Breeding for strength may create frail cows

Abstract Text: The Purebred Dairy Cattle Association Dairy Cow Unified Scorecard assigns 25% of weight to the dairy strength category, which is described as “a combination of dairyness and strength that supports sustained production and longevity.”  Stronger cattle are assumed by many breeders to have longer herd-life, but data from several studies suggest otherwise.  In a 2003 study, relative culling risks (RCR) were assigned based on strength scores (1 = weak to 50 = strong) from 268,008 Jersey cows.  A score of 25 equated to a RCR of 1.00.  RCR values below or above 1.00 indicated low or high culling risks, respectively.  Scores from 11 to 20 were optimal, whereas cows with a score of 41-45 were at most risk of being culled (RCR = 1.30).  A similar analysis was performed in a study of 891,524 Holstein cows (Caraviello et. al., 2003; 2004), and strength scores higher than 25 were associated with significantly elevated RCR.  These two studies indicate that “stronger” cattle have greater culling risk, which may partially be attributed to unfavorable associations with disease. Genetic correlations of strength with displaced abomasum, ketosis, mastitis, and cystic ovaries indicated that selection for strength would elevate disease risks (Zwald et. al., 2004).  A 2015 analysis (Dechow) compared chest width and body condition score (BCS) in Canadian Holstein genetic evaluations with the Canadian dairy strength trait and U.S. productive life (PL) and daughter pregnancy rate (DPR).  In this study, 527 bulls with Canadian and U.S. daughters were divided into groups based on chest width and BCS.  Sires that transmitted wide chests and low BCS had the highest dairy strength scores (+9.5), followed by wide chest and average BCS sires (+6.5).  These groups had the lowest PL (-2.5 and -1.6 months, respectively) and DPR (-2.75% and -1.6%, respectively).  In contrast, sires that transmitted narrow chests and high BCS scored lowest for dairy strength (-5.5), highest for PL (+1.1 months), and highest for DPR (+0.4%).  Therefore, high scores for strength suggest decreased longevity and reproductive efficiency.  This counterintuitive relationship of dairy strength with health and survival may result from poorly defined measures of strength in linear scoring programs; cows with extremely low BCS have minimal muscle mass but are still considered to have high dairy strength if they have a wide skeletal system.  Dairy producers should take caution when breeding for strength.

Keywords: strength, longevity, genetics