Effects of bovine somatotropin and thyroid hormone status on hormone levels, body weight gain, and mohair fiber growth of Angora goats
R. Puchala, I. Prieto, V. Banskalieva, A. L. Goetsch, M. Lachica, and T. SahluJournal of Animal Science 79:2913-2919. 2001.
Growth hormone and bovine somatotropin have had variable effects on wool growth, but their impact on mohair growth is unclear. In this experiment 48 Angora goats (24 wethers and 24 doelings; 5 months old; 16 ħħ 0.5 kg initial body weight) were used to evaluate effects of recombinant bovine somatotropin (bST) administration and thyroid hormone status (euthyroid [normal thyroid levels], hypothyroid [low levels], and hyperthyroid [high levels]) on hormone levels, average daily gain, and mohair growth. The bST used was a slow release zinc-based suspension with sustained delivery (100 g/kg body weight per day) over a 14-day period. Hyperthyroidism was maintained by daily treatment with thyroxine (150 g/kg body weight per day), and hypothyroidism was achieved by feeding 6 mg/kg body weight per day of propylthiouracil. Dry matter intake was greatest for euthyroid-bST (794 g/day), and similar among hypothyroid treatments (693 and 703 g/day for control and bST, respectively) and euthyroid-control (681 g/day), and lowest for hyperthyroid groups (554 and 518 g/day for control and bST, respectively). Average daily gain for hyperthyroid goats (11 g/day) was lower than with hypothyroidism and euthyroidism (72 and 73 g/day, respectively). Mohair fiber growth was greater for hyperthyroidism (0.133 g/100 cm2 per day) than for hypothyroid and euthyroid goats (0.102 and 0.104 g/100 cm2 per day, respectively), and hyperthyroidism also decreased fiber diameter by 7.8%. Results demonstrated a complex interaction between exogenous growth hormone administration and thyroid hormone status in Angora goats. Treatment with bST blocked effects of propylthiouracyl, allowing maintenance of normal concentrations of thyroid hormones. Treatment with thyroxine prevented an increase in insulin-like growth factor-I plasma concentration due to bovine somatotropin. Based on these findings, exogenous growth hormone administration does not appear to influence mohair fiber growth, regardless of thyroid hormone status, and, thus, its effects may differ from those on other tissues/organs. The substantial effect of thyroxine administration on mohair fiber growth, despite decreased feed intake and live weight gain, implies a major role of thyroid hormone status. Such research may lead to future means of enhancing mohair fiber growth.
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