Estimates of food intake using slow-release devices and carcass composition of Angus cows from a control line and a line selected for increasing yearling weight

Abstract

Data from cows in two Angus lines, established in 1971 and comprising a yearling weight selection line (AS1) and its contemporary control (ACO), were recorded over three periods per year to estimate food intakes at pasture in a 3-year study begun in 1991. In each period, daily organic matter intake (DOMI) was estimated using chromic oxide slow-release devices. In total, 126 cows were included over an average of 2.6 periods each. Forty-six cows were slaughtered after weaning to determine if there were any line differences in body composition. The repeatability of DOMI estimates was 0.46¦0.04, whilst the value between cows was 0.24¦0.05. The AS1 cows were 15.0% heavier and ate 10.5% more than the ACO cows (P < 0.001). Estimated intakes per unit body weight or per unit metabolic body weight did not differ between lines. Adjusted calf weaning weights at 6 months of age per unit DOMI of the dam were 20.2 and 19.4 kg/kg respectively ( P < 0.05), a 4.1% increase by the AS1 line. The AS1 cows had 17.7% (P < 0.001) heavier carcasses than ACO cows. When adjusted to equal hot carcass weight on a log-log basis, AS1 cows had significantly more bone and less fat ( P < 0.01) than ACO cows. Kidney weights, but not heart or liver weights, were significantly greater for AS1 cows than for ACO cows (P < 0.01) when adjusted to a constant empty live weight. It was concluded that the cow-calf pairs in the AS1 line had a 4% greater calf weaning weight per unit of DOMI by cows than those in the ACO line, and differences were associated with cows which had relatively larger kidneys, more % bone and less % carcass fat soon after weaning.