THE EFFECTIVENESS OF A SINGLE TREATMENT WITH DORAMECTIN OR IVERMECTININ THE CONTROL OF GASTROINTESTINAL NEMATODES IN GRAZING YEARLING STOCKER CATTLE

Four studies were conducted to a similar experimental design in the U. S. to evaluate the effectiveness of doramectin injectable administered to yearling stocker cattle in the control of gastrointestinal nematod iasis over the subsequent grazing period. Studies were conducted in Wi sconsin (WI) and Arkansas (AR) during the summer season. The other two studies were conducted in Georgia (GA) and Mississippi (MS) during th e winter/spring season. Doramectin was compared with both ivermectin i njectable and ivermectin pour-on in the WI study, with ivermectin inje ctable alone in the GA study and with ivermectin pour-on alone in the other two studies. At each study site, an area of permanent pasture pr eviously grazed by parasitized animals was subdivided by fencing into equal pasture units each with its own water supply. A treatment design ation (non-medicated control, doramectin injectable, ivermectin inject able or ivermectin pour-on) was randomly assigned to each pasture unit . Weaned beef calves with confirmed gastrointestinal nematode infectio ns were randomly allotted to a pasture unit and corresponding treatmen t group, Each treatment group consisted of three replicates of seven a nimals per pasture unit (total 21 animals) in the WI study, three repl icates of four or six animals per pasture unit (total 16 animals) in t he AR study, five replicates of six animals per pasture unit (total 30 animals) in the GA study and three replicates of 12 animals per pastu re unit (total 36 animals) in the MS study. Treatments were 1% doramec tin injectable solution, 1% ivermectin injectable solution, 0.5% iverm ectin pour-on solution or non-medicated controls. The injectables were administered at a dose of 1 ml/50 kg body weight (200 mu g doramectin or ivermectin/kg) by subcutaneous injection in the neck. Ivermectin p our-on solution was administered topically at a dose of 1 ml/10 kg bod y weight (500 mu g ivermectin/kg). After receiving their prescribed tr eatment, animals were placed on their designated pasture unit where th ey remained for the entire grazing period (84-140 days). Fecal nematod e egg counts and body weights were monitored at predetermined interval s throughout each study. Doramectin treatment reduced pretreatment egg counts by between 95 and 100% by 21 days post-treatment. Subsequent r ises in egg output from exposure to infective pastures were delayed by two to four weeks resulting in substantial reductions in total egg de position over the grazing period and, therefore, potential pasture rec ontamination. Doramectin treatment resulted in substantial average dai ly weight gain advantages (0.152-0.272 kg) over the grazing season com pared to non-medicated controls. Advantages were statistically signifi cant (P < 0.05) in three of the four studies. There were no significan t differences (P > 0.05) in average daily gain between the doramectin and ivermectin injectable or ivermectin pour-on treated groups. (C) 19 97 Elsevier Science B.V.