Reducing post-partum anoestrous interval in first-calf Bos indicus crossbred beef heifers. 2. Responses to weaning and supplementation

Abstract

Supplementation designed to increase the supply of glucose and amino acids, was compared with early weaning as strategies for reducing post-partum anoestrous in Bos indicusx Bos taurus (518x318) first-lactation heifers. Twenty-three pregnant heifers were allocated to four treatment groups and fed long-chopped hay (15.8 g N/kg dry matter) ad libitum and a complete mineral mixture throughout the experiment as well as 450 g molasses and 50 g urea per day during the final month of pregnancy. After calving, three groups were supplemented daily for 120 days with either 2 kg cracked rice (E), 0.4 kg formaldehyde-treated casein (P), or cracked rice (2 kg)+formaldehyde-treated casein (0.4 kg) (E+P); the unsupplemented control group had their calves weaned at 60 days post-partum. Supplemented animals had significantly higher intakes of hay (7- 1-7.3 v. 5.9 kg OM/day), weight gain (0.4-0.5 v. 0.0 kg/day) and body condition score (4.6-4.7 v. 4.3) than the unsupplemented heifers, but there were no differences between the supplemented groups in these variables. Unsupplemented animals and heifers fed P partitioned similar amounts of lactose and fat into milk, whereas secretion of both lactose and protein was significantly higher in the E+P and E groups. All control heifers ovulated within 14 days of their calves being weaned, whereas only two of the lactating supplemented heifers (EfP) showed cyclic ovarian activity at that time and three other lactating heifers ovulated several weeks later. The response to weaning occurred in heifers that had a liveweight and condition score of 374 kg and 4.3 respectively, which was 30-50 kg and a half unit in condition below the suckling heifers. In unsupplemented heifers showing ovarian cyclicity, acetate entry rate was less than half that of the supplemented animals, whereas glucose entry rate, adjusted for loss to lactose, was similar for both groups. Supplementation did not affect either the basal concentration or frequency of pulsatile release of luteinizing hormone (LH) at 60 days post-partum, or the LH response to gonadotrophin releasing hormone (5 8g GnRH). It is concluded that under the conditions of the present study, weaning had a greater effect than post-partum nutritional supplements on the resumption of ovarian activity in first-lactation heifers with moderate nutrition.