Differences in body condition and body size affect the responses of grazing dairy cows to high-energy supplements in early lactation

Two experiments of 5-weeks duration, using Friesian cows, were conducted at the Kyabram Dairy Centre during September-October (experiment 1) and November-December (experiment 2), 1998. The aim of the research was to determine whether the level of substitution with the feeding of concentrates to grazing dairy cows would be increased by thinner body condition (3.4 v. 5.2 units) and by larger body size (618 v. 486 kg) because both these factors would be expected to increase the level of unsupplemented pasture intake. In both experiments, there were 2 pasture allowances (targets of 25 and 50 kg DM/cow.day) and 2 weeks of supplement (0 and 5 kg DM/cow. day). In experiment 1, pasture intake increased significantly (P < 0.05) with the increase in pasture allowance and decreased with supplementation. Levels of substitution averaged 0.56 and 0.47 kg DM/kg DM when concentrates were fed to cows with low and high body condition, respectively. However, variation in the data meant that body condition did not have a significant (P > 0.05) influence on the positive relationship between substitution and unsupplemented pasture intake. The only effects of body condition on animal production were that the high body condition cows produced milk with a higher fat content, and they lost mole body condition, than the low body condition cows (P < 0.05). The increase in pasture allowance significantly (P < 0.05) increased milk production and milk urea content, and decreased the loss in body condition. Feeding concentrates significantly (P < 0.05) increased milk production and prevented any losses in body condition, but milk urea content declined with supplementation. In experiment 2, heavy cows ate more pasture than light cows (P < 0.05), and increasing the pasture allowance increased pasture intake while providing cows with concentrates reduced pasture intake (P < 0.05). When substitution was considered relative to unsupplemented pasture intake (kg DM/cow.day), light cows exhibited significantly (P < 0.05) more substitution than heavy cows, but when pasture intake was expressed as a percentage of liveweight, there were no significant (P > 0.05) differences in substitution between light and heavy cows at common intakes. Heavy cows produced more (P < 0.05) milk with a lower protein content than light cows. Increasing the pasture allowance and supplementation with concentrates both significantly (P < 0.05) increased milk production and milk protein content, while the higher pasture allowance reduced body condition loss and supplementation increased (P < 0.05) liveweight. Feed conversion efficiencies were the same for light and heavy cows, at 1.68 kg FCM/kg DM of total intake in both cases. The hypothesis that substitution would increase as intake increased was supported by the results obtained for both body condition and body size, since body condition affected neither intake nor substitution while liveweight influenced both.