ILEIA Newsletter Vol. 11 No. 3 p. 10

Aggressive colonizers work for the farmers: experiences from South-East Asia

Leo Oyen

Weeds undoubtedly cause great damage in agriculture. However, in some situations such plants can be used by farmers to their advantage. They have learned to manage populations of weeds and to strike a balance between competition, soil protection and weeding costs. In this article some examples are given from the book "Auxiliary plants in agriculture and forestry". This book is the eleventh volume in a series called Plant Resources of South-East Asia (PROSEA).

PROSEA is an International Programme in which Indonesia, Malaysia, the Netherlands, Papua New Guinea, the Philippines, Thailand, and Vietnam cooperate. It compiles information on the Plant Resources of South-East Asia. Over 6000 plant species that are used by man will be treated in a 20 volume handbook. Each volume will deal with one commodity group. The texts treating individual species describe aspects of botany, vernacular names, uses, ecology, agronomy or forestry, genetic resources and breeding, and prospects.

Eight volumes have been published sofar. Auxiliary plants in agriculture and forestry is planned for publication in 1996. It will deal with plants with indirect benefits to farmers, like green manure and cover crops, shade trees, trees and shrubs used in agro-forestry and in revegetating degraded land. Fuel wood trees will also be treated in this volume as many of them also play auxiliary roles.

Quick covering

In shifting cultivation with strongly reduced fallow periods and in landuse systems with controlled fallow periods, the rapid establishment of a plant cover to protect the soil is very important. A similar situation occurs in paddy fields. Wherever irrigation water is not permanently available and where management is not sufficiently intensive to grow three crops per year, irrigated rice production always has fallow periods between crops. During these periods, fields are dry but the soil contains enough moisture for upland plants to grow and a fallow vegetation to develop rapidly. When these fields are prepared for the next crop, the mass of vegetation is dug into the soil, where it decomposes quickly.

In his "A dictionary of the economic products of the Malay Peninsula", Burkill lists over 20 species of plants as green manure crops, which are undoubtedly spontaneous pioneer species that rapidly invade fields after the harvest. Very few of these species are nitrogen-fixing legumes, so the field will not benefit from added nitrogen. Some of the species listed by Burkill are well-known noxious weeds and are treated in "The world’s worst weeds" (Holm et al., 1977). It is only with proper management that farmers can make use of them. No soil cover at all is worse than a weedy one, as loss of organic matter from the top soil, erosion and leaching of nutrients would rapidly reduce the capacity of the soil to produce a crop.

In irrigated rice, the most important weed control method is flooding. As long as a rice field is flooded, most other plants cannot grow or only poorly and can be controlled effectively. Here, farmers use certain weed species to their advantage. They are aggressive colonizers and are considered noxious weeds under many circumstances. In this case, however, they are part of the system: their aggressiveness makes that other plants that would be more difficult to control in the rice crop get established.

Perennial species, which are often difficult to remove in a few weedings, are given less chance to get established. Species used this way should be annual plants, not forming storage organs for survival and relying only on seeds for their reproduction. They should decompose quickly and not be a host of diseases or pests of subsequent crops.

In upland crops, these species are also used. If they can be eradicated during land preparation and controlled by weeding, and if they prevent the spread of more persistent weeds, their advantages as green manures and cover crops balance the cost of weeding. Few plants fulfil all these requirements and only through selective weeding can they become dominant "weeds" in a rotation.

Two faces

Ageratum conyzoides"weeds" L., or (billy) goat weed, is one of the best examples of these and among the weeds most commonly seen in warm, humid areas throughout the world. It can flower within two months after germination and produces ample amounts of seed. It quickly forms a dense mat of plants, smothering most other plants. When the plants are ploughed up, a new stand will quickly develop from the large seedbank in the soil.

In some crops, it is a noxious weed. In sugarcane, for example, goat weed gets established late in the rainy season when fields become too wet for cultivation. They remain green longer than the cane and hamper firing the cane before the harvest. The plants also interfere with manual and mechanical weeding. In irrigated rice, however, the aggressive nature and early and profuse production of seed can be used to the farmer’s advantage. The plants do not form long vines, which would interfere with land preparation and decompose quickly releasing nutrients.

There is no evidence of harmful chemicals released during their growth or during decomposition. Although it does not fix nitrogen like leguminous crops, these characteristics make it a good green manure crop. It adds organic matter to the soil, protects the soil from nutrients leaching and reduces soil erosion. In small-scale plantings of upland crops, more intensive weeding is necessary, but with careful management it can mostly be controlled.

Siam weed

A related species Chromolaena odorata (L.) King & Robertson (formerly known as Eupatorium odoratum L.) or Siam weed is a species of the same Compositae family. It is an even more aggressive colonizer, quickly invading open spots in vegetation. Unlike Ageratum conizoides it is a perennial shrub, up to 7 m tall forming a strong and massive taproot (Audru et al., 1988; Slaats, in preparation). It grows into a mass of intermingled stems that eventually sag to the ground, smothering all other vegetation. If left unchecked, it can turn large areas into wasteland.

Rangelands are especially vulnerable to invasion by Siam weed because of the high prevailing light intensity. However, even this plant is used by farmers, also as a green manure crop and fallow species. It has many characteristics of a good cover crop and green manure plant: its extraordinary capacity to cover an area and establish itself, its very high growth rate and its ability to suppress other plants, even the very persistent grass Imperata cylindrica (L.) Beauv. or lalang.

Farmers use it as a cover crop and green manure plant in Laos and Cambodia. Its use also becomes more common in Africa. Shorter rotations in shifting cultivation make traditional bush fallow impossible. A natural fallow of Siam weed is gradually becoming more common in semi-permanent food crop production.

The biomass production of a fallow vegetation dominated by Siam weed at the end of the dry season may range from 15 t/ha after 2 years to 22 t/ha after 4 years, supplying 70-140 kg N, 5–8 kg P, 109-125 kg K, 70-160 kg Ca and 30-50 kg Mg per ha to the succeeding crop. Regrowth of Siam weed after cropping is best when the impact of cultural practices during cropping is modest and the cropping period is limited to 1 or 2 seasons.

Ground covers in plantation crops

In plantation crops like oil palm and rubber, a ground cover consisting of a mixture of legume species is routinely established. Calopogonium mucunoides Desv, or calopo, Centrosema pubescens Benth, or centro, and Pueraria phaseoloides (Roxb.) Benth, or tropical kudzu, have been in common use for decades. Tropical kudzu is the first to establish a dense cover. When the tree crop canopy starts closing, centro and calopo take over. Centro is the most shade tolerant species, but under a closed canopy it also disappears.

In recent years, Calopogonium caeruleum (Benth.) Sauv. is often added to the mixture as it is even more shade tolerant. Under the closed tree cover, some non-legume species will grow and some of them can compete sufficiently with the trees to reduce yields. A number of these species produce chemical substances that affect the growth of other plants. When trees grow older more light reaches the ground, giving these plants an even better position to compete. In this situation, plantation managers and small farmers have to manage the composition of the ground cover.

Establishing a cover of a desired species is not possible because of the shady conditions, but allowing the natural vegetation to grow without any control would lead to the expansion of unwanted species. Here, the soil cover has to be managed by selective weeding, allowing species that do little harm to grow and expand, while eradicating, as far as possible, the more competitive plants (Ochse et al., 1961). Goat weed is used here too, other examples are Alternanthera tenella Colla (A. amoena Voss), Peperomia pellucida Kunth, Oxalis spp., Erechtites valerianaefolia (Wolf) DC.

Leo Oyen, PROSEA Programme, Haarweg 333, 6709 RZ Wageningen, Netherlands.

References
– Audru, J. et al. 1988. L’herbe du Laos: synthèse des connaissances actuelles sur la plante et sur les moyens de lutte. Institut d’Elevage et de Médecine Vétérinaire de Pays Tropicaux. 186 pp.
– Burkill, IH. 1966. A dictionary of the economic products of the Malay Peninsula. 2nd edition. Ministry of Agriculture and Co-operatives, Kuala Lumpur. 2444 pp.
– Ochse, JJ et al. 1961. Tropical and subtropical agriculture. 2 volumes. The Macmillan Company, New york. 1446 pp.
– Slaats, JJP. Forthcoming. Chromolaena odorata. In: FH Ibrahim and LJG van der Maesen (eds). Plant Resources of South-East Asia, Vol. 11: Auxiliary plants in agriculture and forestry." Backhuys Publishers.
– Holm et al. 1977. The world’s worst weeds: distribution and biology. The University Press of Hawaii, Honolulu. 609 pp.
– Zimdahl, RL. 1993. Fundamentals of weed science. Academic Press, San Diego. 450 pp.

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