Intake and digestibility of two tropical grasses fed untreated, urea-treated or supplemented with nitrogen.

Intake and in vivo digestibility of Pennisetum pedicellatum Trin (Pp, trial 1) and of Scheonefeldia gracilis (Sg, trials 2 and 3) were compared. The grasses were harvested either at boot stage and sun-cured, or late at seed dissemination stage, and distributed in one of three modes: untreated, after treatment with 6% urea, or supplemented with nitrogen in amounts equivalent to those of the urea treatment. Amounts refused by the animals were 19 +/- 4%, 21 +/- 4% and 40%, respectively in trials 1, 2 and 3. Urea treatment improved feed intake from 41.1 g per kg metabolic body weight (kg MEW) for untreated Sg to 67.2 g/kg MBW for treated Sg; it also improved chemical component digestibility and increased the nutritive value of treated forage to the level of early harvested forage (tables III, IV). The presence of inflorescences limits the intake of Sg harvested early (34.7 g/kg MEW) and the 2% urea treatment restores it (52.5 g/kg MEW) (table V). The increase in straw dry matter intake and in digestibility of straw organic matter following urea treatment may be explained by the chemical changes related to the breaking of the lignin-polysaccharide links wich favors the release of digestible components. The supplementation of late harvested forage with cottonseed meal increases the intake and digestibility of organic matter (tables III, IV). This may be due to greater stimulation of ruminal flora as a result of increased availability of energy and nitrogen-providing: nutrients. With Pp, supplementation is more efficient than chemical treatment, in contrast with Sg. This is probably linked with differing charasteristics between the two forages. Results indicate that the utilisation of tropical grass straws by sheep may be significantly improved by urea treatment, or by supplementation. The decision of a farmer with regard to which technique is more appropriate will depend on the grass species, and on the specific socio-economic environment.