GROWTH OF RANGE CATTLE PROTECTED FROM HORN FLIES (Haematobia irritans) BY EAR TAGS IMPREGNATED WITH FENVALERATE

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Abstract
An experiment was conducted on a ranch to evaluate the efficacy of polyvinyl ear tags impregnated with fenvalerate to control horn flies and to improve growth of steers on range. An experimental tag, weighing 10.8 g and containing 7.2% fenvalerate by weight, was attached to each ear of yearling steers when they were moved to summer range in May. The tags were evaluated in combination with zeranol implanted as a growth stimulant during a 115-day grazing period. Release of fenvalerate from the tags kept a treated group of animals completely fly-free. At the end of the experiment, liveweight gain due to slow release of fenvalerate was increased by 18.5% for implanted and 17.7% for unimplanted animals. Increases due to zeranol implants were 7.1% for animals protected and 6.3% for those unprotected from horn flies. No statistical evidence was found for interaction between responses to the impregnated ear tags and growth stimulator implants in increasing the rate of gain in weight of the steers on pasture. The results of this experiment under ranching conditions corresponded closely with basic measurements on the impact of horn flies on productivity of cattle in a model for host-parasite regulation. Key words: Cattle, flies, ear tags, Haematobia irritans, fenvalerate