Submitted to: Journal of Federation of American Societies for Experimental Biology
Publication Type: Abstract Only
Publication Acceptance Date: 4/3/1998
Publication Date: N/A
Technical Abstract: Thirty-two beef steers (285 ñ 3 kg BW) were used to determine the effects of chlortetracycline (CTC) and dietary protein level on circulating insulin-like growth factor-1 (IGF-1) concentration, growth, visceral organ mass, and carcass merit. Steers were allotted randomly by weight to a factorial arrangement of dietary treatments consisting of either 10 or 13% CP diets top-dressed with a corn meal carrier (500 g/d) containing either 0 or 350 mg CTC. Feed intake was recorded daily, steers were weighed weekly, and jugular blood samples for IGF-1 analysis were collected on d 0, 7, 14, 28, 56, and 84. After 84 d, steers were slaughtered and visceral organ mass and carcass merit determined. Dry matter intake and rate of gain were not affected by treatment during either period 1 (0 to 28 d) or 2 (28 to 84 d), however gain efficiency was greater (P < .05) in steers fed the 13% CP diet than those fed the 10% CP diet during period 2. In period 1, plasma IGF-1 concentrations were increased by CTC (P < .05) in steers fed the 10% CP diet, but were not affected by CTC in steers fed the 13% CP diet (CTC CP, P < .05). Plasma IGF-1 concentrations were similar across treatments in period 2. As a percent of empty body weight, rumen and abomasum weights and small intestinal (SI) length were greater (P < .05) in steers fed the 13% CP diet than those fed the 10% CP diet, while CTC decreased (P < .05) SI weight. Feeding CTC increased (P < .01) fat over the longissimus muscle and marbling score (P < .08). Feeding sub-therapeutic levels of CTC appears to enhance IGF-1 status during short-term adaptation when dietary protein is limited. Furthermore, prolonged feeding of CTC alters composition of gain by increasing fat deposition.