Brachiaria decumbens (PROSEA)
Brachiaria decumbens Stapf
- Protologue: Flora of Trop. Africa 9: 528 (1919).
- Family: Gramineae
- Chromosome number: 2n= 36
Brachiaria bequaertii Robyns (1932).
- Signal grass (En)
- Malaysia: rumput signal
- Thailand: ya-siknaentonnon, ya-surinam.
Origin and geographic distribution
The origin of signal grass is in East Africa (Uganda, Kenya, Tanzania, Rwanda, Burundi, Zaire). Experimental lines and cultivars have spread very widely to all tropical regions of the world. Thus, signal grass is becoming increasingly naturalized in South-East Asian countries and the Pacific region.
The main use of signal grass is as forage in permanent pastures that are grazed. It has also proved to be useful as grazed ground cover in tree plantations, and is suitable for erosion control.
Signal grass provides a forage that is palatable to ruminants (but not to horses); depending on regrowth age, it is of moderate to high quality. Nitrogen concentrations decrease from 2.7-0.7% and in vitro DM digestibility from 75-50% as age of regrowth increases. When consumed as a pure diet, signal grass can occasionally cause skin-photosensitization combined with hepatic disorders, particularly in sheep and goats, and also in young cattle. There are 200-300 seeds/g.
A low-growing, decumbent, rhizomatous and stoloniferous, apomictic (in cultivation) perennial forming a dense soil cover. Stem prostrate to ascending, 30-150 cm high. Leaf broadly linear to narrowly lanceolate, 5-25 cm × 7-20 mm, sparsely to densely pubescent, bright green. Inflorescence composed of 2-7 racemes on an axis 2-10 cm long; racemes 1-5 cm long, often borne at almost right angles to the axis (hence the name "signal" grass), bearing spikelets usually in 2 rows on a broad, flattened rachis; spikelets elliptic, 4-5 mm long, pubescent at the tip; lower glume _-½ the spikelet length, clasping; upper glume membranous; upper lemma granulose.
B. decumbens intergrades with B. brizantha (A. Rich.) Stapf, and the species are difficult to distinguish. Therefore, in the literature the name B. decumbens probably sometimes actually refers to B. brizantha, and vice versa. The main difference between the cultivars or lines of B. brizantha and B. decumbens is the growth habit: B. decumbens is low-growing and forms a dense cover whereas B. brizantha is tufted with a rather erect growth. The best-known and most widely used cultivar of signal grass is "Basilisk", released in Australia.
Signal grass is well adapted to the humid and sub-humid tropics where it withstands dry seasons of up to 5 months but does not tolerate flooding for more than a couple of days. It grows well on a range of soils, provided they are well drained, including acid, highly Al-saturated soils of medium to low fertility. It is moderately shade tolerant.
Signal grass is easily established by seed, at rates that vary according to seed quality from 2-10 kg/ha. Germination of fresh seed may be poor because of dormancy (hard-seededness), but improves with time (up to one year of storage) or through scarification with sulphuric acid (10-15 minutes). Signal grass can also be propagated vegetatively by stem cuttings that are planted 60-100 cm apart in moist soil, or are broadcast and then disked in. The highly competitive vigour exhibited by signal grass under appropriate conditions helps in suppressing growth of weeds but creates problems with legume persistence. In spite of the competitive vigour of signal grass, persistent associations with legumes are feasible in certain environments. Suitable legumes are mainly stoloniferous species such as Arachis pintoi Krap. & Greg., nom. nud. (Pinto peanut), Desmodium heterophyllum (Willd.) DC. (hetero) and D. heterocarpon (L.) DC. ssp. ovalifolium (Prain) Ohashi, but also the trailing-climbing Centrosema pubescens Benth. (centro) and Pueraria phaseoloides (Roxb.) Benth. (tropical kudzu), the herbaceous to shrubby Stylosanthes guianensis (Aublet) Swartz (stylo), and the shrub/tree Leucaena leucocephala (Lamk) de Wit.
Although it is tolerant of low soil-fertility, it responds well to fertilization with N, P and K. Flowering and seed-setting can occur year-round and high seed yields can be obtained.
The major pest problem of signal grass (which, however, seems to be restricted to tropical America) is spittlebug (mainly the genera Anaeolamia, Deois and Zulia in the Cercopidae family). This pest can very severely affect the productivity and persistence of signal grass.
Signal grass is harvested by grazing animals or is mown in cut-and-carry systems and fed as fresh material. It withstands heavy grazing and trampling and thus can be grazed continuously or rotationally at stocking rates that should be high enough to prevent the grass from becoming old and of poor quality. Depending on soil fertility and moisture regime, DM yields of signal grass range between 6-36 t/ha per year. Animal production varies accordingly (liveweight gain of 250-800 g/steer per day and up to 1200 kg/ha per year).
Genetic resources and breeding
There is no variability within cultivars or lines of signal grass. As their reproduction is apomictic, progenies are essentially clones of the mother plant. However, the major germplasm banks of tropical forage grasses at ATFGRC (CSIRO, Australia) and CIAT (Colombia) hold a range of quite variable, distinct B. decumbens accessions. At CIAT, besides selection within a recently assembled, large B. decumbens/B. brizantha gene pool, a breeding programme is in progress aimed at B. decumbens / B. brizantha cultivars that combine spittlebug resistance, low soil fertility requirements, and high nutritive value.
Until new lines from breeding and selection programmes become available, signal grass cultivars or lines will continue playing a very important role in tropical pasture development in regions where spittlebug is no major constraint, and where other Brachiaria species such as B. brizantha, B. dictyoneura (Fig. & De Not.) Stapf or B. humidicola (Rendle) Schweick. have no comparative advantage.
- Bogdan, A.V., 1977. Tropical pasture and fodder plants. Longman, London. pp. 54-57.
- Loch, D.S., 1977. Brachiaria decumbens (signal grass) - a review with particular reference to Australia. Tropical Grasslands 11: 144-157.
- Oram, R.N., 1990. Register of Australian herbage plant cultivars. CSIRO, Australia. pp. 89-90.
- Skerman, P.J. & Riveros, F., 1990. Tropical grasses. FAO, Rome. pp. 238-242.
- Toutain, B., 1985. Graminées fourragères des genres Brachiaria et Urochloa pour le Pacifique. Revue d'Elevage et de Médecine Vétérinaire de Nouvelle Calédonie 7: 47-56.
R. Schultze-Kraft & J.K. Teitzel