Effects of an oral antibiotic on fertility in range beef cattle

Abstract

In a series of trials in Florida, crossbred beef females were fed different chlortetracycline (CTC) regimes relative to weaning and breeding to determine effects on fertility. In trial 1, alternately sorted crossbred yearling heifers were either given CTC at 1.1 mg/kg/day for 30 days before natural breeding (T, n=225) or no CTC (C, n=225). More pregnancies occurred in the treated group compared with controls (71.5% and 56.8% respectively). In experiment 2, crossbred yearling heifers were placed in 3 comparable groups after 3 months postweaning CTC at 1.1 mg/kg/day; A (n=106) controls, B (n=97) CTC at 22 mg/kg/day for 5 days prebreeding, then 1.1 mg/kg/day for the first 30 days of breeding, C (n=101) CTC at 1.1 mg/kg/day for the first 30 days of breeding. Groups did not differ in numbers of pregnancies (overall 71.6%) although days pregnant tended (P<0.10) to favour both CTC groups. In experiment 3, crossbred beef cows were placed in 2 comparable groups (n=149-214) at each of 3 locations; CTC at 1.1 mg/kg/day for 30 days at the start of breeding, and no CTC. Treated females achieved more pregnancies than non-treated (83.3% and 78.4% respectively) and achieved them earlier in the breeding season. In all trials, body condition score (BCS) at breeding was a significant factor in pregnancy outcome. Overall, feeding of CTC to both heifers and cows at the start of breeding appeared to benefit fertility parameters. Both BCS and prior feeding of CTC influenced results.