An effective strategy for milk production is to supplement silage with small amounts of high-protein concentrate based on mixed carbohydrate sources and fish and soyabean meals (Aston et al 1992). Recent work at Trawsgoed by Sargeant and McAllan (1993) using growing steers given high-digestibility grass silage supplemented with rapeseed meal (a source of highly rumen degradable protein) produced similar live-weight gains to those obtained with fish meal. Jacobs and McAllan (1992) concluded that microbial protein yield is greater when the more degradable protein source is given, indicating a better balance in the supply of nitrogen and energy nutrients within the rumen. Lactating cows given grass silage diets have a high requirement for Metabolisable Protein (MP). The source of rumen fermentable carbohydrate may be important when MP supply is derived from degradable dietary protein and hence microbial growth. This study therefore examined the effects of varying the sources of carbohydrate and protein in low levels of concentrate given with high-digestibility grass silage. The main effects observed in the milk production experiment are presented here.