|Van vleck, Lloyd|
Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/6/2003
Publication Date: 2/2/2004
Citation: KOCH, R.M., CUNDIFF, L.V., GREGORY, K.E., VAN VLECK, L.D. GENETIC RESPONSE TO SELECTION FOR WEANING WEIGHT OR YEARLING WEIGHT OR YEARLING WEIGHT AND MUSCLE SCORE IN HEREFORD CATTLE: EFFICIENCY OF GAIN, GROWTH, AND CARCASS CHARACTERISTICS. JOURNAL OF ANIMAL SCIENCE. 2004. 82:668-682. Interpretive Summary: A selection experiment was initiated in a Hereford population at the Fort Robinson Beef Cattle Research Station, Crawford, NE in 1960. One line selected for weaning weight (WWL), another for yearling weight (YWL), and a third for an index of yearling weight and muscle score (IXL). An unselected control line (CTL) was also established. Results reported as the experiment was conducted over a 25 year period, demonstrating the effectiveness and response to selection, had a significant effect on selection procedures used by seedstock producers in the beef industry over the past 40 years. A mating design was developed for the final phase of the long term experiment to partition selection response for direct and maternal additive genetic effects. Final estimates correlated response to selection in each line are reported here for efficiency of gain, growth and carcass characteristics. Results demonstrated that steady and significant progress can be achieved in closed herds maintained with annual use of only six bulls and 150 cows by selection of beef animals with superior records of performance for moderately heritable traits. Mass selection for growth to weaning or yearling ages was effective in changing weaning and market weight. Selection emphasis on both direct and maternal components of growth, is superior to emphasis on the direct component alone. Selection for two traits does not necessarily reduce rate of progress relative to selection for one of the two traits, provided traits share a favorable genetic correlation, like that between yearling weight and muscling score. Selection for growth improved postweaning feed efficiency, especially over a weight constant intervals. Selection for growth rate to weaning or yearling ages, or for an index of yearling weight and muscling score had neither antagonistic or favorable effects on carcass composition or quality grade. Selection for growth in weight alone is not enough to optimize genetic progress in beef cattle. Other traits, such as carcass and meat characteristics should also be considered. Even greater progress in herds could be expected by using the records of performance and expected progeny differences from many herds that share common ancestors.
Technical Abstract: 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) index of yearling weight and muscle score (IXL). Selection response was evaluated for efficiency of gain, growth from birth through market weight and for carcass characteristics. Heritability and genetic correlations among traits were estimated using animal model analyses. Over a time constant interval, selected lines were heavier, gained more weight, consumed more ME, and had more gain/ME than the control. Over a weight constant interval, selected lines required fewer days, consumed less Mcal of ME, had more efficient gains, and required less energy for maintenance than control. Direct and maternal responses were estimated from reciprocal crosses among unselected sires and dams of control (CTL) and selection lines. Most of the genetic response to selection in all three lines was associated with direct genetic effects and the highest proportion was from post-weaning gain. Indirect response of carcass characteristics to selection over the 20 yr were increased weight of carcasses that had more lean meat, produced with less feed per unit of gain. At a constant carcass weight, selected lines had 1.32 to 1.85% more retail product and 1.62 to 2.24% less fat trim and 10/100 to 25/100 degrees less marbling than CTL. At a constant age, heritability of direct and maternal effects and correlations between them were: market wt, 0.36, 0.14, and 0.10; carcass wt, 0.26, 0.15, and 0.03; ribeye area, 0.33, 0.00, and 0.00; marbling, 0.36, 0.07, and -0.35; fat thickness, 0.41, 0.05, and ¿0.18, kidney-pelvic-heart fat percentage, 0.12, 0.08, and -0.76; retail product percentage, 0.46, 0.05, and -0.29; retail product wt, 0.44, 0.08, -0.14; and muscle score, 0.37, 0.14, and -0.54. Selection criteria in all lines improved efficiency of post-weaning gain, increased the amount of salable lean meat on an age or weight constant basis, but carcasses had slightly lower marbling scores.