A COMPARISON OF THE EFFICACY OF 2 TREATMENTS OF DORAMECTIN INJECTABLE, IVERMECTIN INJECTABLE AND IVERMECTIN POUR-ON AGAINST NATURALLY ACQUIRED GASTROINTESTINAL NEMATODE INFECTIONS OF CATTLE DURING A WINTER-SPRING GRAZING SEASON

Four groups of 18 crossbred beef steer calves (three replicates of six per group) were used to compare persistent efficacy of doramectin inj ectable, ivermectin injectable and ivermectin pour-on against naturall y acquired infections of gastrointestinal nematodes during winter-spri ng grazing in Louisiana. The experiment was initiated on January 11. T reatments administered on Day 0 and again on April 5 (Day 84, 12-week interval) were: Group 1, untreated controls (CONT); Group 2, doramecti n (DOR) at 200 mu g/kg, SC injection; Group 3, ivermectin (IVM-INJ) at 200 mu g/kg, SC injection; Group 4, ivermectin pour-on (IVM-PO) at 50 0 mu g/kg, back midline. The cattle were weighed and feed samples (for egg counts and for culture-larval identification) were collected at r egular intervals throughout the 161 day experiment. In the interval be tween Day 0 and 84, arithmetic mean egg counts of the CONT group avera ged about 890 eggs per gram, but then decreased markedly between Days 119 and 126, and remained at a lower plane for the remainder of the ex periment. From Day 28 to 56, egg counts of the DOR group were consiste ntly lower (P < 0.05) than those of controls and both NM-treated group s. Egg counts of the DOR group were always lowest after the second tre atment, but differed (P < 0.05) only from IVM-PO counts between Days 1 19 and 140 (35 and 56 days after the second treatment). Ostertagia was the predominant genus, followed by Cooperia in all four groups. Oesop hagostomum, Trichostrongylus, Haemonchus, and Bunostomum were other ge nera identified. Bodyweights of the DOR group remained significantly g reater (P < 0.05) than those of all other groups from Day 112 through the end of the experiment. Total gains for the CONT, DOR, IVM-INJ, and IVM-PO groups were 96, 159, 147, and 150 kg, respectively; treated gr oups were significantly (P < 0.05) greater than CONT, but differences among treated groups was not significant (P > 0.05). (C) 1997 Elsevier Science B.V.