The ensiling potential of tropical grasses

Tropical grasses are efficient users of sun energy which results in high biomass production. Their rapid growth, however, predisposes them to high lignification, dead material accumulation and therefore low digestibility when left in the field to mature. When harvested at appropriate maturity tropical grasses may be a reliable feed source for off-season feeding. High humidity in the tropics makes production of good quality hay, without mould growth very difficult. The alternative conserved form is silage, which is less weather dependent. However, tropical grasses are often regarded as poor silage material due to their low water soluble carbohydrate contents, high fibre contents and buffering capacity. This work examined the reliability of these factors as basis for classifying these highly productive tropical grass species as poor silage material. In three studies, the ensiling potential of a number of tropical grass species were examined. In a 5 x 3 x 3 factorial design, Panicum maximum, Chloris gayana and Digitaria decumbens were grown under the glasshouse condition at 17/24C day/night temperature and harvested at 2, 4, 6, 8 and 10 weeks. The grass material were wilted for 2-3 hours in bright sunshine and overnight in overcast days. The wilted herbage were divided into 3 treatments of molasses application 'at 0, 5, and 10%. The wsc ranged between 3-7.5, 4-6, and. 3-7%DM in Panicum, Chloris and Digitaria, respectively. WSC tended to increase with maturity: within the investigated intervals. Buffering capacity varied with species, 'Panicum showed lower resistance to pH change than Digitaria and Chloris (Panicum < Digitaria = Chloris). Addition of molasses reduced fibre contents and improved digestibility of silage in Panicum. Application of molasses at 10% improved digestibility over the control at 4, 6, but not at 8 weeks cut in Chloris. Digitaria had improved digestibility with molasses at all cutting intervals. Digitaria had the lowest NI-13-N among the 3 species. Silage made with Panicum cut at 8 weeks, Chloris at 6 weeks and Digitaria at 6 weeks gave the best silage quality. To investigate whether the inclusion of more stem in the harvested grass will produce higher water soluble carbohydrate (wsc) content in rhizomatous grass species, kikuyu (Pennisetum clandestine) was used in experiment 2. Using a dairy farm paddock at Berry in NSW, a 2 x 2 x 3 design was used. The grass was harvested at 3 and 6 weeks, with inclusion of long and short stem. Smaller amount of molasses, 0, 2.5, and 5% was used in study 2. Extraneous carbohydrate did not improve (P<0.005) the ensiling potential of kikuyu grass, but reduced the fibre content (P