Effect of horn fly (Haematobia irritans) control on growth and reproduction of beef heifers.

The influence of horn fly control with commercially available ear tags was studied on beef replacement heifers (n = 670) for growth and reproductive performance. The study was conducted at five sites in Louisiana over 3 yr. Heifers used were yearling replacement females that were exposed to fertile bulls during a limited spring breeding season that coincided with the horn fly season. In mid to late May of each year, heifers were randomly assigned to one of two horn fly treatments: untreated and treated for horn fly control. The trial continued each year until September or October at the end of fly season. Pregnancy status was determined by rectal palpation. Horn fly populations were controlled on the treated heifers at moderate levels (84%). Total weight gain of treated heifers was 14% greater than for untreated heifers. Horn fly treatment had no effect on pregnancy rate (78% and 75% for untreated and treated heifers, respectively). Treatment differences for weight gain were of greater magnitude for heifers failing to conceive than for heifers that became pregnant. Weight gains of nonpregnant treated heifers were 33% greater than for nonpregnant untreated heifers, whereas weight gains of pregnant treated heifers were 8% greater than for pregnant untreated heifers. In conclusion, horn fly control on yearling beef replacement heifers improved weight gain but had no effect on first exposure reproduction.