In recent years, it has become clear that the amount and type of protein included in diets for dairy cows are crucial factors determining milk production. On many occasions, milk production has been increased by increasing the protein content of the diet. On rather fewer occasions, milk production has been altered by changing the source of dietary protein. However, in all instances, it has been inferred that the milk production response is related to changes in the amounts of total amino acids reaching the small intestine.

No account has previously been taken of the extent to which these changes in milk production, consequent on changing protein inputs, depend on the nature of the basal diet although it is well established that the nature of diets, in particular their influence on the balance of fermentation end-products produced within the rumen, has a big influence on the partition of nutrient use between secretion in milk and deposition in tissue. This experiment was designed to measure the response of dairy cows to protein supplements given with diets designed to maintain widely different patterns of rumen fermentation.