EFFECT OF PROTEIN AND PROTEIN-FREE ENERGY-INTAKE ON PLASMA-CONCENTRATIONS OF INSULIN-LIKE-GROWTH-FACTOR-I AND THYROID-HORMONES IN PRERUMINANT CALVES

We conducted two experiments with preruminant calves weighing 80 to 24 0 kg to study the long-term nutritional regulation of circulating IGF- I, thyroxine (T-4), and triiodothyronine (T-3) The two experiments wer e similar in design but were performed with calves of two live weight ranges: 80 to 160 kg (Exp. 1) and 160 to 240 kg (Exp. 2). In each expe riment, 36 calves were allocated to one of 12 dietary treatments, whic h consisted of six protein intake levels at each of two energy intake levels. Digestible protein intakes ranged between .90 and 2.72 g N.BW- .75.d(-1) in Exp. 1 and between .54 and 2.22 g N.BW-.75.d(-1) in Exp. 2. The energy intake levels were kept constant on a protein-free basis ; increased energy intakes were realized by increasing energy intake f rom fat and carbohydrates in a fixed ratio. The digestible protein-fre e energy intakes were 663 and 851 kJ.BW-.75.d(-1) in Exp. 1 and 564 an d 752 kJ.BW-.75.d(-1) in Exp. 2. Blood samples were taken 5 to 6 h aft er feeding once every 14 d until the calves reached their target weigh t. In both experiments, plasma IGF-I and T-4 concentrations increased with increasing protein intake (P < .01), but they were unaffected by protein-free energy intake (P > .10). In both experiments, plasma T-3 levels were markedly higher at the high protein-free energy intake lev el (P < .01) and increased slightly with increasing protein intake in Exp. 1 (P = .19) and Exp. 2 (P < .01). Results of these experiments su ggest the involvement of IGF-I in the response of protein deposition t o increased protein intakes and the involvement of the active thyroid hormone Ts in the response of protein deposition to increased protein- free energy intakes.