Thrombin as a Potential Proxy to Select for Horn Fly Abundance in Beef Cattle

SIMPLE SUMMARY: Horn flies are a major nuisance to livestock, resulting in reduced productivity and substantial economic losses. Current fly control methods have temporary efficacy, adversely impact the environment, and increase fly resistance to insecticides. Using the animal’s innate resistance and tolerance to horn flies through genetic selection could be an attractive alternative. Unfortunately, measuring horn fly abundance, especially under pasture conditions, is economically and logistically challenging, and alternative approaches are needed. In this study, thrombin, a major player in blood coagulation, was investigated as a potential proxy trait to assess on-animal fly counts. Our genetic analyses showed that the blood thrombin level is negatively correlated with fly count, is moderately heritable, and can be used to select against fly abundance in beef cattle. ABSTRACT: Horn flies are a major nuisance to cattle and induce significant economic losses. Fly abundance varies within and across breeds and genetic analyses have shown sufficient genetic variation to permit selection. A major bottleneck for selecting against horn fly abundance is the complexity of measuring fly attraction phenotypes. Easy-to-measure proxy phenotypes could be an attractive option to indirectly estimate fly abundance. In the current study, thrombin was investigated as a potential proxy to assess fly abundance. Fly counts and blood samples were collected on 355 cows. Pearson correlation between subjective fly count and thrombin was −0.13, indicating a decrease in fly abundance with the increase in thrombin concentration. When thrombin was discretized into three classes, there was a 22% difference in fly count between the top and bottom classes. Heritability estimates of thrombin were 0.38 and 0.39 using linear and threshold models, respectively. The correlation between estimated thrombin breeding values and fly count was around −0.18. There was a noticeably lower density of high fly counts among animals with high breeding values for thrombin. These results indicate that thrombin could be used in combination with other biological factors to estimate fly abundance and as a proxy for selection against fly abundance.