THE EFFECT OF NITROGEN AND PROTEIN SUPPLEMENTATION ON FEED-INTAKE, GROWTH AND DIGESTIVE FUNCTION OF STEERS WITH DIFFERENT BOS-INDICUS, BOS-TAURUS GENOTYPES WHEN FED A LOW-QUALITY GRASS HAY

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 1), and on the digestive and metabolic functions of 16 of the steers that were fistulated in t he rumen (Study 2). All steers were reared in the one subtropical envi ronment. They consisted of backcross Hereford (H) (H x BH), backcross F-1 (BH x BH) and backcross Brahman (B x BH), all of 50% heterosis, an d a first-cross F-1 of 100% heterosis (B x H). Steers in both studies were confined in pens and offered a basal diet of chaffed pasture hay (digestibility 52 +/- 1.4% and nitrogen [N] content of 6.1 +/- 0.2 g/k g dry matter [DM]) supplemented with minerals only (low N diet; 8 stee rs) or with urea or formaldehyde-treated casein and cottonseed meal (h igh 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 d iet x 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) DMI (per kg liveweight) by 14% in H x BH and by 13% in B x BH steers, but there was no significant effect of the supplement on the DMI of B x H and BH x BH steers. Daily liveweight change was increased (P < 0.0 5) by the supplement from -30 to 250 (s.e.d.+/-40) g/steer and there w as no significant difference between genotypes. N supplementation incr eased (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 BH x BH steers, but the increases in other genotypes were not significant. Rumen ammonia conce ntration(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 B x BH than H x BH steers but did not differ significantly between supplemented a nd unsupplemented steers. The H x BH steers had the lowest density of small entodiniomorph protozoa when N-supplemented, which was less (P < 0.05) than that in B x BH steers which had the greatest density. Supp lementation increased (P < 0.05) N retention but only B x BH steers ha d a positive N balance. These experiments indicated that there are som e physiological differences between genotypes. The B x H genotype with the high hybrid vigour had a high DMI on the low digestibility hay di et without the N supplements and it transferred more urea from the pla sma pool to the gut. The backcross steers (H x BH and B x BH) had low DMI which increased when supplemented. The high content B. indicus ste ers (B x BH) had positive net retentions of N, but the results indicat ed that rumen protected proteins may be more usefully fed to steers wi th a lower B. indicus content.