The accurate prediction of the replacement rate of forages by concentrate supplements together with digestive changes induced is essential for the prediction of responses in milk components to changes in feed input. Generalised hypothesis suggest that one feed substitutes for another in proportion to the amounts voluntarily consumed when given alone (Blaxter, 1980) but this is not always the case (Thomas, et al 1981). Little information is available on the metabolic constraints on intake or the effect of supplements on digestive factors that may be limiting intake. The present experiments were designed to study digestive factors that may be limiting the intake of different grass silages in cows given different levels of supplements.

Two formic acid (3 1/tonne) treated silages were prepared from perennial ryegrass harvested in the first week of June (early cut, EC) and at the end of June (late cut, LC). Four diets consisting of the two silages each supplemented with 3 kg (EC3; LC3) or 9 kg (EC9; LC9) dry matter of a dairy concentrate were offered to four lactating Friesian cows in a four by four Latin Square arrangement. The cows were fitted with a large (10cm u.d.) rumen cannula with a simple cannula in the proximal duodenum. The concentrates, which included (kg/tonne) 370 barley, 183 wheat, 139 wheat middlings and 139 maize gluten feed were given in two equal portions/d with silage available ad libitum.