The effects of iodine and thyroxine administration on the Romney Crossbred ewe

Summary Two trials designed to compare the effects of thyroxine implantations and iodine injected as potassium iodide and Neo‐Hydriol (iodised poppy‐seed oil) on the Romney ewe, showed that none of the treatments had any significant effect on fleece weight or grade. The following observations were made in the second trial: Heart‐rate measurements are highly repeatable, and are proportional to body weight, thus giving a reasonable indication of metabolic rate. An April 60‐mg thyroxine implant resulted in a slight but nonsignificant effect on heart rate, body weight, and wool growth. A 90‐mg implant in November increased heart rate 66%, and decreased body weight 12 lb, but did not stimulate wool growth. The two iodine treatments resulted in a highly significant effect on the thyroid glands, indicated by decreased cell heights and increased iodine contents, whereas thyroxine implantation slightly reduced cell height, but did not affect the iodine content. Several observations point to the possibility of a borderline environmental iodine deficiency, which may account for the slight stimulation of wool growth by thyroxine treatment. A daily dose equivalent to 0.7 mg of iodine injected as potassium iodide is adequate for a sheep's nutritional requirement, while an injection of 1 ml of Neo‐Hydriol provides supplementary iodine for a considerable period.