Effects of bovine somatotropin and thyroid hormone status on hormone levels, body weight gain, and mohair fiber growth of Angora goats

Forty-eight Angora goats (24 wethers and 24 doelings; 5 mo old; 16 +/- 0.5 kg initial BW) were used in an experiment with a 2 x 3 factorial treatment arrangement (n = 8) to evaluate effects of recombinant bovine somatotropin (bST) administration and. thyroid hormone status (euthyroid, hypothyroid, a nd hyperthyroid) on hormone levels, ADG, and mohair fiber growth. The bST w as a slow-release zinc-based suspension, with sustained delivery (100 mug/[ kg BW(.)d]) over a 14-d period. Hyperthyroidism was maintained by daily tre atment with thyroxine (T-4; 150 mug/[kg BW(.)d]), and hypothyroidism was ac hieved by feeding 6 mg/(kg BW(.)d) of propylthiouracil. The experiment was conducted in July to September and consisted of a 2-wk pretreatment period and 8 wk of bST treatment. Goats were given ad libitum access to a diet wit h 15% CP and 2.54 Mcal/kg ME (DM basis). Concentrations of T-4 and T-3 were greatest (P < 0.01) among treatments for hyperthyroidism and hyperthyroid- control (T-4: 38.6 and 38.0 mug/dL; T-3: 406 and 385 ng/dL, respectively); similar among euthyroid-control, euthyroid-bST, and hypothyroid-bST (T-4: 1 1.1, 11.5, and 9.8 mug/dL, respectively; T-3: 232, 252, and 226 ng/dL, resp ectively); and lowest (P < 0.01) for hypothyroid-control (T4: 5.1 mug/dL; T 3: 144 ng/dL). Plasma concentration of insulin-like growth factor-I was gre atest (P < 0.01) for euthyroid-bST (596 ng/mL) and hypothyroid-bST (618 ng/ mL); however, concentration for hyperthyroid-bST was similar to those for e uthyroid-control, hypothyroid-control, and hyperthyroid-control (188, 178, 187, and 191 ng/mL, respectively). Dry matter intake was greatest (P < 0.05 ) for euthyroid-bST (794 g/d), similar among hypothyroid treatments (693 an d 703 g/d for control and bST, respectively) and euthyroid-control (681 g/d ), and lowest for hyperthyroid groups (554 and 518 g/d for control and bST, respectively); ADG for hyperthyroid goats (11 gld) was lower than with hyp othyroidism and euthyroidism (72 and 473 g/d, respectively); and mohair fib er growth was greater (P < 0.01) for hyperthyroidism (0.133 g/[100 cm(2.)d] ) than for hypothyroid and euthyroid goats (0.102 and 0.104 g/[100 cm(2.)d] , respectively). Hyperthyroidism also increased mohair length growth rate b y 15% and decreased fiber diameter by 7.8% (P < 0.01). These results demons trate interactions between growth hormone administration and thyroid hormon e status, although these influences had limited effects on ADG and mohair f iber growth.