Subclinical selenium insufficiency. 5. Selenium status and the growth and wool production of sheep supplemented with thyroid hormones

Abstract

Fine wool Merino sheep grazed pastures low in selenium (Se) in 2 experiments, and were supplemented with Se and thyroxine (T4) in experiment 1, and with Se, T4, and tri-iodothyronine (T3) in experiment 2. Both experiments ran for 6 months. In experiment 1, 72 sheep with a mean liveweight of 37 kg were given Se by intraruminal pellet and 6 levels of T4 ranging from 0 to 10 mg sodium L-thyroxine as a subcutaneous injection at 14-day intervals. In experiment 2, 36 sheep with a mean liveweight of 24 kg were given Se by intraruminal pellet, and 3 levels of T4 ranging from 0 to 0.4 mg and 3 levels of T3 ranging from 0 to 0.1 mg by subcutaneous injection at weekly intervals. In both experiments wool production and mean fibre diameter were significantly increased by Se supplementation; the increase in clean fleece production was 7 and 32% in experiments 1 and 2, respectively. In experiment 1, T4 increased greasy fleece weight by up to 8% and reduced yield, mean fibre diameter, and liveweight. In experiment 2, T4 reduced fibre diameter. T3 did not significantly affect wool production, wool characteristics, or liveweight. There was no significant interaction between Se and thyroid hormone supplementation in either experiment. Thyroid hormones were used as effectively for wool production by both Se-deficient and Se-sufficient sheep.