Learning objectives
- Define a weed and list the ways weeds reduce yield
- Describe cultural, mechanical, biological and chemical control
- Explain key husbandry practices from planting to harvest
1. Why weeds matter
Weeds compete directly for nutrients, moisture, light and space; some release growth-inhibiting chemicals (allelopathy). They also act as alternate hosts — Striga parasitises cereal roots, while many broadleaf weeds harbour aphids, whiteflies and the viruses they carry. They increase harvesting cost and contaminate produce, lowering market grade.
The critical period of weed competition is usually the first 3-6 weeks after emergence. Keep the field clean through that window and later weeds cost far less yield.
2. Four control methods
Cultural: crop rotation, close spacing, cover crops, mulching, clean seed and timely planting. Mechanical: hand weeding, hoeing, slashing, tillage. Biological: introducing insects or pathogens that attack a specific weed. Chemical: herbicides, either pre-emergence (applied before weeds appear, e.g. atrazine in maize) or post-emergence, and either selective (kills weeds only, e.g. 2,4-D in cereals) or non-selective (kills all green plants, e.g. glyphosate).
- Always read the label rate — over-dosing scorches the crop and builds resistance
- Rotate herbicide modes of action to prevent resistant weed populations
- Wear gloves, mask and boots; do not spray into wind or before rain
3. Core husbandry practices
Good husbandry is preventive plant health: proper land preparation, correct spacing for airflow, thinning and supplying to keep an even stand, staking climbers such as tomato and yam to keep foliage off the soil, earthing-up, timely fertiliser application, pruning and de-suckering, and harvesting at the right maturity.
Most of these also reduce disease. Wide spacing and staking lower leaf wetness and slow blight and leaf spot; rotation breaks soil-borne pathogen cycles; sanitation removes the infected debris that carries a pathogen into next season.
Key definitions
- Weed
- Any plant growing where it is not wanted, competing with the cultivated crop.
- Selective herbicide
- A chemical that kills specific weed types without seriously harming the crop.
- Allelopathy
- Release of chemicals by one plant that inhibit the growth of neighbouring plants.
Exam tips
- Group your answer under the four control headings — cultural, mechanical, biological, chemical — for full marks.
- Give a named example of each herbicide type; examiners reward specific chemical names.
Practice exam questions & model answers
Q1.State five ways in which weeds reduce crop yield.
They compete for nutrients, water, light and space; they harbour pests and disease pathogens; parasitic weeds such as Striga draw nutrients directly from crop roots; some release allelopathic chemicals that inhibit crop growth; they obstruct and increase the cost of harvesting and contaminate the produce.
Q2.Differentiate between selective and non-selective herbicides with examples.
A selective herbicide kills certain weeds while leaving the crop largely unharmed, e.g. 2,4-D controlling broad-leaved weeds in cereals. A non-selective herbicide kills virtually all green plants it contacts, e.g. glyphosate, and is used for land clearing or directed inter-row spraying.
Q3.Explain three husbandry practices that reduce disease incidence.
Correct spacing improves air circulation and dries foliage quickly, reducing fungal infection; staking keeps leaves and fruits off wet soil, preventing soil-splashed pathogens; crop rotation and sanitation (removing infected debris) break the life cycle of soil-borne and residue-borne pathogens.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.