|Thallman, Richard - Mark|
|GOLDEN, BRUCE - Theta Solutions, Llc|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2018
Publication Date: 10/29/2018
Publication URL: http://handle.nal.usda.gov/10113/6471215
Citation: Snelling, W.M., Kuehn, L.A., Thallman, R.M., Bennett, G.L., Golden, B.L. 2018. Genetic correlations among weight and cumulative productivity of crossbred beef cows. Journal of Animal Science. 97:63-77. https://doi.org/10.1093/jas/sky420.
Interpretive Summary: Beef cows in the U.S. have been getting heavier as a correlated response to selection for calf growth. The heavier cows require more feed, are more expensive to maintain, and may have lower fertility and lifetime production. Heritabilities of cow weight and cumulative production, and correlations between weight and production were estimated in this study of about 70,000 weight and calf production records accumulated by over 13,000 cows. Cumulative production traits examined included counts of number of pregnancies, calves born and calves weaned, as well as totals of calves’ ages at weaning and weight weaned at each opportunity a cow had to produce a calf. Cow weight was estimated to have high heritability, and cumulative production traits were moderately heritable. Cumulative calf age and weight weaned were more heritable than the count traits. Correlations between weight and production were generally negative, but weak. Results suggest that culling young cows for reproductive failure may overcome losses in productivity that might occur in response to selection for calf growth. Cow productivity could be improved slightly by selecting bull calves with expected breeding values predicted from their dams’ productivity. More improvement might be realized by selection among sires predicted by cumulative productivity of several daughters. Selection indexes to project cow profitability may be developed using expected breeding values for cow weight and cumulative production.
Technical Abstract: Mature weight of beef cows in the United States has been increasing as a correlated response to selection for calf growth. Unfavorable genetic correlations between cow weight and various measures of female fertility, stayability, and lifetime production suggest declining cow productivity might also be expected as a correlated response to growth selection. National cattle evaluations, however, show increasing trends for stayability and sustained fertility. Random regression (RR) models were employed to further examine genetic relationships among cow weight and productivity, and to assess cumulative productivity traits observed throughout cows’ productive lives. Records were from 13,707 females born in the Germplasm Evaluation (GPE) project and mated to calve first as 2-yr olds. Weights observed at pregnancy testing (n = 65,086) and calf production from each exposure to breeding (n = 71,583) were included in uni- and bivariate RR analyses. Production following each breeding season was added to previous production to obtain cumulative production records for each season that the female was exposed to breeding. Zero was added if the cow failed to produce after a breeding season. The number of pregnancies, calves born and calves weaned, as well as age and weight of weaned calves, were accumulated. Projected age-specific eritability (h2) estimates for cumulative production were low (<0.1) at age 2 but increased with age (0.12 to 0.26 at age 6; 0.32 to 0.48 at age 10). Estimated h2 for cow weight were high, fluctuating between 0.6 and 0.7 from ages 2 through 10. Genetic correlations (rg) were positive among all ages within each trait. Between ages 3 and 9, estimated rg were negative between cumulative weaning productivity and cow weight. The orrelations were usually weak enough (<-0.2) that small correlated declines from following yearling weight trends might be overcome by culling females after their first reproductive failure. More noticeable increases might be realized by selection among sires with EBV based on productivity of several daughters. The RR EBV for cow weight and cumulative weight weaned represent major sources of variation in cow costs and income, and can be incorporated into economic selection indexes to project differences in cow profitability and value at any age. The RR approach utilizes all available records, enabling later productivity to be projected from observations on young cows.