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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., Thallman, R.M., Snelling, W.M., Kuehn, L.A. 2008. Experimental selection for calving ease and postnatal growth in seven cattle populations. II. Phenotypic differences. Journal of Animal Science. 86(9):2103-2114. Interpretive Summary: Calving difficulty in first-calf heifers results in increased costs and reduces calf and cow survival and productivity. Selection can genetically reduce the incidence of calving difficulty but is partially correlated with lower growth. An experiment to reduce calving difficulty in heifers while maintaining or increasing growth to one year of age was run for seven years. A selection line and a control line were formed in each of seven cattle populations. Selection lines calved 3.0 days earlier, had 1.8 days shorter gestation, were 3.0 kg lighter at birth, had fewer calves assisted at birth, and assisted calves had less difficulty. Growth rates of selection lines were 1.7% higher from birth to weaning and were not different between weaning and one year of age. Results show that selection can be used effectively by cattle breeders to reduce heifer calving difficulty and calving assistance while maintaining or increasing yearling weight. Technical Abstract: Effects of selection for 2-yr-old heifer calving ease (reduced calving difficulty score) on phenotypic differences between select and control lines of cattle for birth, growth, yearling hip height, and pelvic measurements were estimated. The selection objective was to decrease calving difficulty score in 2-yr-old heifers while maintaining or increasing yearling weight. The control line objective was to maintain or increase yearling weight by the same amount as the select lines and to maintain or proportionally increase birth weight. Select and control lines were formed in 4 purebred and 3 composite populations. Selection began in 1992 and select (n = 6,926) and control (n = 2,043) line calves were born from 1993 through 1999. Selection was based on EBV calculated from a 4-trait BLUP with observations on 2-yr-old calving difficulty scores, birth weight, weaning weight, and postweaning gain. Calving difficulty was scored 1 (unassisted) to 7 (caesarean). All birth traits in select lines differed significantly from control lines. Averaged over 7 yr, select lines calved 3.0 ± 0.5 d earlier, had 1.8 ± 0.5 d shorter gestations, were 2.99 ± 0.32 kg lighter at birth, had 5.6 ± 1.5% fewer calves assisted at birth (averaged across dam ages), and 2-yr-old heifers had 0.80 ± 0.08 lower calving difficulty score. Select lines averaged 19.8% lower 2-yr-old heifer calving assistance but there was no difference in calving assistance of older cows resulting in a highly significant interaction of selection and dam classification. Preweaning ADG was 1.7 ± .6% greater in select lines. Greater preweaning gain offset lower birth weights in select lines resulting in weaning weights that did not differ (P = 0.71). Postweaning ADG (P = 0.16) and yearling weight (P = 0.41) also did not differ. Higher preweaning ADG in select lines was not maintained after weaning. Select lines hip heights were 0.70 ± 0.21 cm shorter when measured as yearlings. Pelvic height, width, and area of select heifers measured 25 to 74 d after yearling weights were not significantly different. The differences between select and control lines significantly changed over the course of the experiment for some traits. In the final 2 yr of the experiment, select lines had 3.9 kg lower birth weight and 1.3 cm shorter hip heights. Selection can be used effectively to reduce 2-yr-old calving difficulty and calving assistance while maintaining or increasing yearling weight. |
