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 t o 20 yr of selection for 1) weaning weight (WWL), 2) yearling weight ( YWL), and 3) an index of yearling weight and muscle score (IXL). Mater nal response was estimated from reciprocal crosses among unselected si res and dams of control (CTL) and the selection lines. An Angus line w as added to increase the number of reciprocal cross comparisons. Direc t responses of WWL, YWL, and ML Linebreds compared with CTL were signi ficant for all traits. Maternal genetic responses were much smaller th an direct responses. Direct response in birth weight was largest for Y WL, followed by WWL and IXL. Maternal effect of ML 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; mater nal response was greater for ML than for WWL, which was selected for t his trait, and response was negative for YWL. Responses in maternal ef fects on final weight were much reduced in Hereford crosses because of a negative relation between maternal responses in pre- and postweanin g gains, especially in YWL and IXL. However, in Angus crosses, a posit ive association between pre- and postweaning gains increased maternal responses in final weight. Direct response for postweaning gain was gr eater in IXL than in YWL or WWL in Hereford crosses. In Angus crosses, YWL had larger direct responses for birth weight, preweaning gain, an d postweaning gain than in other lines. The direct response for muscle score from selection in ML, which was selected for muscle score and y earling weight, was greater than in other lines; maternal response was not important. The greatest gain in final weight was obtained when se lection resulted in a favorable change in the total of direct and mate rnal effects pre- and postweaning, which in this experiment was provid ed by including a muscle score along with yearling weight as selection criteria.