Iodine metabolism and thyroid hormone relationships in growing sheep fed on kale (Brassica oleracea) and ryegrass (Lolium perenne)–clover (Trifolium repens) fresh-forage diets

Abstract

1. Kale (Brassica oleracea) and ryegrass (Lolium perenne)–clover (Trifolium repens) pasture, grown under similar soil conditions, were grazed in the vegetative state by growing lambs of 23·6 kg initial live weight for 24 weeks. The kale and pasture contained respectively 20 and 270 μg iodine/kg dry matter (DM). The kale also contained 8 μmol total glucosinolates/g DM and 11·5 g S-methyl-L-cysteine sulphoxide (SMCO)/kg DM, both of which were nondetectable in the pasture diet.2. Intramuscular injections of 1 (475 mg) were given during weeks 1 and 12 to half the forty-eight lambs grazing each forage. Wool growth, live-weight gain and cytochrome oxidase (EC 1.9.3.1) activity of biopsied hind-limb muscle were measured at 6-week intervals. Jugular blood samples were removed every 6 weeks for the determination of haematological factors and serum thyroid hormone concentrations. All animals were slaughtered at the end of the experiment and thyroid weight, thyroid I content, and the weight and cytochrome oxidase activity of heart muscle determined.3. Serum concentrations of thyroxine (T4) increased from 20 to 48 nmol/l during the 24 weeks that control lambs grazed ryegrass-clover pasture. I supplementation increased the concentration and total amount of I in the thyroid gland and increased serum T4 concentration, but did not affect any other values measured in the lambs grazing the pasture herbage. Serum concentrations of triiodothyronine (T3) were stable at 2 nmol/l for both groups.4. Control lambs grazing kale for 24 weeks showed marked thyroid enlargement and depletion of thyroid I. By week 6, serum T4 and T3 concentrations had declined to 2–5 nmol/l and 1 nmol/l respectively and were stable at these values for the remainder of the experiment. I supplementation eliminated the thyroid depletion of this element, caused serum T4 concentration to rise and stabilize at 90 nmol/l by week 18, and T3 concentration to stabilize at 2 nmol/l by week 6. From week 6 onwards, wool growth was increased 13% by I supplementation, whereas empty body growth was unaffected.5. Lambs grazing kale developed haemolytic anaemia, due to rumen fermentation of SMCO. I supplementation enabled the lambs to resist the anaemia better by increasing erythrocyte reduced gluthathione (GSH) content. Relative to pasture-fed animals, lambs grazing kale and supplemented with I showed increased heart muscle weight and cytochrome oxidase activity. This represented a compensatory mechanism for the reduced blood oxygen-carrying capacity caused by the anaemia. I-deficient (i.e. control) lambs grazing kale showed reduced cytochrome oxidase activity in both heart and hind-limb muscle.6. The findings are in accord with T3 having a greater biological potency than T4 for regulating rates of body and wool growth. Increases in heart weight, heart cytochrome oxidase content and erythrocyte GSH content of kale-fed lambs were, however, associated with elevation in serum T4 and not T3 concentration.7. I requirements of growing sheep and cattle consuming the pasture diets are discussed. Because of its better relationship to production traits, it is considered that requirements should be based on the ability to maintain T3 rather than T4 concentrations. On this basis, requirements could be met by diets containing 180–270 μg I/kg DM.