Article Written by Nurain Oladeji
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 importance1. 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 harbor 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 days2.
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 growth3. 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 harvest4. Yield loss attributable to weed infestation can be as much as 100% in crops such as rice1.
Methods of Weed control
A woman hoe-weeding a cassava plot
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 inter-cropped with cassava or yams to control weeds for the tuber crops1. 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-weeding5. This method is unsustainable even for some
smallholder farmers due to high cost of labour and possible unavailability of sufficient labourers when required1. 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 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 kills 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.