EFFECTS OF THE BORN FLY (DIPTERA, MUSCIDAE) ON PHYSIOLOGICAL AND NUTRITIONAL RESPONSES OF BEEF STEERS - CONTINUOUS FLY POPULATION-LEVELS

Twenty-four Hereford-Angus crossbred beef steers were exposed to 0, 75 , 150, and 225 horn flies, Haematobia irritans (L.), per head under co ntrolled environmental conditions. The physiological and nutritional i ndices of all steers were recorded during a 14-d infestation period. O verall rectal temperature increased in steers exposed to 225 H. irrita ns per head. Feed intake and nitrogen consumption by steers exposed to 225 H. irritans was loner than uninfested steers during the first 4 d . Overall serum cortisol was lower in steers exposed to 150 or 225 H. irritans per head compared with unexposed steers. Compared with unexpo sed steers, packed cell volume (percentage) was lower in steers expose d to 150 H. irritans on days 4, 7, and 14, lower in steers exposed to 225 H. irritans on days 7 and 14, and lower in steers exposed to 75 H, irritans on day 14. Blood urea nitrogen (BUN) was higher on day 1 in steers exposed to 75 H. irritans per head compared with uninfested ste ers,whereas BUN was lower in 225 H. irritans-exposed steers on days 7, 10, and 14 when compared with uninfested steers. Our data, in conjunc tion with previously published data, suggest that rectal temperature i ncreases in beef steers exposed to >150 H. irritans per bend, and wate r consumption and urine production increases at infestation levels >22 5 H. irritans per head. Our data were unable to resolve H. irritans ef fects on steer heart rate, respiration rate, urinary nitrogen excreted , nitrogen retained, and serum cortisol levels.