On May 22, 2013, 128 stocker calves enrolled in an internal parasite control study were treated according to structured allocations and placed directly onto treatment-specific, randomly allocated, similar pastures (4 animals/2 acres (0.8 hectare) pasture). Treatment groups were saline injection (8 pastures); 0.09 mg/lb (0.2 mg/kg) BW doramectin injection concomitantly with 4.54 mg/lb (10 mg/kg) BW albendazole oral suspension (12 pastures); and 0.45 mg/lb (1 mg/kg) BW eprinomectin extended-release injection (12 pastures). Over the 119-day grazing period, average daily gains ñ SE were 1.21ñ0.07, 1.46ñ0.04, and 1.52ñ0.04 lb (0.55ñ0.03, 0.66ñ0.02, and 0.69ñ0.02 kg)/day for the saline, combination, and extended-release groups, respectively; calves in the combination and extended-release groups outgained the control group (P<0.05). In comparison to the saline-treated group, there were statistically significant (P<0.05) reductions of strongyle fecal egg counts at 14, 30, and 58 days post-treatment (combination group) and at 14 and 30 days post-treatment (extended-release group). No significant differences in adult Haemonchus placei, Ostertagia ostertagi, Cooperia punctata, and Oesophagostomum radiatum populations between treatment groups were seen at necropsy. The combination treatment group had fewer O. ostertagi early fourth-stage larvae than did controls (P<0.05). Based on results of this study, a single treatment of heavily infected and challenged stocker calves in the spring in Arkansas with either extended-release eprinomectin or doramectin+albendazole was not sufficient for adequate parasitic nematode control.

A fecal egg count reduction (FECR) test was conducted on stocker cattle treated 118 days earlier with saline injection (S); 0.09 mg/lb (200 mcg/kg) BW doramectin injection concomitantly with 4.54 mg/lb (10 mg/kg) BW albendazole oral suspension (DA); or 0.45 mg/lb (1 mg/kg) BW eprinomectin extended-release injection (ERE), and then continuously grazed by treatment group in groups of 4 until the start of the current study. In total, there were 8 S, 12 DA, and 12 ERE grazing groups (pastures of origin). Three animals from each pasture were randomly allocated for injection with ivermectin, doramectin or moxidectin, all at the rate of 0.09 mg/lb (200 mcg/kg) BW. Fecal samples were obtained at days -1 and 15, with treatments on day 0. Additionally, coprocultures were conducted on individual fecal samples collected on days -1 and 15. On day -1 of the FECR test, the arithmetic mean strongyle eggs per gram of feces counts (EPG) across all pastures of origin were 412, 570, and 321 for the ivermectin, doramectin, and moxidectin-treated cattle, respectively. Day 15 egg counts in the same order were 177, 335, and 28. Using the above arithmetic means and by standard equation, the overall mean FECR percentages were 57.0, 41.2, and 91.2 for ivermectin, doramectin, and moxidectin, respectively. Lowest FECR test percentages were seen for animals from the ERE pastures. Coproculture larvae populations harvested on both days -1 and 15 were primarily Haemonchus placei and Cooperia punctata, regardless of animal's pasture of origin or FECR test treatment.