Chemical composition and dry matter degradability profiles of forage legumes Cassia rotundifolia cv. Wynn, Lablab purpureus cv. Highworth and Macroptilium atropurpureum cv. Siratro at 8 weeks of growth (pre-anthesis)

Abstract

Three forage legumes were harvested at 8 weeks of growth before flowering (pre-anthesis) and analysed for their chemical constituencies after either sun- (4 days) or over-drying (60°C for 48 h). An experiment was conducted with three rumen fistulated Friesian steers (440 ± 20 kg live mass) to estimate the rumen degradability characteristics of dry matter in the dried forage legumes using the nylon bag technique. The chemical composition of the legumes showed substantial variations. Crude protein (CP) content varied from 225 to 282 g/kg DM while neutral detergent fibre (NDF) ranged from 328 to 425 g/kg DM. The acid detergent fibre (ADF) ranged from 282 to 326 g/kg DM. There were no significant differences in acid detergent lignin (ADL), calcium (Ca) and phosphorus (P) content of the legumes. The drying method had a significant effect on crude protein, neutral detergent fibre, acid detergent fibre and ash content. The degradation constant b varied significantly from 363 to 431 g/kg DM while there were no significant differences in the degradation constants a and c. The potential degradable fraction, a + b, was highest for lablab, 842, followed by cassia, 779, and siratro, 746 g/kg DM. At a passage rate of 2% per hour, the calculated dry matter degradability differed significantly between 669 and 716 g/kg DM. At higher passage rates of 5 and 8% per hour the effective degradability of the forage legumes did not differ significantly. The drying method had no effect on the degradability of dry matter. These results suggest that the three forage legumes at pre-anthesis stage of growth have a high protein content which is highly degradable in the rumen to an extent which may meet the microbial protein requirements for efficient use of low quality roughages. However, their ability to provide nutrients to post-ruminal sites still needs to be assessed.