PLASMA CHARACTERISTICS OF BEEF-CATTLE CLASSIFIED AS RESISTANT OR SUSCEPTIBLE TO HORN FLIES

Horn flies inflict economic losses on cattle producers. Therefore, the objective of this study was to identify a serological marker for cows that are innately resistant to horn flies. Plasma characteristics (op tical density, cortisol, and protein pattern) were studied in beef cat tle classified (cow-type) as horn fly resistant or susceptible. Cows ( n = 30) of five breed groups were used in this study. Cows were bled v ia jugular venipuncture in May (the beginning of the horn fly season). Plasma optical density at wavelengths 200 and 464 nm was different (P < .05) between resistant and susceptible cattle. Cow-type also affect ed (P < .05) area percentage for proteins with running molecular weigh ts (M(r)) of 74,000 and 54,000 daltons. Breed group affected (P < .05) optical density at wavelengths 200, 280, 320, and 464 nm and concentr ation of cortisol in plasma. When the ratio of area percentage for pro tein bands 7 and 9 (M(r)) 74,000 and 54,000, respectively) was determi ned, cows could be categorized as horn fly resistant or susceptible. T hese data suggest that a serological marker for horn fly resistant cat tle has been identified; however, the marker will need to be tested on a larger population of cattle.