Control of Cattle Grubs with Organic Phosphorus Compounds Administered in Feed1

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Abstract
Three organic phosphorus compounds were evaluated as feed additives for cattle grub control using 200 Hereford steers in a 119-day feedlot trial. Coumaphos was fed at 3 rates, fenthion at 2 rates and Marctin® (N-hydroxy-naphthalimide diethyl phosphate) at 1 rate. Each treatment group consisted of 5 pens of 5 cattle each, and the control group of 10 pens of 5 animals. Grubs identified as the common cattle grub, Hypoderma lineatum (de Villers), and the northern cattle grub, H. bovis (L.), were collected during the trial. Fenthion in feed for 6 days (1 and 2.5 mg/kg per day) gave 96% and 99% control of cattle grubs. Daily administration of coumaphos at 7.5 mg/kg for 3days and 5 mg/kg for 6 days gave 92% and 87% grub control. Two treatments, 33 ppm coumaphos in feed for 119 day and 20 mg/kg Maretin daily for 6 days, were completely ineffective against grubs. Cattle that were fed fenthion (1 and 2.5 mg/kg) and coumaphos (5 mg/kg) daily for 6 days, or coumaphos (7.5 mg/kg) daily for 3 days, had significantly (P<0.05) fewer grubs and significantly (P<0.05) more were grub-free than in similar groups of untreated cattle or those that received coumaphos at 33 ppm in feed daily for 119 days or Maretin at 20 mg/kg per day for 6 days. Based on the ratio of grub-free to total number of animals per group, the 2.5 mg/kg fenthion treatment was significantly (P<0.05) better than all other treatments except 7.5 mg/kg coumaphos. Medicated feed containing fenthion, Maretin, or 33 ppm coumaphos was readily consumed and did not produce anorexia or signs of toxicity. Anorexia and signs of organic phosphorus compound toxicity were observed in animals treated with coumaphos at 7.5 mg/kg daily for 3 days, while mild anorexia and soft feces resulted from treatment with 5 mg/kg coumaphos daily for 6 days. The weight gain and feed conversion by steers in the various treatment groups did not differ significantly from each other or from the control group.