1. Three experiments have been carried out to study the effect of adding trichloroethyl adipate, a compound known to prevent the production of methane both in vitro and in vivo, to the diet of sheep on fermentation in the rumen and on the growth of lambs. In Experiment 1 the additive was incorporated at various levels for 6 weeks, in Experiment 2 it was given at one of four levels for 12 weeks and in Experiment 3 it was included in the food of growing lambs for 15 weeks.
2. Initially, in Experiments 1 and 2, the additive caused a large reduction in methane production and in the molar proportion of acetic acid in the rumen liquor with a corresponding increase in propionic acid. These effects were much reduced after the first 2 to 3 weeks of administration.
3. The total food intake of the animals receiving the additive in Experiment 3 was 4% less than that of the control animals. This difference was negligible at the start of the experiment but increased throughout the feeding period.
4. On average, the relative live-weight gain and the changes in the weights of carcass, viscera and empty body weight of the animals receiving the additive were also 4% less than those of the control animals.
5. The observed differences in weight gain resulted from the differences in food intake and the additive had no beneficial effect upon lamb growth. This lack of effect on growth was due to rapid adaptation by the rumen fermentation to the presence of the additive.