By Professor Sheunesu Mpepereki

IN this article we shall examine the control of weeds in soyabean production using chemicals called herbicides as well as touch on mechanical means such as use of hand hoes and tractor or ox-drawn cultivators.
For those who have already planted their soyabean crops, some of the information may be coming a little too late, but better late than never.
Weeds can severely reduce yields if left uncontrolled, thereby lowering profits. Weeds compete with crop for space, moisture, nutrients and light.
Weed-infested soyabean crops have jokingly been referred to as sora-beans.
Traditionally weeding with hoes is effective for small plantings of one or two hectares or where labour is plentiful.
However, herbicides offer a cost effective method of weed control in soyabean production.
While small areas can be weeded with hoes, we encourage farmers taking up soyabean as a cash crop to use appropriate herbicides.

Pre-emergent herbicides
Herbicides can be applied immediately after planting, but before the crop germinates.
These are called pre-emergent herbicides.
They are of two kinds: herbicides for broad leaf weeds and those for grass-type weeds.
They prevent the seeds of weed plants from germinating.
Generally, we apply lower rates of pre-emergent herbicides in sandy soils and higher rates in heavier or clay soils. Higher rates in soils with high clay content are meant to make up for some of the chemical absorbed by the clay. It is important to read the label on the herbicide container.

Post emergent herbicides
The second group of herbicides are applied after both the crop and the weeds have germinated.
These are called post-emergent herbicides.
They are said to be selective as they kill weeds, but not the soyabean crop.
There are two types of post-emergent herbicides; those that target broad leaf weeds and those for grass type weeds.
In all cases the soyabean crop must first develop at least two trifoliate (three leaflets) leaves before post-emergent herbicides can be applied.
Too young crops may be killed by the herbicides.
Equally too old weeds may fail to respond to the herbicides; three – four leaf stage weeds are easily controlled by herbicides.
Farmers have the option to apply either pre-emergent or post emergent herbicides. Where pre-emergent herbicides have been properly applied and the weeds effectively controlled, there will be no need to purchase post-emergent herbicides.

In practice, however, small quantities of post-emergent herbicides may be needed for spot application in areas where weeds may have been poorly controlled e.g. where the boom spray swaths may fail to overlap leaving weedy bands in the soyabean field.

Herbicides against grass weeds
Pre-emergent herbicides are applied after planting, but before crop germinates. Rates depend on the clay content of the soil; more clay in the soil will require higher herbicide application rates.
Soils with less than 20 percent clay content are normally not recommended for herbicide application; otherwise the herbicide will damage and prevent crop seeds from germinating.
Here are some examples of pre-emergent herbicides for use in controlling grass weeds in soyabeans. The rates are indicative; read the container label to be guided.

For controlling grass weeds before crop germination
l Lasso (Alachlor) @ 1,1 – 3L per hectare.
l Metolachlor @ 1,5 – 2 L per hectare
l Dual 960 @ 0,7 – 1,2L per hectare.
l Harness @ 0,75 – 1L per hectare
l Command 4EC @ 1,5 – 2L per hectare

It should be noted that some of the above are trade names given as examples.
The products may contain the same or different active ingredients.
Their mention here is not an endorsement as other products may be available and be just as effective.
Farmers are encouraged to deal with reputable suppliers.

Post emergent herbicides are applied after the soyabean crop has germinated and has grown to at least two trifoliate leaves. After herbicide application, it may take some days before the weeds show signs of herbicide damage; usually a yellowing of the growing tips followed by drying out of stems over time. The weeds however stop taking up water and nutrients within 24 hours of spraying.

Spraying conditions
Spraying post emergent herbicides must be done when there is little or no wind to avoid excessive drift of spray off the target field. Rains must not fall within at least two hours after spraying, otherwise the chemical is washed off. A special wetting agent (detergent or type of soap) must be mixed with the herbicide to improve penetration of chemical into the leaves of the weeds.

Below are some examples of post emergence herbicides against grass weeds. Application rates vary with the growth stage of weeds. An overdose should always be avoided as it can kill the entire crop. Read the label on the container carefully especially the recommended rates of application.

For controlling grass weeds after crop germination
l Fusilade @1,0 – 1,5 L per hectare’
l Agil @ 0,5 – 2L per hectare.
l Nabu @ 2 – 3L per hectare.
l Before planting
l Trif @ 1,5 – 1,6L per hectare.

Trif is applied before planting and disked into the soil immediately.
It controls Shamva grass, volunteer wheat, barley and other grass species.

Herbicides against broad leaf weeds
Pre- emergence herbicides
These are applied after planting but before the soyabean crop germinates.

For controlling broad leaf weeds before soyabean crop germinates
l Authority 75WP @ 300 – 400g per hectare.
l Igran 500FW @ 1,5 – 2L per hectare.
l Metribuzin 480SC @ 0,6 -1,1L per hectare.
l Ronstar @ 3L per hectare

Post emergence herbicides
These are applied after emergence of the soyabean crop and weeds. Application rates must be confirmed by reading the container label.

For controlling broad leaf weeds after soyabean crop has germinated
l Classic @ 30 g per hectare
l Basagran @ 3L per hectare ( Apply only after second set of trifoliate leaves)
l Pursuit @ 700ml per hectare.
l Lynx @ 300ml per hectare

Farmers may need to take note of the following general points on herbicide use.

1. Rates of application are usually lower when herbicides are applied as mixtures. Some herbicides cannot be mixed. Read the label carefully.
2. Alachlor (or Dual or metalachlor) to control grasses can be mixed in the same tank with metribuzin to control broad leaf weeds.
3. Metribuzin must be diluted in smaller water quantity before adding to tank.
4. Half fill the tank before adding the herbicide; stir thoroughly; fill to required volume; stir thoroughly again for uniform mixing.
5. Do not mix post emergence herbicides in the same tank. The stress of two chemicals applied at the same time can kill your soyabean crop. Each herbicide is applied separately with a minimum 7 days separating the application dates. If in doubt, read the label.
6. For example you can apply ‘Classic’ to control broad leaf weeds this week and wait until next week to apply say Fusilade for grass type weeds.
7. Where some green grasses have emerged prior to planting, Round-up (glyphosate) may be included in the tank mix in 2 above to burn out the weeds.
8. It is always advisable to consult the company that manufactures the chemical or their agents when in doubt about its use.
9. Never buy unlabelled chemicals; you might damage your crop after applying a wrong chemical. Avoid buying unlabelled products; check if containers are sealed.
10. Never buy chemicals where the expiry date is not shown. Expired chemicals do not work.
11. Seek advice from knowledgeable people e.g. agricultural experts from AGRITEX, Universities, Department of Research and Specialist Services, seed suppliers or experienced soyabean farmers.

In the next article we shall discuss further aspects on managing a soyabean crop incuding pests and diseases.