The process of ensiling whole-crop maize is associated with major changes in composition which appear to be detrimental to nutritive value. Of particular significance is the production of organic acids and the degradation of plant protein to non-protein nitrogen (NPN). Intake may be increased by restricting fermentation, and since less fermentation occurs in drier crops than in wet, immature crops, the ensiling of crops at dry matter contents of 30–35% is recommended. There is also an improvement in net energy as the crop matures, which is related to an increase in starch content and a decrease in cell wall content. The extent to which supplementary NPN can be exploited in diets based on maize silage is limited by low levels of energy intake, and a relatively high content of NPN in the silage as a result of the fermentation process. In the presence of supplementary energy, differences in utilisation between sources of supplementary nitrogen are minimal. Many experiments have confounded energy intake with source of supplementary N, and more research is needed to understand fully the factors affecting rumen microbial protein synthesis in productive cattle.