Effect of horn fly (Diptera: Muscidae) control during the spring on calf production by fall-calving beef cows.

In the spring of three consecutive years, horn fly, Haematobia irritans (L.), populations on fall-calving cows were reduced 82% for a 12-wk period using pyrethroid-impregnated ear tags. Horn fly populations on control cows exceeded 200 flies per animal for an average of 6 wk. Although the weights of cows and calves and body condition scores of both control and treated groups varied from year to year, a year x treatment effect (P < 0.01) on cow and calf weight gain was observed. However, differences in cow and calf production between treatments were not associated with differences in exposure to horn flies, and no significant effect on weaning weight of fall-born calves was associated with horn fly control. Calf body condition scores were also unaffected by horn fly control.