Thirty six Friesian castrates, initially 17 months of age and 249 kg live weight (LW), were given silage of whole-crop maize (cv. '611C') (M) either alone or supplemented with corn and cob meal at 33 per cent of total dry matter (DM) offered. Each of these two basal diets was given either without nitrogen supplement or with urea (U) or cottonseed cake (CSC) in iso-nitrogenous quantities (1·2 and 8·4 per cent of DM for U and CSC respectively). Voluntary daily intake (g) of total DM per kilogramme LW and daily LW gains (kg/head) for the maize alone treatments were: 14·7, -0·05; 24·4, 0·88; 25·0, l·18; for treatments M, MU and MCSC respectively. For the diets supplemented with corn and cob meal the corresponding values were: 19·0, 0·49; 27·2, 1·28; 25·8, 1·22. The result showed that the nutritional potential of maize silage in the tropics in severely limited by the low nitrogen content of the crop. High rates of LW gain can be achieved when the silage is supplemented with plant protein, but additional energy is required to achieve similar rates of gain with urea.