Crop production: Do not neglect weed control


By Digital team | 3 July 2018
herbicide; resistance; weed
Photo: Chris Jooste

Poor weed management is perhaps the single most important factor leading to greatly reduced yields from the fields of the small-scale farmer. Here’s how to manage weeds in your fields.

Following conventional tillage practices can lead to on-going depletion of your soil’s fertility, and this, along with poor plant populations, can lower even further the reduced yields that result from poor weed control.

It has always been difficult for small-scale farmers to use modern chemical technology, either because the implements needed were not available, or no attempt had been made to modify them to suit the smaller scale.

HOW TO CONTROL WEEDS

Conservation agriculture (CA) is defined as the simultaneous application of the following:
1. No-till planting.
2. Rotational cropping.
3. Keeping a permanent cover of plant material on the soil surface.

One of the greatest misunderstandings in CA is the idea that weeds can only be controlled by using herbicides and that nobody uses any other control methods.

In many parts of the world, farmers begin their conservation agriculture system using mechanical (manual) control only, without herbicides. But chemical control does make life a lot easier. Each farmer has to assess his own budget and situation and decide which combination suits him best.

Also read: Conservation agriculture: No-till and controlling weeds

Weed control in conservation agriculture is achieved by a combination of the following:
• Mechanical.
• Chemical.
• Biological practices.

MECHANICAL CONTROL

Possibly the only real benefit of ploughing is that it leads to very effective weed control, but this obviously has to be replaced by other methods in conservation agriculture which leaves the farmer with rolling, “skoffel”, hoeing, chopping and pulling.

For farmers who are extremely resource poor, there are few, or no, options other than mechanical control methods, plus biological control.

It does sometimes happen that herbicides do not work properly and the farmer has to revert to mechanical control to ensure his yields will be up to expectations.

Rolling:
This is an effective tool, especially for controlling plants that have more of an upright growth habit; a roller becomes a “must have” for a conservation agriculture farmer. The most effective is the knife roller, but anything that will flatten the weeds will do. The weight of most knife rollers can be adjusted by simply filling it with water. Set the roller so that it bends and bruises the plant stems effectively, but does not break them up into small pieces. For this reason also, the edge of the knives should be rounded rather than sharp.

“Skoffel”:
The instruments available locally are designed to cause serious soil disturbance and for this reason should only be used as a last resort. Rather use a swallow-tail tine, because it can be changed as needed. Grind away a portion of the section where the bolts go through, making this as narrow as possible so it will only open a narrow furrow. Use the “skoffeler” slowly so that the weeds’ roots are cut off below soil level, but the soil is not turned over.

Hoeing:
A hoe is essential for a small-scale farmer who is practising conservation agriculture, but it should only be used to chop off the weed at soil level. Do NOT turn the soil over. Leave the weeds to rot on top of the soil, except for Wandering Jew and couch grass, which should be removed from the field. A CA farmer should always have a hoe with him when he goes to the field, so that he can remove any weeds he sees.

Chopping:
The recommended – and easiest – way to remove plants that have grown quite tall is to chop them down. But remember that this should only be done if the weeds are unlikely to regrow from the parts left after chopping. So, for example, chopping is not an effective method for controlling nutgrass or couch grass, but it is effective for stink¬blaar and burr weed. Once again, leave the plant material on top of the soil.

Pulling:
This a very effective way to control weeds, but it takes a long time and demands lots of energy. It is, however, very effective against some of the late-developing weeds, such as burr weed and morning glory.

Also read: Vegetable production: The basics of biological control

CHEMICAL CONTROL

The most important advice for using chemical control methods is: “READ THE BROCHURE”.

In an article such as this it is impossible to cover all aspects of every chemical so it is perfectly possible that something important is not mentioned. Atrazine is a very popular herbicide, which has a residual action and also broadens the spectrum of weeds controlled.

I generally avoid it – and any other herbicides that have a long residual action – because the farmers I work with are likely to include in their system crops such as beans, pumpkin and other vegetables. Including atrazine or other triazines is, however, recommended if a farmer is likely to grow mainly maize and/or sorghum.

Site preparation:

  • It is assumed that maize and sorghum are the most likely crops, with a possibility of dry beans being planted or even inter-cropped.
  • This means that all plants on the field have to be controlled before planting begins.
  • An inspection of the field will determine the type of weeds, their stage of development and therefore the most effective herbicide and its application rate.
  • The most popular herbicide to use at this stage is glyphosate (Roundup, Springbok, Mamba), because it is systemic and has a very broad control spectrum.
  • Paraquat is never recommended because it is extremely toxic and proper control among small-unit farmers is likely to be problematic.
  • A huge spectrum of herbicides is available and there are also many advisors, each of whom has his own preference.
  • The recommendations given here do not try to cover all possible options and are likely not to be the choice of all advisors.

Pre-emergent control:

  • This is aimed mainly at controlling annual grass weeds.
  • There is no chemical control option for controlling grass seeds in maize and sorghum once the weeds have germinated, which means that the farmer must get it right first time.
  • Many herbicides can be used at this stage, but the cheapest will be one having alachlor as the active ingredient.
  • Check your soil’s clay content and use the herbicide as recommended.
  • If nutgrass is a problem, apply alachlor at the highest recommended rate, BEFORE the weed seeds germinate.
  • If more than a week has passed since the glyphosate was sprayed, it would be “additional insurance” to spray another 1 to 2 litres/ha; if more than 14 days have passed, doing so is essential.

Also read:
How to calibrate a boom sprayer
How to effectively use knapsack sprayers

Post-emergent control:

The hope is that the measures used will give sufficient control so that the farmer has only to do some chopping or pulling to control the weeds, but this is not always the case.

Problem plants:

  • Nutgrass is the most likely plant to appear.
  • If it is likely to develop into a major problem, the farmer will have to take special care.
  • The first option to consider is replacing the alachlor with a pre-emergent herbicide which is thought to provide more effective control.
  • And remember that the application rate should always be set at the highest recommended rate, if nutgrass is a problem.
  • The next option is to use a post-emergent herbicide such as Basagran or Servian. Basagran controls a range of broad-leaf weeds in addition to nutgrass.
  • It does, however, have very specific climatic requirements and must also be sprayed when broad-leaf weeds are still small.
  • Servian controls only nutgrass in maize or sorghum.
  • The third alternative is to include the herbicide Callisto in the spraying programme.
  • It is very effective, but the problem with this herbicide is that it only comes in a 5 litre container, which is sufficient for 20 to 25 ha, and the cost is beyond the budget of most small-unit farmers.

Late-developing weeds:

  • Stinkblaar, burr weed and morning glory often develop after the residual effects of the herbicides have stopped being effective.
  • For the small-scale farmer, the most effective way to control these weeds is to ensure that hoeing or chopping is carried out regularly.
  • Witchweed is a serious problem where it occurs and this will be dealt with in a separate article.
  • Basically, the most effective control for small-scale farmers is to introduce a rotation of crops resistant to witchweed.

Also read:
Conservation agriculture builds a better life for Phumelele Hlongwane

SA farmer Cosmas Xaba investigates options in conservation agriculture

  • This article was written by Werner Ristow and first appeared in Farming SA.