In recent years the use of complete diets has increased markedly in the UK. Although figures are difficult to obtain, it is estimated that approximately 20% of the National Herd is now fed complete diets. This represents 400,000 cows and the number is steadily increasing . Complete diets are often associated with rations where the forage component contains a significant proportion of maize silage, and where sodium hydroxide treated wheat (Soda Grain) is often included. Although it is widely believed, within the farming community, that the inclusion of Soda Grain (SG) results in a marked increase in milk protein content, there is limited scientific evidence to support this view . At the same time there has been a concomitant decrease in the inclusion of molassed sugar beet feed (MSBF) as this by-product is associated with increased milk fat content. Recent feeding studies at the Centre for Dairy Research have also established that the use of distillery by-products will reduce milk fat content. The aim of the current study was to determine the effect on forage intake, milk yield and composition of lactating dairy cows fed a complete diet based on grass and maize silage, when SG was replaced by either MSBF and cracked wheat (CW), or MSBF and distillery by-products, which were used to replace a conventional protein supplement of soya bean and rape seed meal.