Evidence of asynchrony in the onset of signs of oestrus in zebu cattle treated with a progestogen ear implant.

Thirty-two zebu cows aged three to nine years were given a progestogen implant to determine their oestrous response. The experiment was carried out in two parts: in the first, 15 cows were induced to display oestrus, one animal per day during the dry season; in the second, 17 cows were selected for daily induction during the wet season. The animals were observed continuously from 24 hours after the removal of the first implant until 48 hours after the withdrawal of the last. There were no significant differences between the durations of the pre-receptive phase (pro-oestrus), receptive phase (oestrus) and post-receptive phase (metoestrus) in the two trials. The average expression of oestrus after the withdrawal of the implant was 60 per cent in the dry season and 35.2 per cent in the wet season (P < 0.05). Furthermore, only 60 per cent of the animals responded by displaying sexual activity within the expected range of 31 to 57 hours after withdrawal of the implant. The distributions of the animals displaying sexual behaviour in the wet and the dry seasons were similar. It is suggested that zebu cows under field conditions tend to manifest synergistic sexual behaviour.