EFFECTS OF SAMPLE PREPARATION ON NITROGEN DEGRADABILITY OF PANGOLA GRASS (DIGITARIA-DECUMBENS) AND TROPICAL TREE LEGUMES

The effects of particle size on the nitrogen degradability of four tropical forages were studied in 1991 in Guadeloupe. Samples of pangola grass (Digitaria decumbens) 22 and 47 days old, Gliricidia sepium and Leucaena leucocephala were prepared as follows: grasses were (1) freshly cut with scissors to 0.5-1.0 cm in length and frozen at -18-degrees-C; or for later study were dried at 80-degrees-C and ground to pass a (2) 0.5 mm, (3) 1.0 mm or (4) 2.0 mm screen. Nitrogen degradability (ND) was determined by placing samples in nylon bags with two different pore sizes (25 and 46 mum) which were then put into the rumen of cows for 2, 4, 8, 16, 24 and 48 h. The kinetics of nitrogen degradation were examined using Orskov's model. Particle losses through the nylon bags, dry matter (DM) and nitrogen solubility of the samples were also measured in vitro. The sample preparation and the type of forage were the main sources of variation in the rapidly degradable nitrogen fraction, the slowly degradable nitrogen fraction, the degradation rate, the potentially degradable nitrogen fraction and ND. Nitrogen degradability was 55.8, 46.7, 64.0 and 46.5% for pangola grass (at 22 and 47 days regrowth), Gliricidia and Leucaena samples, respectively. Mean ND was 47.9, 59.4, 56.1 and 49.6% for freshly cut and 0.5, 1.0 and 2.0 mm dried ground samples, respectively. Sample preparation had little effect on nitrogen solubility. For samples dried and ground at 0.5 and 1.0 mm, particle losses were 18.8 and 15.0% of DM, respectively. The insoluble but degradable fraction was 60.8, 51.9, 42.5 and 42.7% for freshly cut and 0.5, 1.0 and 2.0 mm dried ground samples, respectively. The freshly cut material appeared to be suitable for the estimation of ND in tropical forages.