Effect of eu-, hypo- and hyperthyroidism and bst on mohair growth, ADG, and hormone status
R. Puchala , I. Prieto, V. Banskalieva, A. L. Goetsch, M. Lachica, and T. Sahlu
E (Kika) de la Garza Institute for Goat Research, Langston University, Langston, OK.
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 × 3 factorial treatment arrangement (n = 8) to evaluate two levels of recombinant bST (0 and 100 g/d) and three thyroid hormone statuses (euthyroid, hypothyroid, and hyperthyroid) on ADG and mohair growth. The bST was a slow release zinc-based suspension designed to sustain delivery of bST over a 14-d period. Hyperthyroidism was maintained by treatment with thyroxine (T4; 150 g/kg BW/d) and hypothyroidism by treatment with propylthiouracil (6 mg/kg BW/d). The experiment that consisted of a 2-wk pre-treatment period and 8 wk of bST treatment was performed during the summer (July to September). Goats were given ad libitum access to a mixed diet (15.0% CP; 2.34 Mcal/kg ME; DM basis) and were housed in raised, individual indoor stalls under ambient lighting. There was an interaction between bST and thyroid hormone status (P < 0.01) in plasma T4, triiodothyronine (T3) and insulin-like growth factor I (IGF-I). Hypothyroid-bST and hypothyroid-control animals had highest (P < 0.01) concentrations of plasma T4 (38.6 and 38.0 g/dL, respectively) and T3 (406 and 385 ng/dL, respectively). No differences were observed between euthyroid-control, euthyroid-bST, and hypothyroid-bST animals in plasma T4 (11.1, 11.5, and 9.8 g/dL, respectively) or T3 (232, 251, and 226 ng/dL, respectively). The hypothyroid-control group had the lowest (P < 0.01) concentration of both T4 (5.1 g/dL) and T3 (144 ng/dL). Increased plasma IGF-I (P < 0.01) was observed in euthyroid-bST (1,080 ng/mL) and hypothyroid-bST (1,028 ng/mL) animals; however, hyperthyroid-bST animals had plasma IGF-I concentrations similar to those observed in euthyroid-control, hypothyroid-control, and hyperthyroid-control groups (78, 74, 74, and 78 ng/mL, respectively). The ADG for hyperthyroid goats (11.3 g/d) was lower (P < 0.01) than for hypothyroid and euthyroid goats ( 72.4 and 72.6 g/d, respectively). Mohair production was higher (P < 0.01) in hyperthyroid goats (0.13 g/100 cm2/d) than in hypothyroid and euthyroid goats (0.10 and 0.10 g/100 cm2/d, respectively). Strong interactions exist between growth and thyroid hormones in the body; however, manipulation of their status had a very limited effect on productivity of Angora goats.
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