Performance and carcass merit of growing beef steers with chlortetracycline-modified sensitivity to pituitary releasing hormones and fed two dietary protein levels

This paper reports the effects of reduced sensitivity to growth hormone-rel easing hormone and thyrotropin-releasing hormone through feeding a subthera peutic level of chlortetracycline (CTC; 350 mg CTC/d) and two levels of die tary CP (10% and 13% of diet DM) on growth performance and carcass merit ch aracteristics. Thirty-two steers (initial average BW, 286 kg) were adapted to a common 13% CP diet consisting primarily of grass hay, corn, and soybea n meal fed to gain 1.25 kg/d. The steers were assigned to four treatments ( with or without CTC and 10% or 13% dietary CP in a factorial arrangement) a nd fed ad libitum amounts of diet for 91 d. Feed intake was determined dail y and steers were weighed weekly. Steers were killed at the end of the feed ing period for carcass merit determinations. Efficiency of BW gain was grea ter (P < .05) for steers fed the 13% CP diet than for the 10% CP diet and t ended to be less for CTC-steers when the 10% CP diet was fed and greater fo r the CTC-steers when the 13% CP diet was fed (CTC x dietary CP interaction , P < .10). Feeding CTC increased (P < .01) fat over the longissimus muscle and marbling. This study is interpreted to indicate that the sustained eff ect of subtherapeutic feeding of CTC to cattle appears to increase fat depo sition consistent with a reduced growth hormone and thyroid status reported earlier for these same steers. This would tend to increase energy utilizat ion but may not necessarily produce a measurable increase in BW gain.