A field trial was performed under commercial feedlot conditions in western Canada to compare the efficacy of a new formulation of long-acting
oxytetracycline (LA 30) to a standard long-acting
oxytetracycline formulation (LA 20) and
florfenicol (FLOR) for the treatment of undifferentiated
fever (UF) in calves that received metaphylactic
tilmicosin upon arrival at the feed-lot. Seven hundred and ninety-seven recently weaned, auction market derived, crossbred, beef calves suffering from UF were allocated to 1 of 3 experimental groups as follows: LA 30, which received intramuscular long-acting
oxytetracycline (300 mg/mL formulation) at the rate of 30 mg/kg
body weight (BW) at the time of allocation; LA 20, which received intramuscular long-acting
oxytetracycline (200 mg/mL formulation) at the rate of 20 mg/kg BW at the time of allocation; or FLOR, which received intramuscular
florfenicol administered at the rate of 20 mg/kg BW at the time of allocation and again 48 hours later. Two hundred and sixty-six animals were allocated to the LA 30 group, 265 animals were allocated to the LA 20 group, and 266 animals were allocated to the FLOR group. The relative efficacy of the LA 30 group, as compared with the LA 20 and FLOR groups, was assessed by comparing relapse, chronicity, wastage, and mortality rates. The overall mortality (RR = 0.50) rate in the LA 30 group was significantly (P < 0.05) lower than in the LA 20 group. However, the overall chronicity (RR = 2.56) and overall wastage (RR = 6.97) rates of the LA 30 group were significantly (P < 0.05) higher than in the LA 20 group. There were no significant (P > or = 0.05) differences in UF relapse rates or cause specific mortality rates between the LA 30 and LA 20 groups. In the economic analysis, there was an advantage of $28.59 CDN per animal in the LA 30 group compared with the LA 20 group. The overall chronicity (RR = 2.25) and overall wastage (RR = 2.80) rates of the LA 30 group were significantly (P < 0.05) higher than the FLOR group. There were no significant (P > or = 0.05) differences in UF relapse rates, overall mortality rates, or cause specific mortality rates between the LA 30 and FLOR groups. In the economic analysis, there was an advantage of $12.90 CDN per animal in the LA 30 group compared with the FLOR group. In summary, the results of this study indicate that it is more cost-effective to use a new formulation of long-acting
oxytetracycline (300 mg/mL formulation administered at a rate of 30 mg/kg BW) than a standard long-acting
oxytetracycline formulation (200 mg/mL formulation administered at a rate of 20 mg/kg BW) or
florfenicol for the treatment of UF in feedlot calves that have previously received metaphylactic
tilmicosin upon arrival at the feedlot.