Five Romney wethers, fitted with rumen fistulae, were each fed five pelleted diets high in starch (30–50%) containing 0·72, 1·22, 1·72, 2·47 and 3·72% N respectively. Samples of rumen liquor were incubated with glycerol-tri-[1-14C]oleate or with [1-14C]- linoleic acid. The rates of lipolysis or hydrogenation of these substrates were measured. In addition rumen contents of the sheep when fed the 1·22% N diet were incubated with radioactive substrates plus 0, 100 and 200 mg finely ground casein.

It was found that there was an approximately linear increase in the rate of lipolysis between 0·7 and 2·5% N. The rate of hydrogenation did not increase below l·2% N and tended to decrease above 2·5% N. The potential for hydrogenation of unesterified linoleic acid was calculated to be 4–6 times greater with all diets except that containing 1·2% N. This may be one explanation for the elevated concentration of the rumen unsaturated fatty acids in sheep fed diets containing ca. 1–1·5% N.

It was also established that the addition of finely ground casein to the incubate was without effect.

It was concluded that microbial composition is important in regulating the relative rates of lipolysis and hydrogenation which determined the amount of unsaturated dietary fatty acid present in the rumen.