Efficacy of monensin as a cattle feed additive against the face fly and horn fly

Four groups of eight Hereford and Hereford crossbred steers were monitored for the efficacy of monensin as a feed additive against the face fly and horn fly. Groups were assigned randomly to 3-ha grass-clover pastures. Two groups were supplemented with 1.1 kg ground corn/head daily (untreated), while the remaining groups were given 1.1 kg ground corn containing 100 mg monensin for 14 d and 200 mg monensin/head daily for the remainder of the 112-d trial. Fresh feces were collected from four steers in each group and bioassayed with newly hatched face fly and horn fly larvae. Fly mortality occurred in the larval stages, and the bioassay for feces from cattle given monensin at 200 mg/head daily showed an average of 19.9 and 23.3% fewer (P less than .01) face fly and horn fly pupae, respectively, than were recovered from feces of the untreated cattle. Surviving face fly and horn fly pupae were smaller (P less than .01) than the untreated pupae.