The effects of L-thyroxine on sheep and wool production
Authors: Labban F. M. 
Published: March 2009

1. Three experiments were carried out in this study to investigate the effects of L-thyroxine on sheep and wool production. In the first experiment L-thyroxine was orally administered to ewe lambs from weaning time for a period of 48 weeks. In the second experiment, ten uniform yearling ewes were selected and were randomly distributed into two groups of five; one group was fed daily 7 mg. L-thyroxine per ewe for 7 weeks from 1 April 1954. In the third experiment L-thryoxine was fed as a supplement to lactating ewes over the second to the sixteenth week of lactation at the rate of 7 mg. daily per ewe.

2. Differences in both body weight and rectal temperature between the experimental and control groups were not statistically significant. The treatment had also no effect on the body weight of the ewes during lactation.

3. Wool growth, as measured by the weight of keratine production per unit of surface area, was increased significantly during the period of thyroid treatment. This increase in wool production was due to an increase in the length of the wool fibres but not to any observable increase in the diameter of the fibres. This effect is different from that produced by a high plane of nutrition where extra wool production is characterized by increase in both length and diameter.

4. It was observed that the treated ewes exhibited hypersexual activities characteristic of male behaviour, i.e. jumping and mounting other animals.

5. There was a significant increase in the rate of milk secretion over the period. 2–8 weeks. This increase was reflected in an improved live-weight increase of the lambs which were suckling these treated ewes. The difference in milk yield between treated and untreated ewes was not maintained over the nine to ten weeks period of the lactation.

6. As a result of these studies it has been established that the normal milking performance of Border Leicester × Scotch Blackface ewes is high as compared with that of other breeds or crosses reported in the literature. The shape of their lactation curve suggests, however, that it is possible that after the eighth week of lactation the ewes may become competitors with their lambs for available grazing. Expressed in another way ewes become less efficient as converters of their food into milk for lambs, for which milk is becoming proportionately a less important component of the diet. This finding raises the question of the possibilities of providing creep grazing or supplementary feeding for lambs so that their growth potential is fully realized. Alternatively, there may, under farm conditions, be grounds for weaning lambs at about 12 weeks so that they may have preferential treatment in regard to available grazing rather than to adopt customary weaning at about 16–18 weeks of age.

7. It has been established that a close relationship exists between the milk yield of dams and the growth, rate of their lambs under normal grazing conditions. Because of this strong relationship a regression formula has been tentatively advanced which may permit workers to select ewes for milk yield on the basis of the live-weight gain of their lambs.