Cover Image
close this book Agricultural development workers training manual - Volume III Crops
close this folder Chapter III: Technical guidelines and references for the crops training component
Open this folder and view contents Units of measure and conversions
Open this folder and view contents Surveying and interpreting the agricultural environment
View the document A guide to troubleshooting common crop problems
Open this folder and view contents Guidelines for vegetable growing
Open this folder and view contents Introduction to insects and insect control
View the document Some "organic" (non-chemical) pest controls
Open this folder and view contents Using chemical insecticides
Open this folder and view contents Disease control
Open this folder and view contents Nematodes and their control
View the document Weed control

Weed control


1. They compete with the crop for water, sunlight, and nutrients.

2. They harbor insects, and some weeds are hosts for crop diseases (especially viruses).

3. Heavy infestations can seriously interfere with machine harvesting.

4. A few weeds like Striga (witchweed) are parasitic and cause yellowing, wilting, and loss of crop vigor.

Extent of Yield Losses

Numerous trials in the U.S. have shown maize yield losses ranging from 41-86% when weeds weren't controlled. One trial in Kenya yield only 370 kg/ha of maize with no weed control compared to 3000 kg/ha for clean weeded plots. A CIAT trial with beans in Colombia a yield drop of 83% with no weeding.

Of course, all farmers weed their fields to some extent, but most of them could significantly increase their crop yields if they did a more thorough and timely job. A University of Illinois (U.S.) trial showed that Just one pigweed every meter (40") along the row reduced maize yields by 440 kg/ha (390 lbs./acre). By the time weeds are only a few inches tall, they have already affected crop yields.

Relative competitive ability of the reference crops: Slow starters like peanuts, millet,- and sorghum compete poorly with weeds during the first few weeks of growth. Low growing crops like peanuts, bush beans, and bush cowpeas are fairly effective at suppressing further weed growth once they are big enough to fully shade the inter-row spaces. However, tall growing weeds that were not adequately controlled earlier can easily overtake these "short" crops.


Broadleaf vs. Grassy Weeds

Broadleaf weeds have wide (broad or oval shaped) leaves with veins that form a feather-like pattern. Grassy weeds are true grasses and have long, narrow leaves with veins that run up and down in a parallel pattern. A few weeds like nutsedge (nutgrass) belong to neither category but are sedges, all of which have triangular stems. Some chemical herbicides are more effective on broadleaf weeds, while others give better control of grassy types.

How Weeds Reproduce and Spread: Annuals vs. Perennials

Annual weeds live only a year or so and reproduce by seed: they are the most common weeds in may fields. In the tropics, annuals may live more than a year if rainfall is sufficient. Most annuals produce tremendous amounts of seed, some of which may not germinate for years. When you stir the soil with a hoe, harrow, or cultivator to kill weeds, you destroy one crop of them but encourage another by moving more weed seeds closer to the surface where they can sprout.

You can help lower a field's population of annual weeds by controlling them before they produce seed. Permanent eradication of annual weeds isn't possible because most fields contain millions of weed seeds waiting to germinate, and the supply is continually replenished by more seeds brought in by wind, water, animals, animal manure, and by contaminated crop seeds.

Perennial weeds live more than 2 years. Most produce seed but many also propagate by means of creeping above-ground stems (stolons) and creeping underground stems (rhizomes). Johnsongrass, Bermudagrass, quackgrass, and nutsedge are some of the more aggressive perennial weeds. Hoeing or mechanical cultivation may actually aid in spreading them around the field. Most herbicides will kill only the topgrowth, and there is enough food in the underground parts to continue propagation.

Identifying Weeds

Where weeds are being controlled by hoeing or mechanical cultivation, their specific identification is usually not important. However, where chemical weed control is used, you and the farmer should have a good idea of which specific weeds are present, because most herbicides do not give broadspectrum control. The following extension publication is an excellent identification guide and has pictures and descriptions of some 150 common weeds found throughout the tropics and sub-tropics:

"Weeds of the Southern United States, available from the Cooperative Extension Service of Clemson University, Clemson, South Carolina, U.S.A. 29631


Let's look at the pros and cons of the following weed control methods:

1. Burning

2. Mulching

3. Shading (the row crop principle)

4. Hoe and machete cultivation

5. Animal and tractor-drawn cultivation

6. Herbicides

1. Burning

When land is cleared by burning, standing annual weeds are killed along with weed seeds very near the soil surface. However, burning will not kill weed seeds or reproductive underground parts of perennial weeds if they are deeper than 4-5 cm (2"). Furthermore, as the brush is often placed in windrows or piles before burning, much of the soil may not be affected by the fire. Some perennial tropical grasses such as Guinea (Panicum maximum) and speargrass (Imperata cylindrica) are actually stimulated into dense regrowth by burning. On the other hand, weeds may be less of a problem under slash and burn farming, because the soil is usually not turned by plowing or cultivation to bring up more weed seeds.

2. Mulching

Mulching the soil surface with a 10-15 cm (4-6 ) layer of crop residues, dead weeds, or grass can give very effective weed control and provide a number of other benefits:

a. Erosion is greatly reduced on sloping soils.

b. Soil water loss by evaporation and runoff is greatly reduced.

c. In very hot areas, soil temperatures are reduced to a more beneficial level for crop growth.

d. Organic matter is eventually added to the soil.

In trials conducted by IITA in Nigeria, mulching increased maize yields by 23-45% and greatly reduced the heavy labor requirement for hand weeding which accounts for a 50-70% of the hours needed to grow maize in that area.

3. The Row Crop Principle

Arranging crops in rows facilitates hand weeding but also makes possible Technical cultivation (weeding) with tractor or animal-drawn equipment. The rows also permit the crop to exert better shade competition against the weeds.

4. Hoe and Machete Cultivation

Weeding with hand tools is an effective method if sufficient labor is available. However, small farmers who rely on this method commonly fall behind in weeding, and crop yields often suffer.

5. Animal and Tractor-drawn Cultivation

Disk harrows, field cultivators, and spike tooth harrows can provide excellent pre-planting weed control. The spike tooth harrow can also be used to control emerging weeds up until the crop is about 7.5-10 cm (3-4 ) tall without serious damage.

Animal and tractor-drawn row cultivators can be used from the time the crop is a few inches tall; they do a much more rapid job than hand weeding, and a one-row animal drawn model can easily cover 3-4 hectares/day (7.5-10 acres) unless the rows are very narrow. They can be adjusted to throw soil into the row itself to kill small weeds by burying them. If operated too deeply or too close to the row, serious root pruning may result.

6. Herbicides

Herbicides can greatly reduce labor requirements and permit a farmer to grow a larger acreage; they also avoid root pruning damage, soil compaction, and stand reduction which are caused by hand tools or mechanical equipment. In a number of cases, herbicides like Gesaprim (atrazine, see herbicide section) and 2,4-D have proven competitive with hand labor in maize production in the LDC's. IITA is working on improved methods for small farmer application of herbicides such as granular forms and ultra low volume sprayers.

Herbicides do have some very definite disadvantages that must be considered when working with small farmers:

1. They are less reliable than hand tool or mechanical weeding and most require careful and accurate application. This can be achieved by small farmers using backpack sprayers, but it requires some training.

2. Weed control is seldom complete; most herbicides are not broadspectrum, and it's important to analyze the type of local weed species present before choosing a product.

3. Most soil applied herbicides require a certain amount of rain within a week after application in order to move the chemical into the zone of weed seed germination. Others need immediate incorporation into the soil with a disk harrow or rototiller.

4. Improper application may damage the crop.

5. Nearly all herbicides are unsuited for use in intercropping involving cereals and legumes due to crop injury; these products are crop-specific as well as weed-specific.