Supplementing narasin or monensin to control coccidiosis in naturally infected calves

Abstract

This experiment compared narasin and monensin as anticoccidials for calves naturally infected with Eimeria spp. Twenty-four weaned, non-castrated male calves (Bos indicus × B. taurus cross) were assigned to this experiment (d -8 to 42). All calves were infected by Eimeria spp. according to oocyst count per gram (OPG) from fecal samples collected on d -8 and -7 (average 1,059 ± 101 oocysts/g). Calves were housed in individual pens, received corn silage, mineral mix and water for ad libitum consumption, in addition to a grain-based supplement at 200 g/head daily. Fecal samples were collected on d -2 and -1 for OPG, and results averaged as initial OPG value. Calves were blocked according to initial OPG into 8 blocks of 3 calves each, ranked within each block according to BW recorded on d -1, and assigned to receive narasin (NAR; 0.8 mg/kg of BW), monensin (MON; 1 mg/kg of BW), or no ionophore (CON; negative control). Ionophores were added to the grain-based supplement, and offered from d 0 to 42 of the experiment. Calf BW was recorded on d 7, 14, 21, 28, 35 and 42. Fecal samples were collected on d 6 and 7, 13 and 14, 20 and 21, 26 and 27, 34 and 35, 41 and 42 for OPG analysis, and results from samples collected in consecutive days were averaged. Aliquoted fecal samples were also pooled across calves from the same treatment and collection days, and used to determine prevalence of individual species of Eimeria. No treatment effects were detected (P ≥ 0.51) for calf BW or growth rate. A treatment × day interaction was detected (P < 0.01) for OPG, as NAR and MON calves had less (P < 0.01) OPG compared with CON calves beginning on d 7. The OPG was also less (P ≤ 0.03) in MON compared with NAR calves on d 7, 14, and 28, but did not differ (P ≥ 0.48) on d 21, 35 and 42. The anticoccidial efficacy of NAR and MON did not differ (P ≥ 0.16) when calculated across all Eimeria spp., or according to prevalence of E. bovis and E. alabamensins. A treatment × day interaction was detected (P = 0.04) for anticoccidial efficacy to E. alabamensis, which was greater (P < 0.01) in MON calves on d 7 and 14 and did not differ (P ≥ 0.40) afterwards. Collectively, both ionophores were similarly effective in controlling coccidiosis upon completion of the 42-d study, although the anticoccidial effects of monensin were noted earlier in the experiment. Nonetheless, these results corroborate narasin as an efficient anticoccidial ionophore for naturally infected calves.