Abstract
It has been reported that differences in horn fly tolerance in beef cattle could be associated with variation in blood enzymes. Thrombin (TH), a major enzyme for hemostasis that plays an important role in the activation of several pro-coagulation factors through the conversion of fibrinogen to fibrin, has been associated with horn fly prevalence on cattle. Although this assumption is reasonable, as fast-coagulating blood tends to complicate the feeding process for flies, there is little meaningful field data that clearly supports this hypothesis. Therefore, the objective of this study was to assess TH levels in relation to horn fly abundance using 360 cows at two University of Georgia farms (Northwest Georgia REC, 34.5°N, 85.0°W and Eatonton Beef Research Unit, 33.4°N, 83.5°W). Horn fly counts were taken twice by trained evaluators during the summer of 2019 (June and August). Additionally, blood samples were collected twice on each cow to quantify serum TH levels using an ELISA assay (MyBioSource, San Diego, CA). The Pearson correlation between serum TH level and horn fly count was -0.19, indicating a decrease in the number of flies with increasing TH in the blood. In order to reduce the impact of the uncertainty associated with the subjective assessment of horn fly abundance and the potential nonlinear relationship between both parameters, TH was categorized into four classes (1=TH >500 ng/mL; 2=250< TH< 500 ng/mL; 3=100< TH< 250 ng/mL; 4=< 100 ng/mL). Average fly counts were 336, 353, 391, and 464 for the four classes, respectively. Between the first (high TH) and fourth (low TH) classes, there was a 38% reduction in fly abundance. Across sire families, fly abundance ranged between 190 and 650. Collectively, these results indicate that selection for higher serum TH levels could be used as a proxy for improving horn fly tolerance in beef cattle.