effect of zinc, copper and chlortetracycline supplementation on fat thickness and longissimus muscle in young holstein bulls

three hundred eighteen young holstein bulls (initial bw = 375 ± 17.5 kg) were randomly allotted to one of six treatments in a completely randomized design for 56 d the treatments of which consisted of 1) the basal diet (control); 2) basal diet plus 30 mg of cu/kg of dm as cuso4; 3) basal diet plus 150 mg of zn/kg of dm as znso4; 4) basal diet plus chlortetracycline (200 mg/animal/d; ctc); 5) basal diet + cu + ctc; and 6) basal diet + zn + ctc within the same dose. effects of treatments on blood metabolites and back fat thickness, rump fat thickness, longissimus muscle area and hip height were evaluated. serum urea nitrogen (p < 0.001) and plasma total protein (p < 0.001) were different among treatments. copper supplementation improved ultrasound index, however, zinc supplementation negatively affected (p < 0.001). as a result, the control of cu and zn levels, as replacers for ctc, in the diet of young bulls can increase longissimus muscle area and can decrease fat thickness.