Tetronasin (ICI 139603) is an ionophore which improves feed efficiency in ruminants (Bartle et al., 1988). Its nutritional effects are at least partly derived from its influence on nitrogen and energy metabolism in the rumen. The mode of action of tetronasin is therefore similar to monensin and although it is more potent than monensin, it has a similar spectrum of antimicrobial activity (Newbold et al., 1988). The aim of the present study was to examine the effects of the inclusion of tetronasin in the diet on the ability of the resultant microbial population in the rumen to degrade casein, peptides and amino acids.

Thirty castrate Friesian & Hereford crosses were fed a diet of barley, ammonia treated straw, soya bean meal, molasses and mineral/vitamin mix (62.5, 20, 12.5, 2.5 and 2.5% of dry matter respectively) ad lib, with the animals being split into three treatment groups, each of ten animals, receiving 0, 6, and 10 mg tetronasin/kg of total diet. Rumen samples were taken by stomach tube from all animals 42 and 84 d after the start of the trial. Samples were strained through two layers of muslin before use.