A comparison of the efficacy of two 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 injectable, ivermectin injectable and ivermectin pour-on against naturally acquired infections of gastrointestinal nematodes during winter-spring grazing in Louisiana. The experiment was initiated on January 11. Treatments administered on Day 0 and again on April 5 (Day 84, 12-week interval) were: Group 1, untreated controls (CONT); Group 2, doramectin (DOR) at 200 micrograms/kg, s.c. injection; Group 3, ivermectin (IVM-INJ) at 200 micrograms/kg, s.c. injection; Group 4, ivermectin pour-on (IVM-PO) at 500 micrograms/kg, back midline. The cattle were weighed and fecal samples (for egg counts and for culture-larval identification) were collected at regular intervals throughout the 161 day experiment. In the interval between Day 0 and 84, arithmetic mean egg counts of the CONT group averaged 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 experiment. From Day 28 to 56, egg counts of the DOR group were consistently lower (P < 0.05) than those of controls and both IVM-treated groups. Egg counts of the DOR group were always lowest after the second treatment, but differed (P < 0.05) only from IVM-PO counts between Days 119 and 140 (35 and 56 days after the second treatment). Ostertagia was the predominant genus, followed by Cooperia in all four groups. Oesophagostomum, Trichostrongylus, Haemonchus, and Bunostomum were other genera identified. Bodyweights of the DOR group remained significantly greater (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 groups were significantly (P < 0.05) greater than CONT, but differences among treated groups was not significant (P > 0.05).