Recent work (Sinclair et al. 1993) has suggested that synchronising the rate of rumen breakdown and availability of dietary energy and nitrogenous components can increase capture of rumen degradable nitrogen and improve efficiency of microbial protein synthesis. For rumen bacteria to function efficiently they require a supply of both energy and nitrogen sources together i.e a synchronous diet. Nitrogen recycling between the blood and the rumen is a major contributor to the nitrogen economy of the animal and supplies substantial quantities of nitrogen to the rumen bacteria (Kennedy & Milligan 1980). The amount of nitrogen recycled to the rumen is diet dependant. This dependency may not solely be due to the levels of dietary nutrients but also to their rate of breakdown and availability to the rumen bacteria. The predominant mechanism controlling nitrogen recycling between the blood and the rumen appears to be rumen ammonia concentration It has been suggested that the recycling of nitrogen between the blood urea pool and the rumen ammonia pool may be able to compensate for periods of dietary nitrogen deficiency in the rumen due to the asynchrony of availability of the nitrogen and energy components to the rumen bacteria.