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Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/14/2008 Publication Date: 9/1/2008 Citation: Bennett, G.L. 2008. Experimental selection for calving ease and postnatal growth in seven cattle populations. I. Changes in estimated breeding values. Journal of Animal Science. 86(9):2093-2102. Interpretive Summary: Calving difficulty in two-year-old heifers can result in significant costs and lost revenue for cattle producers. Calving difficulty increases labor and veterinary costs. Calves experiencing calving difficulty are more likely to die and the heifers are more likely to be culled as they age. Select and control lines were formed in each of seven breeds. The goal of the select lines was to maintain or increase yearling weight and to improve calving ease. The goal of the control lines was to make the same amount of genetic change in yearling weight as in the select lines and proportional genetic changes in birth weight. Estimated breeding values were calculated from data including heifer calving difficulty score, birth weight, weaning weight, and weight gain from weaning to yearling age. The calculated breeding values included information from all relatives and all traits. Selection was successful in improving calving ease while maintaining or increasing yearling weights. These results demonstrate that producers can use calculated breeding values for difficult traits and selection goals with confidence. Technical Abstract: Experimental selection was used to create beef cattle lines with similar yearling weight direct and weaning weight maternal EBV and either lower 2-yr-old heifer calving difficulty score EBV or proportionally average birth weight EBV. Select (low heifer calving difficulty score EBV) and control lines were created within 4 purebred and 3 composite populations. Select (n = 6,926) and control (n = 2,043) line calves were born from 1993 through 1999 and selection began with the 1992 mating. High replacement rates resulted in 2,188 births to select line and 598 births to control line heifers. Data used to calculate EBV came from these populations and 15 yr of data preceding the experiment. Calving difficulty was scored from 1 (no assistance) to 7 (caesarean). Calving difficulty scores from all twins, malpresentations, and cows 3-yr-old and older were eliminated. Except for the first year when single trait BLUP was used, a multiple trait BLUP was used to calculate direct and maternal EBV for calving difficulty score, birth weight, and weaning weight, and direct EBV for postweaning gain. Sires (n = 498) were selected from those born in both the preceding populations and the select and control lines. In purebred populations, some industry sires (n = 88) were introduced based on their EPD. Tests of mean select and control line EBV differences of calves born in the final 2 yr were were based on among population variation. Select line direct EBV were 1.06 lower for heifer calving difficulty score (P < 0.001) and 3.5 kg lower (P < 0.001) for birth weight than control. Average differences for other EBV were small and not significant. Yearling weight EBV was intentionally increased in both select and control lines of purebred populations. Angus, Hereford, Charolais, and Gelbvieh yearling weight EBV in control lines increased 32.4, 27.2, 21.0, and 10.5 kg, respectively, from 1991 and 1992 to 1998 and 1999 compared to an average increase of 2.7 kg in composite populations. Birth weight direct EBV in purebred control lines increased by approximately 8% of yearling weight EBV increases. Selection based on multiple trait BLUP was able to create lines differing in calving difficulty score and birth weight EBV but not in weaning weight and postweaning gain EBV. Differences between lines should be useful for evaluating BLUP and other traits and identifying potential limitations of genetically decreasing calving difficulty score and birth weight. |
