To satisfy the increased nutrient requirements of high genetic merit dairy cows, diets of increased nutrient density and intake potential are required. This can be achieved through increasing the feed value of the forage component of the diet (grass silage or grazed grass) and by increasing concentrate feed levels. However both approaches tend to reduce the fibre concentration of the diet, and the physical effectiveness of the fibre in terms of its ability to stimulate ruminal processes. The incorporation of small quantities of straw into the diet has been suggested as a means of overcoming this problem and hence improving animal performance. This study was undertaken to examine the production response by dairy cows to straw inclusion in high quality grass silage and zero grazed grass diets.