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Direct and maternal genetic responses to selection for weaning or yearling weight or for yearling weight and muscle score in Hereford cattle.

An experiment involving crosses among selection and control lines was conducted to partition direct and maternal additive genetic response to 20 yr of selection for 1) weaning weight (WWL), 2) yearling weight (YWL), and 3) an index of yearling weight and muscle score (IXL). Maternal response was estimated from reciprocal crosses among unselected sires and dams of control (CTL) and the selection lines. An Angus line was added to increase the number of reciprocal cross comparisons. Direct responses of WWL, YWL, and IXL linebreds compared with CTL were significant for all traits. Maternal genetic responses were much smaller than direct responses. Direct response in birth weight was largest for YWL, followed by WWL and IXL. Maternal effect of IXL on birth weight was larger and that of WWL and YWL was smaller than CTL. Direct responses in weaning weight did not differ greatly among selection lines; maternal response was greater for IXL than for WWL, which was selected for this trait, and response was negative for YWL. Responses in maternal effects on final weight were much reduced in Hereford crosses because of a negative relation between maternal responses in pre- and postweaning gains, especially in YWL and IXL. However, in Angus crosses, a positive association between pre- and postweaning gains increased maternal responses in final weight. Direct response for postweaning gain was greater in IXL than in YWL of WWL in Hereford crosses. In Angus crosses, YWL had larger direct responses for birth weight, preweaning gain, and postweaning gain than in other lines. The direct response for muscle score from selection in IXL, which was selected for muscle score and yearling weight, was greater than in other lines; maternal response was not important. The greatest gain in final weight was obtained when selection resulted in a favorable change in the total of direct and maternal effects pre- and postweaning, which in this experiment was provided by including a muscle score along with yearling weight as selection criteria.