Abstract 397

Hunter,RA; Siebert,-BD

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

BritishJournal-of-Nutrition. 1985, 53: 3, 637-648; 18 ref.

Six Hereford and 6 Brahman steers were fed to appetite on pangola grass (Digitaria decumbens) and spear grass (Heteropogon contortus) hay alone and supplemented with rumen-degradable nitrogen and sulphur and minerals. The rumen digestion of the 2 feeds was estimated by reference to the disappearance of substrate from nylon bags suspended in the rumen and withdrawn after intervals of 8 to 120 h. The digestion of the unsupplemented pangola grass diet occurred more rapidly in Brahmans than in Herefords and was associated with higher rumen ammonia concentrations in Brahmans (40 against 16 mg/litre). The rumen NH3 concentrations were increased to over 100 mg/litre by supplementation. The digestion rate increased in both breeds after supplementation and the breed difference disappeared. Increases in digestion rate were not achieved above NH3 concentrations of 60 to 80 mg/litre. Spear grass, especially the cell-wall-constituent fraction, was more resistant to digestion than pangola grass. Digestion of the unsupplemented spear grass diet proceeded more rapidly in Brahmans than in Herefords. The digestion rate in Brahmans was similar irrespective of whether the diet was supplemented or not. Supplementation increased digestion rate in Herefords.