Two experiments were conducted to evaluate the performance of stocker calves
grazing high-endophyte fescue (68 to 70%) and offered oxytetracycline in a free-choice
mineral supplement. In Experiment 1, 24 yearling crossbred heifers were utilized in a
112-day grazing study beginning on June 18. Performance of both control calves and
those receiving oxytetracycline was extremely poor, making data interpretation
difficult. Average oxytetracycline consumption was 327 mg per head, daily.
Approximately 8296 of the total grazing time occurred during the daylight hours (6arn
- l0pm) for both treatment groups. Heifers offered the medicated mineral mix grazed
20 minutes more than heifers offered the control mineral mix. Hair scores and rectal
temperatures were unaffected by oxytetracycline supplementation.
In Experiment 2, fifty-three crossbred steers were utilized in an 83-day
grazing study beginning on September 15. Steers were randomly divided into groups
of 25 and 28. One group received a control mineral mixture, whereas the other
received a similar mineral mix containing oxytetracycline, 250 mg per ounce. Steers
fed oxytetracycline gained faster (P<.05) and tended to have lower (P=.12) rectal
temperatures. Therefore, response of cattle to consumption of oxytetracycline from a
mineral mixture may be variable. Heifers grazing endophyte infected tall fescue
(Experiment 1) may not have gained sufficiently to support treatment differences.
However, when cattle were gaining weight (Experiment 2), treatment differences were
observed.
