Abstract Dry matter yields of two energy grass crop species grown on low productivity arable crop land in South East England were significantly increased by application of nitrogen fertiliser (switchgrass, yield increase of 43% with 100 kg ha −1 N; reed canary grass, yield increase of 87% with 150 kg ha −1 N). Furthermore, application of N at these concentrations had significant effects on cell wall composition. Growth year and time of harvest also had significant effects on composition. The effect of N application was greater in reed canary grass which showed small but significant increases in neutral detergent fibre and cellulose content (increases of 3.5% and 7.5% respectively compared with plots not treated with N). Both species were significantly increased in Klason lignin content (an increase of 10.0% in reed canary grass and 4.4% in switchgrass compared with plots not treated with N). Analysis of acid detergent lignin also showed lignin content increased with nitrogen application rate. We discuss the implications of these changes on feedstock utility and the degree to which increases in yield may be offset by decreased feedstock utility.