Relationship between levels of iodine and cyanogenetic glucoside in pasture and the productive performance of sheep

This paper describes an experiment designed to test the goitrogenic effect of white clover (Trifolium repens L) in ewe lambs grown and taken through a complete reproductive cycle. The sheep were setstocked on four pastures made up of perennial ryegrass and short-rotation rye-grass, both alone and together with cyanogenetic white clover. Since these species differ in iodine content when grown on the same soil, there were probably differences in dietary iodine intake of the sheep in the four groups. Half the animals in each group were injected intramuscularly with an iodinated poppy-seed oil to serve as an iodine depot.

At slaughter, 19 months from the commencement of the experiment, thyroid weights indicated a goitrogenic action in the non-injected ewes grazing short-rotation rye-grass and white clover and to a lesser extent in the non-injected ewes grazing perennial rye-grass and white clover. There was evidence of a slight goitrogenic action in noninjected ewes grazing perennial rye-grass. The thyroids of non-injected ewes had similar iodine contents, lying within the range of 0·23–0·29% of the dry weight.

In injected ewes, the total iodine content of the serum was three to four times higher than with non-injected ewes and the iodine content of the thyroids was three times greater. All injected ewes had thyroids of normal weight.

No effects of iodine supplementation on growth, reproduction or wool production were found.

Lambs born to the ewes in the fifteenth month of the experiment were slaughtered when 3–5 months old, the age varying with pasture treatment. Thyroid weights indicated a goitrogenic effect from the clover-containing pastures.