Fresh forages may result in the loss of up to 40% of dietary nitrogen as rumen ammonia, as the microbial population is unable to utilise all the non-protein nitrogen released from rapidly degradable plant proteins. This may be due to the characteristically low levels of readily available energy released in the rumen, primarily as a result of the low levels of water soluble carbohydrate (WSC) in traditional forages. In a previous experiment Lee et al, (1999) found an increase in liveweight gain of pre-weaned lambs grazing Lolium perenne selected for high WSC concentrations. This study examined whether the enhanced performance on high WSC grass may be related to increased supply of nitrogen to the small intestine.