Relationships between Measures of Feed Efficiency and Transmitting Ability for Milk of Holstein Cows

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Abstract
Feed efficiency measures, gross energy (solids-corrected milk/estimated energy intake) and N (milk N/N intake) utilization, and apparent digestibilities of ration dry matter, N acid detergent fiber, and starch were calculated for 134 samples of 75 daughters of 31 Holstein sires. Cows were in 5- to 9-day N balance trials in 1st and 2nd trimesters of lactation and were fed for ad lib consumption corn silage-based complete rations formulated to meet average nutrient requirements. The hypothesis that feed efficiency diminishes as milk increases from selection was tested. The sample of cows closely approximated the distribution of genetic ability for milk of the Holstein breed. Nonsiblings of same parity (1, 2, 3 or more) calving within 2 wk of each other and differing by at least 140 kg estimated transmitting ability, were paired in N balance trials. Evidence for diminishing returns to increased transmitting ability for milk was nil for any measure of feed efficiency. Energy efficiency showed linear increases when regressed on cow transmitting ability for fat-corrected milk, but other measures of dietary utilization were unrelated to milk yield.