Simply put, weeds are uncultivated plants. And like most things with a good measure of spontaneity, there is an unending conversation around their significance. Because weeds do not strike as violently as, say, insects, there is a tendency to underestimate their economic importance. Weeds normally compete with crops for nutrients, space, light, water, air and other growth resources, thus reducing crop yields. Most weed species reproduce rapidly and exhibit seed dormancy, which enhances their long-term survival and makes long-term control nearly impossible. They can serve as hosts for plant diseases and can harbour pests such as insects. Some species can produce chemical substances toxic to crops, animals and humans.
Weed competition is usually most serious when the crop is young, especially during the first third of its life cycle. Tropical crops have been studied at experimental farms in order to define the weed-free period required to prevent yield reduction: maize – 56 days; rice – 42 days; sorghum – 35 days; cassava – 84 days; cowpea – 40 days.
Crops like cassava, yam, cocoyam, Irish potato have a slow rate of initial growth and this makes them poor weed competitors at their early stages of growth. For example, in cassava production, the most damaging effect on yield is weed competition during canopy and tuber formation (third month after planting) and less from the 4th month until harvest. Yield loss attributable to weed infestation can be as much as 100% in crops such as rice.
Methods of Weed control
There are a number of ways weeds can be controlled on crop fields. Weeds may also be controlled through the manipulation of plant population, spatial arrangement and ground cover management. Egusi melon is traditionally intercropped with cassava or yams to control weeds for the tuber crop. However, for large-scale crop cultivation, monocropping is usually practised to ease farm mechanization and, under such conditions, these methods are often unsuitable for weed control.
Physical/mechanical removal of weeds by hand or simple farm tools such as cutlass and hoe or even more sophisticated farm machines is also practised. Smallholder farmers can spend up to 50–70% of their total labour time hand-weeding. This method is unsustainable even for some smallholder farmers due to high cost of labour and possible unavailability of sufficient labourers when required. The use of materials such as black plastic films or other mulching materials such as wood shavings and plant residues is also known to be effective in weed control as it keeps sunlight away and hinders weed seeds from germinating. This is more popular in the management of horticultural crops such as pineapple, cucumber, watermelon, etc. which are cultivated on relatively smaller areas of land.
Use Of Weeder on crop fields
Chemical weed control, which involves the spraying of herbicides, has grown to be perhaps the most popular method of weed control. Herbicides are generally classified by mode of action as pre-emergent and post-emergent (contact and systemic). Pre-emergent herbicides target weed seeds and kill them before they can germinate. Pre-emergent herbicides applied to the soil before the crop and weeds emerge from the ground remain active in controlling germinating weed until the critical period of weed competition has passed1. Atrazine is a popular active ingredient in many pre-emergent herbicides. Popular brands include Sun-Atrazine, Atraforce, Merlin Total, Sencor plus, etc.
The use of plastic film and the use of crop residue as mulching materials
Post-emergent herbicides are applied after weed growth. They can either be contact or systemic herbicides. Contact herbicides are fast-acting, killing off plant tissues in contact with the herbicide, but may be ineffective against weed species that can regrow from undamaged roots. Paraquat is an active ingredient in contact herbicides. Common brands include Para force, Reliquat, Paracot, etc. Systemic herbicides are more methodical. They are assimilated into the plant, disrupting crucial growth processes which eventually kill the plants. Glyphosate is an active ingredient found in most systemic herbicides. Popular brands include Hwura Wara, Sunphosate, Force Up, etc.
Weeds are generally classified into grasses, broad leaves and sedges, and on this basis, herbicides have been developed to be selective in their actions. In a maize field, for example, a broad-leaf herbicide could be used if the weed population is predominantly broad leaves.
The use of herbicides, however, required certain measures to safeguard workers and crops. Wearing protective clothing during application is essential, as well as applying the right dosage as recommended on the label of herbicides or by specialists, and adhering to other instructions such as re-entry period into farm after herbicide application.
Due to over-reliance on chemical and mechanical weed control methods, especially by new farms, weed pressure can become even more severe. Availability of herbicides on the market, skill in weed identification, correct matching of herbicide to specific weeds and resistant of weed species to certain herbicides can lead to inefficient weed control. Over-reliance on these two methods can also lead to damage soil structure and the environment. A safe, cheap and effective option for farmers could be integrating all methods available.
Integrated weed management (IWM) is neither a method nor a system of weed control, but a philosophy whose goal is to use all available knowledge in weed science to manage weeds so that they do not cause economic loss to humans. An appropriate IWM is one that economically combines two or more weed management systems at low inputs to obtain an even superior weed suppression. IWM is even more effective in root and tuber crops which have long growing seasons that make dependence on herbicides inappropriate, particularly as none of these herbicides selective in yam and cassava persists in the soils at dosages that will give good weed control for more than three months after planting.
Despite their more popular undesirable characteristics, weeds may also serve some beneficial purposes. They could serve as food and feed for humans and livestock, provide nectar for insect pollinators, protect soil structure by protecting soil from extreme climate, and improve soil organic matter.
Common weed species such as Mexican sunflower (Tithonia diversifolia) are useful ingredients in organic fertilizer and pesticide production. Plants such as the neem (Azadiracta indica), commonly referred to as dongoyaro also possess attributes that make them excellent ingredients in pesticides and organic fertilizer production, as well as a host of other uses including the manufacture of beauty products and pharmaceuticals.
Source: Agriculture Nigeria