Abstract

The effect of a supplement of formaldehyde-treated casein on partition of nutrients and productivity of Bos indicus x Bos taurus (50% x 50%) first-calf heifers was studied. Sixteen pregnant heifers were fed a long-chopped roughage diet (13 g N/kg organic matter) ad lib. for approximately 3 weeks before calving and for 16 weeks after calving. Half the animals were supplemented with formaldehyde-treated casein (300 g per day). There were also 8 non-pregnant, non-lactating controls, half of which were supplemented with formaldehyde-treated casein. Supplementation had no significant effect on intake, irrespective of the physiological state of the heifers. There was a significant (P < 0.05) positive multiple regression between peak intake during lactation and the weight change of both heifer and calf in the first 8 weeks of lactation. Milk yield was not significantly affected by supplementation for the first 8 weeks. In the second half of lactation supplemented heifers produced significantly (P < 0.05) less milk, but not less milk solids, than unsupplemented heifers. During the latter half of lactation there was a highly significant (P < 0.001) positive relationship between milk yield and lactose secretion. The mean (¦ s.e.m.) length of the period of anoestrus after calving was 17 ¦ 2.5 and 12 ¦ 1.7 weeks for unsupplemented and supplemented heifers respectively. The length of the period of anoestrus was positively related to milk production (P < 0.01) and peak intake during lactation (P < 0.05). Supplemented lactating heifers had significantly (P < 0.05) higher plasma insulin concentrations and significantly (P < 0.01) lower plasma growth hormone concentrations than unsupplemented lactating heifers. In supplemented heifers in the second 8 weeks of lactation plasma insulin concentration was significantly (P < 0.01) negatively correlated with milk yield and yield of milk constituents. Supplementation with formaldehyde-treated casein did not increase roughage intake but did affect the partition of nutrients via a positive insulin and negative growth hormone response. Nutrients were preferentially diverted into maternal body growth rather than milk synthesis in the second half of lactation.