FEED-INTAKE, NITROGEN-RETENTION AND LIVEWEIGHT OF HEREFORD AND CROSSBRED HEREFORD STEERS OFFERED FORAGE DIETS DIFFERING IN DIGESTIBILITY AND NITROGEN-CONTENT

Forty-eight steers, comprising purebred British cattle (Hereford) and their crosses with Bos indicus (Brahman × Hereford), dairy (Friesian × Hereford) and large European (Simmental × Hereford) sires were used to record feed intake and estimate nitrogen retention of each genotype. In the first experiment, the feed intake of steers fed on 3 forage diets (H, M and L, metabolizable energy 9.3, 8.4 and 7.0 MJ/kg DM and nitrogen 22.4, 14.9 and 5.6 g/kg DM, respectively) was recorded over 80 days, with feed digestibility and N retention estimated over two, 10-day periods, at the beginning and end of the experiment. The second experiment followed immediately on from the first with the feed intake of steers of each genotype being recorded over 14 days, when each steer was given the H-diet. Steers were then taken off pasture for 70 days and the third experiment completed in pens by recording the feed intake of all steers over 14 days when given the H-diet. Steers fed on the H-diet in experiment 1 had higher (P ≤ 0.01) feed intakes and N retentions than steers fed on the M- and L-diets; M-diet steers had significantly higher values than L-diets steers for both these measures. Consequently, H-diet steers were heavier (331±3.6 kg liveweight) than M- (295±5.1 kg) or L-diet (225±3.6 kg, standard error) steers after adjusting for initial differences in liveweight by covariance. During experiment 1, Brahman cross steers ate 22% less (P <0.01) hay on the L-diet (as g organic matter (OM)/kgsup(0).sup(75) liveweight) than Friesian cross steers, and 17% less (P <0.05) than steers of the other genotypes. Brahman cross steers had a 12% lower intake of digestible OM than Friesian cross steers only on the M-diet in the early part of experiment 1, but not the later part. When all steers were given the same diet (H-diet) in experiments 2 and 3, feed intake did not differ between genotypes. There were differences between diets in the excretion and net retention of N by steers, and in the apparent digestibility of N, but differences were not attributed to genotype. It appears that diet was only partially responsible for differences between genotypes in liveweight recorded at grazing. © 1990 CSIRO. All Rights Reserved.