Abstract 398

Hunter,-RA; Siebert,-BD

CSIRO, Division of Tropical Animal Science, Tropical Cattle Research Centre, Rockhampton, Queensland 4701, Australia.

BritishJournal-of-Nutrition. 1985, 53: 3, 649-656; 15 ref.

In a number of experiments the voluntary intake, by Brahman (Bos indicus) and Hereford (Bos taurus) steers, of 3 low-quality roughages without or with rumen-degradable nitrogen and sulphur and minerals was measured. The chaffed hays were spear grass (Heteropogon contortus) containing N at 6.2 g/kg organic matter and pangola grass (Digitaria decumbens, N 7.9 or 12.0 g/kg). Rumen characteristics relating to rate of fluid outflow from the rumen were also studied. There was no significant difference between breeds in the DM intakes of the unsupplemented diets which varied from 11.3 to 17.8 g/kg bodyweight by Herefords and from 11.8 to 16.1 by Brahmans. Adding N and S to spear grass significantly increased intake by Herefords by 24%, but not by Brahmans. When the lower-N pangola grass was supplemented there was a significant increase in intake by both breeds with the magnitude of the response in Herefords being greater than that in Brahmans, 42 vs. 15%. Herefords ate more of the supplemented spear grass and the lower-N pangola grass than did Brahmans, but there was no breed difference in intake when the higher-N pangola grass was supplemented. Both breeds had an 8% intake response to supplementation, although the increase was only significant in Herefords. The mean retention time of fluid in the rumen on the supplemented pangola grass diet of lower N content was 12.7 h in Brahmans and 17.5 h in Herefords. With the higher-N pangola grass diet without and with supplements, the mean retention times were similar on both diets (10.5 and 9.9 h for Herefords, and 9.5 and 8.1 h for Brahmans). Plasma urea concentrations were higher in Brahmans than in Herefords on all diets. Rumen ammonia concentrations were significantly higher in Brahmans than in Herefords on the unsupplemented lower-N pangola grass diet.