Ammonia concentration and protein synthesis in the rumen

The effect of rumen ammonia concentration on microbial protein synthesis and fermentation was studied in three separate experiments. In each experiment three separate sheep were fed semi‐purified diets [designated A (‘concentrate’), B (‘roughageconcentrate’) or C (‘roughage’)] and infused intra‐ruminally with five graded amounts of urea according to a randomised block design. Rumen ammonia concentrations remained low until the total nitrogen intake was about 10 g day −1 after which rumen ammonia concentration rose rapidly. Rumen and duodenal ammonia concentrations were linearly related ( r =0.90, 0.98 and 0.84 for diets A, B and C, respectively; P < 0.001). Microbial protein production did not increase when rumen ammonia concentrations exceeded 2.8 mM for diet A , 6.0 mM for diet B and 1.6 mM for diet C . Diets A and C produced a propionate‐type fermentation while diet B was characterised by an acetate‐type fermentation. Rumen ammonia concentration had no apparent effect on either concentration or the molar proportions of volatile fatty acids. There were no systematic trends in digestibility in relation to rumen ammonia concentration.