) and variances of 303.2 and 1651.1 (P<0.05) for Groups T1 and T3, respectively, could be attributed to the presence of a male. From the 16th week onwards, all the cows were regrouped in a grazing system together with vasectomised bulls. A greater grouping of oestrus (P<0.05) was detected on LL cows in Group T4 than in Group T2 (calving-oestrus intervals and variance: T4 132.4 days, 337.8; T2, 145.8 days, 1220). The progesterone pattern showed that in Group T1 a large number of cows presented a preovulatory peak. Progesterone production between the first and the second post-partum ovulation showed an area under the progesterone concentration vs. time curve significantly higher (P<0.05) in the Group T1 (119) compared with any of the other Groups (T2, 61 units per area; T3, 67 units per area; T4, 80 units per area). Our results show that the presence of the male stimulates post-partum reproductive activity and that the response is modified by the nutritional condition of the cows.