Introduction
Weeds constitute the single greatest cause of crop lose in Sierra Leone. Because their effect on yields tends to be underestimated, their presence is often tolerated Weeds are defined simply as "plants out of place." This definition may be expanded to include any plants that interfere with the cultivation of desirable plants, that cause economic loss through increased production costs, or that reduce the yield and/or quality of the crop. This chapter describes the nature of weed competition with ice plants, briefly reviews common weed types, and recommends management practices which can help control weeds in rice.
I. Weed Competition
The most obvious action of a weed is competition with desirable plants. Competition denotes a relationship between the same or different species which leads to the flourishing of one at the expense of the other ((or at the expense of both). While weed competition with rice does not normally lead to the death of either species, it almost invariably results in decreased yields in rice. Weeds arc known by everyone but the extent of the problem they pose is not necessarily recognized. Farmers tend to acknowledge the weed problem in their swamps, but the high labor cost of hand weeding (the predominant form of weed control) discourages adequate response.
The precise nature of the competition between weeds and l ice depends on the following influences:
a) relative growth stages of rice and weed,
b) nature of stand establishment (transplanting vs. direct seeding)
c) density of planting
d) rice variety (short variety vs. tall, leafy variety)
e) moisture and nutrient availability
In most irrigated swamp farming systems, rice seedlings are transplanted into puddled soil. This practice gives rice a substantial head start on weeds, and competition is minimal initially. However, competition greatly increases as growth progresses. (This is particularly true in stands of direct-seeded rice, since weeds germinating at the same time compete for light and nutrients with the rice seedlings.)
Weed competition generally takes three forms:
1 Competition for Light Weeds that arc shorter than rice throughout all stages Or growth compete little or not at all with rice for light. However, weeds that are taller can reduce the light available to rice by as much as 50%. Since sunlight pro tides the main source of energy utilized by plants for manufacturing food material ( through photosynthesis), shading by tall weeds can significantly stunt growth and reduce yields.
2. Competition for Water
Under lowland conditions where water is plentiful, competition between rice and weeds for water is minimal. But whenever there is a shortage of water - during the dry season in some swamps, and on the uplands generally - the situation is quite different. If weeds consume a significant portion of the available water supply, processes such as tillering, flowering, and grain filling can be delayed or impeded.
3. Competition for Nutrients
Weeds have a large nutrient requirement. They are voracious feeders and can if left uncontrolled, absorb more of the soil nutrients than the crop. Increases in fertility brought about through fertilization generally are accompanied by increases in weeds which can result in larger yield reductions.
Note: In addition to competing with rice for sunlight, water, and nutrients, weeds pose another problem for the swamp farmer. Many weed species act as alternate hosts for insect pests and disease-causing organisms, and the presence of weeds among the crop or along bunds and swamp peripheries can significantly increase crop losses due to insect and/or disease attack.
II. Common Types of Weeds
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| Common weeds in rice fields |
1. Grasses
Grasses are characterized by long, narrow leaves with parallel veins; the leaves arise alternatingly at colic nodes between hollow internodes (similar to rice, except for the absence in grasses of both auricles and lingule). Grasses are generally the most difficult weeds to control because during the early growth stages they are often mistaken for rice seedlings.
2. Sedges
Sedges are similar to grasses, but they have three ranked leaves, triangular solid stems, and closed leaf sheaths. Most sedges are semi-aquatic preferring moist or flooded soils such as are common in rice paddies, and they are a frequent problem in swamps. Some of the most common sedges have razor-edged leaves and therefore are very difficult to handle.
3. Broad-leaf Weeds
As the name implies, broad-leaf weeds are broad-leaved plants with net veins (as opposed to parallel veins). In lowland ecosystems they can further be distinguished as floating or rooted types. The latter can be a particular problem to the rice farmer, since hand weeding tends to leave behind in the soil root sections which later regenerate. Broad-leaf weeds are commonly found choking water control channels.
4. Algae
Algae comprise a large and diverse group of simple photosynthetic plants. All are aquatic or semi-aquatic. They occur particularly where water is extremely slow-moving or stagnant, and they compete with rice plants chiefly for nutrients.
III. Management for Weed Control
Weeds can be controlled in irrigated rice through the use of management practices, including the application of herbicides. Most weed control in Sierra Leone is affected through non-chemical means, which although labor-intensive can be extremely effective. Management practices for weed control include:
1. Land Preparation
Thorough land preparation can significantly decrease the incidence of weeds in rice by a) destroying all weeds and weed seeds to provide weed-free conditions at the time of planting, and b)providing a good environment for rapid growth of rice seedlings.
2. Water Management
Many weeds cannot germinate and/or grow in flooded soils, making water management an extremely effective tool for controlling weeds --particularly grasses and sedges. When the transplanted seedlings have established themselves (approximately one week after transplanting), completely flood the plot to a depth of 3"-4" to inhibit weed growth. As the rice grows, gradually increase the depth to 6". Important: the soil must be submerged completely and uninterruptedly if flooding is to be effective.
3. Hand Weeding
Hand weeding, although time consuming and tedious, remains the most important weed control method currently in use in Sierra Leone. When the weeds are large enough to be gripped, they are pulled out of the soil and discarded. Smaller weeds can be hand puddled deep into the soil. The earlier hand weeding is carried out the better, since any delay will enable the weeds to absorb nutrients. A common fallacy among swamp farmers is that small weeds do not affect the rice - but they certainly do, as a simple weeding demonstration will show.
4. Hand Hoeing
Hand hoeing is used widely as a method of weed control in several rice-growing countries, particularly where line-planting is practiced. Hand hoeing is faster than hand weeding and works well against creeping perennials.
5. Brushing Bunds/Peripheries
All weeds originate somewhere; many, particularly anual grasses and sedges, reproduce by wind-spread seed. One very effective means of controlling the spread of weed seeds into rice paddies is to brush bunds and swamp peripheries. If brushing is carried out regularly annual weeds will be prevented from completing their reproductive cycles, and the production of weed seeds will be inhibited.
6. Crop Rotation
Since every crop has its own characteristic weeds, continued cultivation of the same crop in one plot allows these weeds to build up. Rotation of swamp rice with an upland crop (during the dry season) may result in reduced infestations of water-tolerant weeds in the subsequent rice crop(s).
7. Herbicides
Herbicides are chemicals used for killing or for inhibiting the growth of plants. In certain instances they can be used effectively and economically for weed control, but as with any pesticide the use of herbicides poses potential health and environmental problems.
Herbicides are divided into three groups, depending or the time of application:
a) Pre-planting (application before crop is planted)
b) Pre-emergence (application after planting, but prior to emergence of weeds)
c) Post-emergence (application after emergence of weeds)
Herbicides also can be distinguished according to the mode of action:
1) Contact herbicides - kill plant tissues at or very close to point of contact.
2) Systemic herbicides - move wither the plant to expert affects away from the point of contact.
3) Selective herbicides - kill or stunt come plant species, with little or no effect on others
Because of a lack of knowledge about the long-term effects of herbicides in swamp farming systems, their use generally should be discouraged in Sierra Leone among local farmers.
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