Author: Daniel U. Thomson, DVM, PhD; R. Spencer Swingle, PhD; Mark Branine, PhD; Steven J. Bartle, PhD; David A. Yates, PhD
Abstract: Steer calves (n = 1690) were used in a 220-day study to evaluate the effects of chlortetracycline (CTC) plus decoquinate (DEQ) on health and performance of feedlot cattle. Treatments were 1) control (CON); 2) CTC + decoquinate early (CTC+DEQ Early), and 3) CTC + decoquinate delayed (CTC+DEQ Delayed). Ten mg of CTC/lb (22 mg/kg) body weight (BW) was fed daily for 5 consecutive days beginning either on day 0 (Early) or day 6 (Delayed); decoquinate was fed at 22.7 mg/100 lb (45.4 kg) BW for 28 days. There were 6 pens per treatment, and 85 to 105 steers per pen. Steers in the CTC+DEQ groups had higher (P < 0.01 to 0.05) dry matter intake and average daily gain than CON steers, but feed efficiency did not differ among treatments. Respiratory morbidity and re-treatment rates were lower (17 vs 23%, and 11 vs 22%, respectively, P < 0.01) for steers in the CTC+DEQ groups than for those in the CON group. Timing of the initial CTC treatment had minimal effect on health and performance; however, feeding CTC for at least 10 days during the receiving period reduced morbidity and improved overall performance.