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-releasing hormone and thyrotropin-releasing hormone through feeding a subtherapeutic level of chlortetracycline (CTC; 350 mg CTC/d) and two levels of dietary CP (10% and 13% of diet DM) on growth performance and carcass merit characteristics. Thirty-two steers (initial average BW, 286 kg) were adapted to a common 13% CP diet consisting primarily of grass hay, corn, and soybean 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) and fed ad libitum amounts of diet for 91 d. Feed intake was determined daily and steers were weighed weekly. Steers were killed at the end of the feeding period for carcass merit determinations. Efficiency of BW gain was greater (P < .05) for steers fed the 13% CP diet than for the 10% CP diet and tended to be less for CTC-steers when the 10% CP diet was fed and greater for 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 effect of subtherapeutic feeding of CTC to cattle appears to increase fat deposition consistent with a reduced growth hormone and thyroid status reported earlier for these same steers. This would tend to increase energy utilization but may not necessarily produce a measurable increase in BW gain.