Applications of high concentrations of fertiliser nitrogen (N) to silage swards are known to alter the chemical composition of herbage by increasing crude protein (CP), decreasing dry matter (DM) and altering water soluble carbohydrate (WSC) concentrations. Many studies have illustrated that increasing the CP concentrations of concentrates increases silage intake. However, there is little data available to illustrate the effects, if any, of silage CP concentration on forage intake and animal performance. Information is also lacking on the value of silage CP relative to concentrate CP in terms of animal performance. Therefore the present study was initiated, using different levels of formic acid to equalise fermentation, to evaluate the effects of N fertiliser on silage CP concentration, forage intake, digestibility and animal performance of lactating dairy cattle. It also provided an opportunity to compare the value of silage CP to that obtained from concentrate.