Pasture type in relation to live-weight gain, carcass composition, iodine nutrition and some rumen characteristics of sheep III. Effects of treatment with iodine

The mean thyroid weights of ewes grazed on the different pastures (perennial rye-grass (P), perennial rye-grass plus white clover (P + C), short-rotation rye-grass (S), short-rotation rye-grass plus white clover (S + C)) did not differ significantly, but those injected with iodinated poppy-seed oil had lighter (P < 0·01) thyroids than the non-injected ewes grazed on the same pastures. 2. Neither the live weights of the ewes nor their lambing performances were affected significantly by treatment with iodine. 3. Lambs from ewes grazed on pastures containing white clover had thyroid glands significantly heavier than those of ewes grazed on rye-grass pastures. The largest thyroids were from the lambs of ewes not treated with iodine and grazed on S. 4. The iodine contents of the thyroids of the lambs at slaughter, measured as percentage dry weight, did not differ significantly. It seems likely that the differences in thyroid weights were caused by factors operating early in the lives of the lambs. 5. The lambs of ewes injected with iodine grew faster after weaning than those of non-injected ewes on the same pastures. The interpretation that this difference was in fact caused by the iodine treatment of the ewes was not wholly acceptable, because the differences in growth rate were not greatest in those groups of lambs showing greatest differences in thyroid weight.