Effects of Selection for Body Size on Feed Efficiency and Size of Holsteins

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Abstract
First lactation records from a herd of Holstein cows bred for either high production and large size or for high production and small size were examined to estimate effects of selection for size and feed efficiency. Selection was among progeny-tested bulls available from the artificial insemination industry. Three rations were fed that differed for ratios of concentrate to milk. After three generations, groups of cows bred for large or small size differed by 50.2 kg in weight, 5.6 cm in wither height, 6.4 cm in length, 2.1 cm in depth, and 5.9 cm in chest circumference; most differences were due to a decrease in the size of the small cows. On the basis of total lactations, cows bred for small size were 2.8% more feed efficient than cows bred for large size. During the fist 90 d of lactation cows fed the low concentrate ration were more efficient than those fed the high concentrate ration. Season of calving had an effect on feed efficiency between d 30 and 120 of lactation. During this period cows calving between December and May were more efficient.