Abstract
Among other regulations, organic cows in the United States cannot receive antibiotics and preserve their organic status, emphasizing the importance of prevention of illness and benefit of high genetic merit for disease resistance. At the same time, data underlying national genetic evaluations primarily come from conventional cows, drawing concern to the possibility of a genotype by environment interaction whereby the value of a genotype varies depending on the environment, and potentially limits the relevance of these evaluations to organic cows. The objectives of this study were to characterize the genetics of and determine the presence of genotype by environment interaction for health traits in US organic dairy cows. Individual cow health data were obtained from 16 US Department of Agriculture certified organic dairy farms from across the United States that used artificial insemination and maintained detailed records. Data were obtained for the following traits: died, lameness, mastitis, metabolic diseases (displaced abomasum, ketosis, and milk fever), reproductive diseases (abortion, metritis, and retained placenta), transition health events (any health event occurring 21 d before or after parturition), and all health events. Binary phenotypes (1 = diseased, 0 = otherwise) for 38,949 lactations on 19,139 Holstein cows were used. Genotypes from 2,347 cows with 87.5% or greater Holstein breed-based representation were incorporated into single-step multitrait threshold animal models that included stayability (1 = completed lactation, 0 = otherwise). Gibbs sampling was used. Genomic predicted transmitting abilities (gPTA) from national genetic evaluations were obtained for sires for production, fitness, health, and conformation traits. We approximated genetic correlations for sires using these gPTA and our estimated breeding values. We also regressed health phenotypes on cow estimated breeding values and sire gPTA. Heritabilities (± standard error) ranged from 0.03 ± 0.01 (reproductive diseases) to 0.11 ± 0.03 (metabolic diseases). Most genetic correlations among health traits were positive, though the genetic correlation between metabolic disease and mastitis was -0.42 ± 0.17. Approximate genetic correlations between disease resistance for our health trait categories and disease resistance for the nationally-evaluated health traits generally carried the expected sign with the strongest correlation for mastitis (0.72 ± 0.084). Regression coefficients carried the expected sign and were mostly different from zero, indicating that evaluations from primarily conventional herd data predicted health on organic farms. In conclusion, use of national evaluations for health traits should afford genetic improvement for health in US organic herds.
Highlights
In the United States, genetic selection for improved disease resistance has been practiced indirectly on a national scale through emphasis on productive life and SCS since 1994 (VanRaden and Klaaskate, 1993; Shook and Schutz, 1994; Wiggans et al, 1994) and for udder conformation traits before that time (Thomas et al, 1984)
We considered the following health traits: metabolic diseases (METB), including displaced abomasum (DA), milk fever, ketosis, and fatty liver; mastitis, consisting only of recorded cases of clinical mastitis; LAME, including any foot disease; any reproductive diseases (REPRO), including abortion, metritis, prolapse, and retained placenta; died with death listed as a health event or culling reason; any event during the transition (TRAN) period defined as 21 d before or after parturition (Drackley, 1999), and all events including those such as “pink eye” not meeting criteria for inclusion into a previous mentioned category or general statements of disease such as “sick.” For TRAN, health events occurring 21 d before parturition were associated with the calving date of the subsequent lactation
Even though we did not use a random sample of herds, we believe that our results can still provide valuable insight into the improvement of the genetics of health among organic Holstein cows because we included cows from across the United States, a variety of herd sizes, and a range of levels of pasture inclusion in the diet
Summary
In the United States, genetic selection for improved disease resistance has been practiced indirectly on a national scale through emphasis on productive life and SCS since 1994 (VanRaden and Klaaskate, 1993; Shook and Schutz, 1994; Wiggans et al, 1994) and for udder conformation traits before that time (Thomas et al, 1984). Direct selection for improved health has become available through large-scale genomic evaluations (Parker Gaddis et al, 2014, 2020; Vukasinovic et al, 2017), genetic parameter estimates for health traits in US Holstein cows existed prior (Zwald et al, 2004a). The prohibition of antibiotics and other USDA organic regulations such as the necessity for cows to graze, the prohibition of synthetic hormones, and the necessity of providing outdoor access, open the possibility of a genotype by environment interaction (G × E) and a subsequent effect on response to selection when contrasting organic to conventional production. There can be a scaling effect, in which case the magnitude of the response to selection differs between environments. Multiple methods are commonly used to determine the presence of G × E including comparing heritabilities and variance components across environments, identifying genetic correlations across environments that differ from unity, and regressing phenotypes from one environment on PTA or EBV established in the second environment (Weigel et al, 1999; Mathur, 2002; Kearney et al, 2004; Shabalina et al, 2021)
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