EFFECT OF DIVERGENT SELECTION FOR YEARLING GROWTH-RATE ON THE MAINTENANCE FEED REQUIREMENTS OF MATURE ANGUS COWS

The maintenance feed requirements of mature non-pregnant, non-lactating Angus cows from lines divergently selected for yearling growth rate, and from a control line, were determined in two experiments. In 1990, cows from each line were individually fed at 1 of 4 levels for six weeks. High-line (fast growth line) cows were 14% and 23% heavier than Control and Low-line (slow growth) cows, but required no more metabolisable energy (ME) per day to maintain constant liveweight (LW) or constant body energy. In 1992, cows were individually fed to maintain LW for six weeks. High-line cows maintained LWs 7% and 24% heavier than Control and Low-line cows but there was no difference in their average daily ME-intake. In 1990 High-line cows required only 74%, and in 1992 82%, of the ME per kg LW of Low-line cows for maintenance. About one-quarter of this difference might be expected on the basis of the difference in size. In both years, High-line cows were proportionally fatter and contained less protein per kg LW than Low-line cows. Standardising maintenance requirements for the differences in body composition of cows from the three lines removed the remaining observed differences in ME required per kg metabolic LW.