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

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Abstract
Specific physiological and nutritional responses of beef steers to laboratory infestations of stable flies, Stomoxys calcitrans (L.), were measured. Measurements used as 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 25 or 50 stable flies per head had significantly increased heart rates and respiration rates. Rectal temperatures were significantly increased when steers were exposed to 50 stable flies per head. Urine production was significantly increased when steers were exposed to 50 stable flies per head and urinary output of nitrogen was significantly increased when steers were exposed to both levels of stable flies. 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 days 4 and 10 in steers exposed to 25 stable flies per head and were numerically higher on most sampling days at both levels of fly infestation. Higher cortisol levels, vital signs, and steer activity contributed to reduced nitrogen retention in steers exposed to stable flies. No significant difference existed among treatments in levels of other blood components and all values were within ranges considered normal.