Research studies have found that no weed control for the first 6 – 8 weeks of the life of any crop can reduce the yield by as much as 50%, i.e. half the crop. For weed control to be effective in terms of its ability to assist the yield, weed control practices must be good during this initial period.

Another aspect of weed control is that of weeds producing seed. There is an old axiom that ‘one year’s seeding means nine year’s weeding’. This may be an exaggeration, but the meaning is clear. It is important that even though weeds may have little effect on a current crop, the cost of allowing a crop of weeds to seed may be detrimental as they will germinate in the future.

Techniques for controlling weeds are to use labour with hoes, mechanical cultivation equipment, or the use of chemicals called herbicides.

  • The first planting rain will bring up what is called the first flush of weeds (there may be numerous flushes as different species germinate under different conditions, i.e. day length). Effective weed control will be ensured if this flush can be destroyed within 10 days of the first planting rain. This can be achieved by using rotary hoes (millipedes), or spike-tooth harrows. The first planting rain, if it fails in late October to late November, is normally followed by sufficient sunny days to make this feasible. It will also be necessary to cover the entire crop in 3 – 4 days as the weather in some seasons may not permit much more than this.
  • Weather permitting; hand labour may follow this initial cultivation to clean up anything which may have been missed. The earlier this is done, the more susceptible are the weeds, and the more effective it will be.
    • The next operation is likely to be an inter-row cultivation as most crops soon get too tall for the use of rotary hoes and spike-tooth harrows.
  • Equipment used for this includes tined cultivators. This operation tends to be slower than the rotary hoes and spike-tooth harrows, so a little more time may be needed. Speed depends on the height of the crop and small plants may get buried by the lateral movement of soil.
  • A point to consider here is that a low level of damage to the crops should be tolerated, as it is almost impossible to avoid this and still get a satisfactory kill of weeds. This operation should pick up any weeds that were missed initially and also the second flush of germinating weeds. The timing of this obviously relates to the pattern of rain, as more rain is needed to bring up subsequent flushes of weeds. December is traditionally a month of few sunny and dry periods and so these cultivations need to be done at every opportunity if weeds are appearing.
  • As the previous method of control is an inter-row cultivation, it needs to be carried out in conjunction with band weeding on the rows themselves.
  • Once a crop ‘canopies’, no further machinery can get into the land and so hand labour may go in to clean up any late weeds which may appear.

Picture 1: Shows weed control threw in-row cultivation with a tined implement.

Source: landoll


Herbicides are used to supplement conventional control, in order to give management an insurance factor against excessively wet weather, or the ability to cope with expanded crop lands. Herbicides, very roughly speaking, could be two to three times as expensive as conventional control and they  are not always effective, especially with resistant species of weeds. Herbicides, therefore, need to be used for specific reasons, and the cost of this action should be calculated.

Knowledge of the clay content of a soil is necessary and has been discussed in the Soil Science Course.

Herbicides can be divided very broadly into two categories, namely, those which control grasses and those which control broadleaf weeds (monocotyledons).

Herbicides may be applied:

  • Pre-planting                                   –                                    before planting
  • Pre-emergence                            –                                    before a crop emerges
  • Post emergence                           –                                    after emergence
  • 2.    CROP PESTS

The Zoology Course will cover the life cycles of all important agricultural pests. The most important class of animals from a crop pest point of view is


Table 1: Shows a list of economically important Crop Pests.

CottonAmerican, Red, Spiny and Pink Boll Worms; Cotton Stainers; Jassids; Aphids and Red Spider Mites.
TobaccoCutworms; Aphids; Leafminers; Laceworm; Eelworm; White Grubs.
Maize & SunflowerStalkborers; Snoutbeetles; Cutworm; American Bollworm.
Sorghum and WheatAphids; Cicadulina; Stalkborer; Quelea (birds).
Soy & GroundnutSemi-loopers; Aphids; Hilda; Snout beetles.
Potatoes & TomatoesTuber Moths; Eelworm; Leaf-eating Ladybirds; Fruitworm.
Coffee and TeaStemborers; Leafminers; Australian Bug; Root Mealie Bug.
Deciduous & Citrus FruitWasps; Fruitfly; Aphids; Scales and associated Ants.
Stored ProductsWeevils; Grainbeeties; Tobacco Beetle; Rodents.
GeneralCutworms and Army Worms.
  • Direct Attack: This involves the use of chemicals about which more will be said later. For this purpose we must learn how to calibrate sprayers, because we cannot overstress the necessity for applying the prescribed quantity of chemical per hectare and as evenly as possible.
  • Crop Rotations: Pests multiply rapidly if a favorable environment for their multiplications occurs repeatedly. Hence, the necessity for breaking this cycle by the use of crop rotations and periods where the land has been left fallow. Eg: root knot nematodes in sandy soils with warm climates are a serious problem. If the land is continuously cropped with the same crop there is a buildup of nematodes which can damage the crop and thus effect yields significantly. By growing a non-hosting crop one can reduce nematode infestation enough to plant a susceptible crop the following year without using fumigants.
  • Parasitism: This involves the use of other species to parasitise the pest species however it involves an active human management role and can play a major role in integrated pest management systems. There are three basic types of this management and those are biological control, augmentation and conservation. A practical use of this is the virus which affects semi-loopers and some species of wasps prey on caterpillars, butterflies and moths. There are many other species and situations that aid in effectively reducing crop pests.
  • Many other approaches: These will be discussed in other lectures and are not of practical significance at present.
  • The approach to pest control must be based on the premise that we do not want to eliminate a pest completely (in fact, we have never actually managed to do this yet), but to reduce it to low and manageable proportions. Researchers are finding that there is a balance in nature and we need to bear this in mind. To overkill one pest appears merely to encourage other problems as, very often the beneficial species, of which there are plenty, are also wiped out, It is a question, today, of management and not complete control

There are four desirable properties in a Pesticide which are:

  • It must be highly poisonous to the Pests.
    • It must not be harmful to the Plant.
    • It must be as selective as possible, i.e. poisoning the pest without harming the plant or other beneficial pests.
    • It must leave no undesirable residue.

Table 2: Shows the forms in which pesticides are made:


The active ingredients should be mixed with a carrier, e.g. chalk, clay or Talcum Powder. Some dusts like boric acid are the insecticide themselves. It must NOT be mixed with water and used as sprays because it will clog up the nozzles of the sprayer. Therefore it is unsuitable for low volume spraying (see wettable Powders).

Very little equipment is required for the use of dusts. However, on windy days, it is not easy to use, and it is easily washed from plants by rain or irrigation. To inhale a pesticide dust can be very dangerous.


Many dangerous pesticides are manufactured in the form of granules in order to lessen the health hazards. They are made up of the pesticide and a dry carrier that has large particles. Normally it is ground up organic material and is an inactive ingredient. The granules are used in funnels for application to the soil. This is easy to apply, and no water needs to be carried.

  1. Wettable Powders: These are mixed with wetting and stabilizing agents, but they do need constant stirring. They are not suitable for low volume spraying due to the paste clogging up the nozzles. However they are easy to transport and store as they do not have water added and thus the weight is less.
  • Disposal Powders: These are similar to wettable powders, but have a finer particle size and they are suitable for low volume spraying.
  • Emulsions: These are suspensions of oil droplets in water, i.e. pesticide dissolved in an oily organic solvent, together with an emulsifying agent. They are sold as concentrates, and need to be diluted before use. However, they do tend to be more expensive than powders and are less easily washed off by rain. They break down more quickly than powders, but are likely to be poisonous. They are suitable for low volume spraying.
  • Aerosols, Smokes and Fogs: These are not really for agricultural use. They are mainly for use in sheltered places, e.g. the home.
  • Fumigants: These act in a gaseous form and are usually used for stored products and soil pests and they must be used in confined spaces to be effective.

Pesticides are normally a chemical which can discourage, kill or incapacitate pests. However they can also be dangerously poisonous to humans and animals. Pesticides are a class of biocide and are divided into different types, depending on whether they are made from natural materials or manufactured from chemical compounds. The following list gives some examples of the different types of Pesticides:

  • Manufactured from plant material: Nicotine made from the tobacco plant (withdrawn from use as a stock remedy) and Pyrethrum made from the Pyrethrum flower.

Manufactured from chemicals:

  • Organochlorine compounds: These were commonly used in the past however most have been banned due to their health and environmental effects.
    • Organophosphate compounds: This form of pesticide affects the nervous system. It affects humans the same way as it does insects. Some are very poisonous but they do not usually persist in the environment.
    • Carbamates: there are a few subgroups to carbamate pesticides. The effect is the same as for organophosphates affecting the nervous systems but the effects are usually reversible.
    • Pyrethroids: These are a synthetic form of pyrethrum which is manufactured in a laboratory from the flower chrysanthemum. These have been modified to increase environmental stability. Some pyrethroids are poisonous to the nervous system.

The containers holding Pesticides are ‘colour coded’ by means of a colour strip or colour code on the bottom of the label. These shows visually how dangerous or not the contents are. Set out below is a list showing the different categories:

Table 3: Shows the different colour coding for chemicals depending on their toxicity levels.

  GreenClass IV (4) or U: The contents less hazardous (unclassified) and are safe to use. The chemicals should be locked away and stored out of reach from children, animals and food.
  BlueClass III (3): The contents slightly hazardous can make you sick, and need to be handled with caution and protection should be used. Use in accordance with the manufacturer’s instructions.
  YellowClass II (2): The contents are moderately hazardous. It is the second most toxic pesticide sold in S.A. This means the contents are dangerous, and could be harmful if not handled carefully. Protective clothing and equipment must be used at all times.
      RedClass Ia (1a): The contents of this class are extremely hazardous. Class Ib (1b): The contents are highly hazardous. These are the most toxic chemicals sold in S.A. This colour denotes that the contents are lethal; they can kill if protective clothing is not worn. Necessary instructions regarding its use will be written clearly on the outside of the container. All precautions must be used at all times.

Pesticides act in different ways in order to kill the insects which are regarded as pests. Pesticides are available in various forms and these can be used in different ways depending on the type of pest to be killed and the time when an attack is most likely to occur. Here are some of the ways in which pesticides affect insects.

Systemic Poisons: have to be swallowed by the pest and are absorbed into the body through the digestive tract. These are applied as sprays or dusts onto the leaves of the crop however they are been replaced with synthetic organic pesticides which are less harmful to humans and animals.

Contact Poisons: enters the body through the epidermis and carried into its body once it has come into contact with the poison. It does not have to be eaten and can be sprayed directly onto the pest.

Fumigant Poisons: kill when they are inhaled by the insect or animal. They are carried into the bloodstream via the lungs.

Seed Dressings: are usually powders which are applied to seeds just before they are planted. This operation is easy to carry out by mixing the seed and the seed dressing in a rotating drum. Seed which is bought from a Seed Firm is usually mixed with a seed dressing before being sent to the farm. Seed Dressings are used to prevent the seed and the seedling being attacked before and after germination.

Pictures 2 and 3: Show the difference between seed that has been treated with a pesticide (purple

hue) and seed that has been untreated.

Source: alibaba                                                            Source: cepolina

Baits: are used as a method of killing certain specific pests. The bait, which can be something like finely ground maize meal, is mixed with the poison and small quantities are put out in bait stations or around plants. This method is used for killing cutworms before the crop has been planted. There are also bait traps that have poisons ready mixed into an edible substance that is placed in a trap of sorts. The traps vary according to what pest is been targeted.

Pictures 4 and 5: Shows bait stations. Picture 4 (left) is a rat trap and picture 5 (right) is an ant trap.

Source: gardexinc                                                                  Source: spmglobal

Sprays: This is the most common and the most effective method for killing insect and other pests. The poison is mixed with water and sprayed over the crop by hand, tractor, or aerial spraying. This method gives each plant a good cover of spray and will effectively kill most insect pests. Some poisons are sprayed onto the soil, are washed down into the topsoil and are taken up by the plant. These are Systemic poisons which will kill the harmful insects attacking the plant.

Pictures 6 and 7: Show a knapsack sprayer (left) and a tractor mounted boom sprayer (right).

Source: hymaticagro                                                       Source: schabenindustries

Pictures 8 and 9: Show a self-propelled boom sprayer (left) and an aeroplane spraying lands (right).

Source: applylikeapro                                               Source: gregjordanphotography



  • Mark out 100m.
    • Select correct gear and revs (rpm). This should be the same as when spraying crops.
    • Time in seconds, the time it takes to cover the distance.
    • The sprayer should be attached and at least half full.

Calculate the speed into km/h

3.6 (constant) X distance covered (100m) Seconds (step1)

= km/h


  • Set tractor at operating revs (rpm) and not in gear.
    • Measure the output of water from 1 nozzle in a minute.
    • Multiple the output from 1 nozzle by the number of nozzles on the boom. The answer will give you the total litres per min = L/min


600 (constant)          X                                              total L/min (step2) Boom width (m)                                        X                                        Speed (km/h, step1)

= litres per hectare or L/ha

This is the total amount of water that is sprayed over one hectare.


Tank capacity (litres) X Chemical rate (ml or L/ha)

Output L/ha (step3)

= chemical rate / tank (ml or L)