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

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Abstract
Specific physiological and nutritional responses of beef steers to laboratory infestations of horn flies, Haematoma irritans (L.), were measured. Indicators of physiological stress were changes in heart rate, respiration rate, and rectal temperature. Blood cholinesterase, cortisol, creatine phosphokinase, glucose, hemoglobin, lactic dehydrogenase, packed cell volume, total protein, urea nitrogen, and white cell count were measured to assess metabolic changes in response to fly infestation. Dry matter and nitrogen digestibilities and nitrogen retention were used as indices of feed utilization. Beef steers exposed to 100 or 500 horn flies per head had significantly increased heart rates, respiration rates, and rectal temperatures. In addition, water consumption and urine production were significantly increased after exposure of steers to 500 horn flies per head. Urinary nitrogen excretion was significantly increased when steers were exposed to 500 horn flies per head and the value for steers exposed to 100 horn flies was intermediate between these and control steers. Dry matter digestibility 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 1 in steers exposed to 500 horn flies per head, and were numerically higher on most sampling days at both levels of fly infestation. Higher cortisol levels, combined with increased body temperatures and steer activity, contributed to reduced nitrogen retention in steers exposed to horn flies. No significant difference existed among treatments in levels of other blood components measured and all values were within ranges considered normal.