SUMMARY

Organic farmers give first priority to prevention of the introduction and multiplication of weeds. The management practices aim at keeping the weed population at a level that does not result in economic loss of the crop cultivation or harm its quality. The goal is not to completely eradicate all weeds, as they also have a role to play on the farm. For example, weeds provide cover that reduces soil erosion. In addition, most of the biological diversity in our crop fields comes from the presence of weeds. They provide habitat for both beneficial biocontrol insects and mycorrhiza fungi. Because weeds offer pollen and nectar they allow biocontrol insects to maintain their populations and, therefore, serve as a valuable instrument in controlling pests.

However, weeds may also alter the environment of the crop in a negative way. Light and air circulation, for example, are reduced between the crop plants. In this darker and more humid environment, diseases find ideal conditions in which to spread and infect plants.

As we have seen many times up to this point, a basic working principle in organic farming is to prevent problems, rather than to cure them. This applies equally to weed management. Good weed management in organic farming includes creating conditions which hinder weeds from growing at the wrong time and in the wrong place and then become a serious problem for the crop cultivation. Competition by weeds doesn’t harm the crop throughout the whole cultivation period in the same way. The most sensitive phase of a crop to weed competition is in its early growth stage. A young plant is vulnerable and depends highly on an ideal nutrient, light, and water supply for a good development. If it has to compete with weeds at this stage, the crop may grow weak, which also makes it more vulnerable to pest and disease infections.

Weed competition later in the cultivation period is less harmful. However, some weeds may cause harvesting problems and reduce the crop yield in that way. Therefore, weeds should not be completely ignored after the most critical growth period of the crop, but in general, they become less important.

These considerations should influence the selection and timing of weed management measures. In general, such measures aim at keeping the weed population at a level which doesn’t result in economic loss of the crop cultivation or harm its quality.

A.Preventative Practices

Several preventive measures may be applied at the same time (Figure 9-2 & 9-4). The importance and effectiveness of the different methods depend to a large extent on the weed species and the environmental conditions. However, some methods are very effective for a wide range of weeds and are therefore regularly used:

Choice of crops and varieties: tall crops and varieties with broader leaves will compete better with late occurring weeds than small varieties with narrow leaves. Some varieties will inhibit and suppress weeds while others will tolerate them. For example, there are witchweed (Striga) resistant maize and cowpea cultivars in many countries of Africa, which give better performance at the same level of weeds where other varieties are more affected (Figure 9-1).

Mulching: the weeds find it difficult to receive enough light to grow and may not be able to pass through the mulch layer. Dry, hardy material, that decomposes slowly, keeps its effect longer than fresh mulch material.
Living green cover: The cover competes successfully against the weeds for light, nutrients, and water and therefore helps to prevent weed growth by winning the competition for resources. The cover crops usually used are legumes, which improves soil fertility on top of suppressing weeds. For example, a ground cover of desmodium (Desmodium uncinatum) or silver leaf, inter-seeded among maize, reduces striga weed and fixes nitrogen at the same time.
Crop rotation: Rotation of crops is the most efficient measure to regulate seed and root weeds. Changing the conditions of the crop interrupts the living conditions of the weeds thus inhibiting their growth and spread.
Intercropping (mixed cropping and under-sowing): Intercropping with fast growing weed-suppressive species (“smoother crop” or “living mulch”) between rows of main crop species is effective in weed control. There are different examples known to work in Africa, for example, sowing cowpeas and egusi melons or pumpkins as intercrops in cassava to reduce weed occurrence.

Sowing time and density: Optimum growing conditions enhance the optimum crop plant development and their ability to compete against weeds. Proper crop spacing will ensure that minimum space is available for the growth of weeds and will minimize competition with weeds. This will effectively restrict weed development. In order to apply this approach, the limiting weeds must be known and the seasons in which they occur. A weed calendar of the area or region, if available, might be of help. It will be used to manage weeds in a targeted fashion with proper timing and effect.
Balanced fertilization: it can support an ideal growth of the crop, which promotes the growth of the crop over the weeds.
Soil cultivation methods can influence the total weed pressure as well as the composition of weeds. For example, minimum-tillage systems can increase the weed pressure. Because weed seeds can germinate between soil cultivation and sowing of the crop, weed cures before sowing can be effective at reducing weed pressure. Use of superficial stubble treatment works against persisting weeds. It should be done under dry weather conditions to allow the weed roots which have been brought to the surface to dry out.
Pasturing: in perennial crops like coffee, mangoes, avocadoes or cocoa, the use of sheep and goats to reduce rampant weed growth is becoming common. In case of cattle, broadleaf weeds tend to predominate due to the cattle preference for grasses. Therefore, it is necessary to rotate with sheep and goats which prefer broadleaves to overcome this selective grazing (Figure 9-3).

Prevent dissemination of weeds by eliminating them before seed dispersal.
Prevent insemination of crops by weeds by avoiding the introduction of weed seeds into the fields through tools or animals; and by using only weed free seed material.

B.Biological Control of Weeds

The soil-borne fungus Fusarium oxysporum (different isolates from Burkina Faso, Mali and Niger) is very effective in reducing the witch weed (Striga hermonthica and S. asiatica) in different cereal crops, leading to yield increases in scientific trials. Other Fusarium species found in Sudan and Ghana are very effective, too (Fusarium nygamai, F. oxysporum and F. solani). This mycoherbicide is on the way to being formulated and registered in different countries in Africa.

Rhizobacteria capable of suppressing germination of witch weed (Striga spp.) seeds or actually destroying the seeds are particularly promising biological control agents since they can be easily and cheaply formulated into seed inoculants. Pseudomonas fluorescens putida isolates significantly inhibited germination of Striga hermonthica seeds. However, currently no biocontrol product is available.

C. Mechanical Control

With the necessary preventive measures, weed density can be reduced, but it will hardly be enough during the critical periods of the crop at the beginning of cultivation. Therefore, mechanical methods remain an important part of weed management (Figure 9-5).

Manual weeding is probably the most important one. As it’s very labour intensive, reducing weed density as much as possible in the field will bring less work later on and should therefore be aimed at. There are different tools to dig, cut and uprooting the weeds; hand, ox-drawn and tractor-drawn tools. Using the right tool can increase work efficiency significantly.

Weeding should be done before the weeds flower and produce seeds.

Flame weeding is another option: Plants are heated briefly to 100°C and higher. This provokes coagulation of the proteins in the leaves and a bursting of their cell walls. Consequently, the weed dries out and dies. Although it is an effective method, it is quite expensive, as it consumes a large amount of fuel gas and needs machinery. It is not effective against root weeds.

REFERENCES

FiBL. 2011. African Organic Agriculture Training Manual – Pest, Disease and Weeds. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick

IFOAM. 2003. Training Manual for Organic Agriculture in the Tropics. Edited by Frank Eyhorn, Marlene Heeb, Gilles Weidmann, p 186-188, http://www.ifoam.bio/

SOURCES

IFOAM

ON TECA

Weed Management in Organic Agriculture: http://teca.fao.org/read/8375

Prevention and Management of Weeds