Two studies were undertaken at Grafton, NSW, to determine the effects of supplementing a subtropical hay diet with a mixture of non-protein nitrogen (urea) and protein (protected casein), on the feed intake and growth of 20 steers of four genotypes (Study I), and on the digestive and metabolic functions of 16 of the steers that were fistulated in the rumen (Study 2). All steers were reared in the one subtropical environment. They consisted of backcross Hereford (H) (H × BH), backcross F1 (BH × BH) and backcross Brahman (B × BH), all of 50% heterosis, and a first-cross F1 of 100% heterosis (BxH). Steers in both studies were confined in pens and offered a basal diet of chaffed pasture hay (digestibility 52f 1.4% and nitrogen [N] content of 6.1 ± 0.2 g/kg dry matter [DM]) supplemented with minerals only (low N diet; 8 steers) or with urea or formaldehyde-treated casein and cottonseed meal (high N diet; 12 steers) for 49 days. There were eight steers, for each of the two diets in Study 2, which were fed for 30 days. There was a diet × genotype interaction (P < 0-05) in the daily DM intake (DMI) of hay by steers in Study 1. The mixed N supplement increased (P < 0.05) DM1 (per kg liveweight) by 14% in HxBH and by 13% in BxBH steers, but there was no significant effect of the supplement on the DM1 of BxH and BHxBH steers. Daily liveweight change was increased (P < 0.05) by the supplement from-30 to 250 (s.e.d ± 40) g/steer and there was no significant difference between genotypes. N supplementation increased (P < 0.05) rumen volume (63 to 87 ± 7.6 L) and fluid residence time (491 to 822 ± 76.9 min) (P < 0.05) in BHxBH steers, but the increases in other genotypes were not significant. Rumen ammonia concentration (30 to 61 ± 3 7 mg N/L) and plasma urea concentration (56 to 94 ± 6-0 g N/L) were increased (P < 0.05) by supplementation. Total protozoa density in rumen fluid was greater (P < 0.05) in BxBH than HxBH steers but did not differ significantly between supplemented and unsupplemented steers. The HxBH steers had the lowest density of small entodiniomorph protozoa when N-supplemented, which was less (P < 0.05) than that in BxBH steers which had the greatest density. Supplementation increased (P < 0.05) N retention but only B × BH steers had a positive N balance. These experiments indicated that there are some physiological differences between genotypes. The BxH genotype with the high hybrid vigour had a high DM1 on the low digestibility hay diet without the N supplements and it transferred more urea from the plasma pool to the gut. The backcross steers (HxBH and BxBH) had low DM1 which increased when supplemented. The high content B. indicus steers (BxBH) had positive net retentions of N, but the results indicated that rumen protected proteins may be more usefully fed to steers with a lower B. indicus content.