Physiological and Nutritional Response of Beef Steers to Combined Infestations of Horn Fly and Stable Fly (Diptera: Muscidae)

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Abstract
Specific physiological and nutritional responses of beef steers to combined infestations of horn flies, Haematobia irritans (L.), and stable flies, Stomoxys calcitrans (L.), were measured in the laboratory. Measurements used as indicators of physiological stress were changes in heart rate, respiration rate, and rectal temperature. Various blood components were measured to assess metabolic changes in response to fly infestation. Dry matter and nitrogen digestibility and nitrogen retention were used as indices of feed utilization. Beef steers exposed to 100 horn flies plus 25 stable flies or 500 horn flies plus 50 stable flies per head had significantly increased heart rates, respiration rates, and rectal temperatures. Dry matter and nitrogen digestibility did not differ significantly among treatments, but nitrogen retention was significantly reduced in both fly-infested groups. Blood cortisol concentrations were significantly increased on day 12 in steers exposed to 100 horn flies plus 25 stable flies per head and on days 1, 8, and 10 in steers exposed to 500 horn flies plus 50 stable flies per head. Higher cortisol levels, vital signs, and steer activity contributed to reduced nitrogen retention in steers exposed to horn flies plus stable flies. No significant difference existed among treatments in levels of other blood components, and all values were within ranges considered normal.