Andropogon gayanus (PROSEA)
Andropogon gayanus Kunth
- Protologue: Enum. Pl. 1: 491 (1833).
- Family: Gramineae
- Chromosome number: 2n= 20 (var.tridentatus(Hochst.) Hack.), 40 (other varieties)
Andropogon bisquamulatus Hochst. (1842), A. squamulatus Hochst. (1842), A. tridentatus Hochst. (1842).
- Gamba grass (En)
- Philippines: batag (Tagalog), batad (Bikol), bukakau (Ilokano)
- Thailand: ya kumba
- Vietnam: hu[n]g th'ao.
Origin and geographic distribution
Gamba grass originates in Africa where it extends throughout the tropical region from 15°N to almost 25°S. Var. bisquamulatus (Hochst.) Hack., which is of particular interest as a forage, only occurs as an important component of fire-climax savanna vegetation north of the equator from Senegal to Sudan in regions with annual rainfall of between 400-1500 mm. Commercial or semi-commercial gamba grass cultivars or lines have spread to most tropical regions of the world, particularly in tropical America.
The main use of gamba grass is as forage in permanent pastures grazed by ruminants. The stems are used for thatching, and when flattened, for coarse matting.
Gamba grass provides a palatable forage when young but feeding value declines rapidly with forage age and a decreasing leaf/stem ratio. Nitrogen concentrations range from 0.5-2%, and in vitro DM digestibilities from 40-55%. Mineral contents are low (0.08-0.14% P and 0.27-0.39% Ca). Seed of commercial quality contains 60-120 viable seeds/g.
An erect, coarse, and tussock-forming, perennial bunch grass with short rhizomes and stems 1-3 m high. Root system consists of fibrous, horizontally growing roots, vertical roots penetrating as deep as 2-3 m and strong cord roots containing starch granules. Leaf-sheath up to 20 cm long, densely hairy at base; leaf-blade linear-lanceolate, up to 100 cm × 4-30 mm, acute, usually narrowed to the prominent midrib at the base to form a pseudopetiole; pubescent on both sides, particularly when young. Inflorescence consists of paired racemes, 4-9 cm long, bearing about 17 spikelet pairs; sessile spikelet up to 8 mm long, with a long (10-30 mm) conspicuous awn; pedicelled spikelet 5-8 mm long, hairy (var. bisquamulatus), with a short (1-10 mm) awn; each spikelet with 2 florets, but only upper floret of sessile spikelet is bisexual and fertile. Caryopsis oblong, plano-convex, 3 mm × 0.75 mm, purple or hyaline (var. bisquamulatus ). Germination of fresh seed is often reduced by dormancy, but under appropriate storage conditions, improves within nine months after harvest. Initial growth is slow but once established, the grass is very competitive and, despite its erect growth habit, it may consequently affect persistence of associated legumes. It has a short-day flowering response with a critical photoperiod of 12-14 hours.
In A. gayanus four botanical varieties are distinguished by morphological and ecological characteristics. Of these, the most important one as a forage is var. bisquamulatus. Most A. gayanus literature refers to this variety, with the possible exception of cultivar "Kent" in northern Australia. All commercial cultivars released in tropical America belong to var. bisquamulatus; they are derived from accession CIAT 621, an introduction from Shika, Nigeria.
Gamba grass is best adapted to the subhumid tropics with an annual rainfall between 800-1500 mm. It still grows well in environments with annual rainfall up to 2500 mm with a marked dry season. Due to its deep rooting, it is remarkably drought-tolerant and survives up to 6 dry months. It is also tolerant of fire. It is adapted to a wide range of soils, but grows best on those that are medium-textured. It has low nutrient requirements and exhibits excellent growth on acid, highly Al-saturated oxisols and ultisols of low fertility, including low available P.
Gamba grass is established by drilling or broadcasting viable pure seed at a rate of 0.75-1.25 kg/ha (equivalent to 10-15 kg/ha of average-quality commercial seed). Seed should not be sown immediately after it is harvested, as dormancy will result in poor establishment. If the seed-bed is not too fine, adequate plant populations can be obtained even by surface-sowing. Fertilization with P and K is generally necessary for establishment on low-fertility soils. Newly sown gamba grass pastures are quite susceptible to weed infestation because of slow initial growth.
Gamba grass can be successfully associated with legumes of creeping, climbing, semi-erect to erect growth habit such as Centrosema spp., Pueraria phaseoloides (Roxb.) Benth. or Stylosanthes spp. The grass requires a grazing management which prevents it from becoming mature and stemmy (reduced quality), and, in mixtures with legumes, from becoming too vigorous and too tall (increased competition affecting legume persistence). Rotational grazing at appropriately high stocking rates is recommended. Although its soil-fertility requirements are low, gamba grass responds to fertilization with N, P, and K.
The major pest problem of gamba grass, which is restricted to tropical America, is leaf-cutting ants of the genera Atta and Acromyrmex. Not only can they completely destroy a pasture in the seedling stage, but they can also severely affect the persistence of an established pasture.
Gamba grass is usually harvested by grazing animals but can be used to make hay. Depending on soil fertility, moisture and cutting regimes, it produces DM yields ranging from 1-18 t/ha per year. Under favourable conditions, however, 12-week DM yields as high as 5-15 t/ha are possible. Gamba grass is a prolific seeder with potential pure seed yields as high as 350 kg/ha. Animal production from pure gamba grass pastures is low in savanna climates (90-120 kg liveweight gain per head per year) because of liveweight losses during the dry season; by introducing a legume, annual weight gains of about 150 kg per head can be achieved.
Genetic resources and breeding
There is considerable variation within the gamba grass cultivars. As the grass reproduces sexually by cross-pollination, there is a continuous recombination of genes. Additional variability within var. bisquamulatus seems to be adequately represented in the major germplasm collections of tropical forage grasses at CIAT (Colombia) and ATFGRC (CSIRO, Australia). Several exploratory breeding efforts aimed at improving gamba grass cultivars are under way in Colombia and Brazil.
Because of its adaptation to acid, low-fertility soils, to burning and drought, and also its high productivity, gamba grass is likely to continue playing an important role in pasture development in low-input systems of the subhumid tropics characterized by acid soils and savanna climate. In more humid environments, other grasses (such as Brachiaria spp.) seem to have a comparative advantage.
- Bogdan, A.V., 1977. Tropical pasture and fodder plants. Longman, London. pp. 34-38.
- Bowden, B.N., 1963, 1964. Studies on Andropogon gayanus Kunth. 1: Empire Journal of Experimental Agriculture 31: 267-273 (1963); 2: Journal of the Linnean Society (Botany) 58: 509-519 (1964).
- Skerman, P.J. & Riveros, F., 1990. Tropical grasses. FAO, Rome. pp. 185-190.
- Toledo, J.M., Vera, R., Lascano, C. & Lenné, J.M. (Editors), 1990. Andropogon gayanus Kunth: a grass for tropical acid soils. CIAT, Cali, Colombia. 381 pp.