Divergent Selection for Postweaning Feed Conversion in Beef Cattle: Predicted Response Based on an Index of Feed Intake and Gain vs Feed:Gain Ratio3

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Abstract
As part of a divergent selection experiment (high vs low feed conversion) designed to obtain realized genetic parameter estimates for postweaning feed conversion, 35 Angus bull calves were selected each year from 1979 through 1983 to be fed in a 140-d postweaning performance test. From these 35 individually-fed bulls, the three most efficient and three least efficient (in terms of kilograms of total digestible nutrients required per kilogram of gain) were selected each year to each be mated to approximately 20 purebred Angus cows in a test herd. In the last 2 yr of the experiment, feed:gain ratios were adjusted for differences in maintenance requirements before selection. An alternative to selection based on feed/gain or adjusted feed/gain would have been to select on an index of feed intake and gain (Index3 = −1.0 feed intake + 3.87 gain) where feed intake was either unadjusted or adjusted (Adjlndex3) for differences in maintenance requirements. Residual correlations (year effects removed) among the four selection critera were large (|r|⩾.76; P<.01) as were correlations among ranks of the bulls based on the four selection criteria (|n|⩾.75). Predicted genetic change for feed:gain ratio in both the high and low directions was greatest when selection was based on feed:gain ratio or Index3. Genetic changes expected to result from selection for feed:gain ratio or Index3 were similar, as were genetic changes expected due to selection for adjusted feed:gain ratio or Adjlndex3. Results indicated little advantage to use of an index of feed intake and gain in place of the feed:gain ratio if the goal is to bring about generic improvement in postweaning feed conversion of beef cattle.