Four groups of seven dairy cows were given hay plus high-fibre concentrates based on sugar-beet feed (hay-concentrate, 40:60 w/w) or high-starch concentrates based on flaked maize (hay-concentrate, 20:80 w/w), with a crude protein (nitrogen x 6.25) content of either 160 or 220 g/kg dry matter, over weeks 4–18 of lactation. Performance during week 3 of lactation, when all cows were fed on a standard ration, was used as a covariate. For diets with a high-fibre content, higher protein concentrations led to increases in yields of milk and milk fat, with no effect on live-weight loss. For diets with a high-starch content, higher protein concentrations did not affect milk yield or composition but resulted in an increase in live weight rather than a decrease. Diets with a high-starch content led to increased proportions of propionic acid in the rumen and increased concentrations of insulin in the blood. It is concluded that the source of carbohydrate needs to be taken into account when predicting the response to protein supply by dairy cows.
