Oestrus and Ovarian Activity in Lactating Sahiwal-shorthorn Beef Cows in South-eastern Queensland

Oestrus and ovarian activity were studied in a group of crossbred SahiwalShorthorn beef cows over four lactation periods of varying length throughout different seasons of the year. In the first lactation, cows were in poor condition (body weight 303 kg) and the mean interval from calving to first oestrus was 2202 16 days. Five COWS were still anoestrus 18 months after calving and were culled. The interval to first post partum oestrus was correlated with the length of the suckling period, but not in the subsequent three lactation periods when the body’ weight (438 kg) was significantly higher (P<O.Ol ) . The shortest mean interval from calving to first oestrus was recorded in the summer period (93 * 3 days). Follicular growth and regression occurred in about 30% of cows about 36 days before the first oestrus post partum. Bos indicus crossbred cows exhibited oestrus between 90 to 140 days post partum irrespective of whether the calves were weaned at 95 or 188 days. If the calves were weaned at 14 days or less, oestrus occurred at a mean of 50 days post partum.