The effects of including monensin in the diet on the dynamics of protozoa in the rumen of sheep were investigated using injections of protozoa labelled with 14 C-choline.The half time of rumen fluid was higher in sheep receiving monensin but neither the pool size nor the composition of protozoa species (mainly smaller entodiniomorph ciliates) was significantly affected.More protozoa passed out of the rumen as liquid outflow rate increased and there was a tendency for the apparent production rate of protozoa to be higher with the resulting increase in washout of protozoa.

The viability of protozoa in whole rumen fluid was assessed by measuring the incorporation of Me-14 C-choline in vitro. The use of the technique as an assay for testing antiprotozoal agents was evaluated with a variety of surfactant detergents which have previously been shown to have anti protozoal activity in vivo. A good correlation was obtained between the potency of these compounds in vitro and in vivo.

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The importance of ruminants in the world agricultural economy needs no emphasis. Greater understanding of the complex digestive processes peculiar to the rumen suggests various ways in which the effectiveness of ruminant nutrition can be substantially increased.