PRE-EMERGENT HERBICIDE OPTIONS IN MAIZE AND SOYABEANS

WEEDS – FACTS 

1| The annual yield loss in maize as a result of weed problems can reach 50% or more.

2| The crop must have a good head start against weeds and hence must be weed free for the first 10 weeks of the maize crop cycle.

3| If a farmer fails to control weeds in the first 5 weeks of a maize cycle, then 50% of the yield could be lost.

4| If a farmer fails to control shamva grass (Rottboellia cochinchinensis) throughout the maize crop cycle, then 57-80% of the yield can be lost.

5| One year of seeding makes 7 years of weeding. Do not allow weeds to bear seed, lest it will create future weed management problems.

6| There are more than 25 herbicides registered in Zimbabwe, which can be sprayed in maize.

Time to automate agriculture

Hand weeding has no place in commercial agriculture. It is bad and terrible economics and definitely unsustainable. It is expensive, hard and inefficient and it does not guarantee a weed free field. It actually agitates the soil to produce more weeds. Enlightenment of the above startling weed statistics, cost-effective weed management strategies become very critical to the farmer. There is a very important weed management concept in crop productivity called the critical weed free period.

But what is the critical weed free period?

It is the critical point in time during the growing season when weeds cause the largest yield loss.

Weed free period in Maize.

In maize, this period occurs from V1 to V6 (1 to 6 leaf collars). Some sources will mention that it even begins at the VE stage (emergence). During this time the maize crop needs to stay clean. Weeds will definitely affect yield if control is not provided.

Weed free period in Soyabeans.

For soybeans the period is the V1-V3 (1 to 3 trifoliates). If you can control weeds during this time, losses should be assumed to be under 5%.

Why are these periods important?

It is critical to stay weed-free during these periods if a grower wants to maximize yield potential. Maize and soybeans on average can have a yield loss of up to 50 or 60% during this time if control is not provided and even higher depending on the weed spectrum. Timeous and correct use of pre-emergent herbicides can go a long way in as much as weed management options are concerned. Any successful weed management program must include pre-emergent herbicides as these are very critical with quite a number of unique advantages some of which are listed:

Pre-emergent herbicides:

• Offer an alternate mode of action to many post-emergent options; can reduce selection pressure on subsequent post-emergent herbicide applications.
• Remove much of the early season weed competitive pressure on a crop and can protect yield better than post-emergent herbicides.
• Can save costs, especially in the fallow here multiple knockdown applications may be required.
• Can reduce the time pressure on spraying operations, especially in situations when double knocking is a requirement.

Here are some of the effective pre-emergent herbicide combinations/options that a farmer can count on in Soyabeans and Maize.

Soyabeans pre-emergent herbicide combinations:

• Metolachlor 1.5 Litres/ha + Metribuzine 0.6 -1 Litres/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist.
• Metolachlor 1.5 Litres/ha + Strongarm 30grams/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist.
• Metolachlor 1.5 Litres/ha + Terbutryn 1.8 – 2.4 Litres/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist.

1. You can alternatively use Kalif/Clomazone/Command in place of Metolachlor.

Maize pre-emergent herbicide combinations:

• Metolachlor 1.5 Litres/ha + Atrazine 3 -5 Litres/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist. However note that Atrazine is both a pre-emergent herbicide as well as an early post emergent herbicide. This combination is suitable for those growers who intend to repeat growing maize in the next season, bearing in mind that Atrazine has a residual period of up to 18 months from point of application upon which NO any other crop other than maize or sorghum can be grown. Failure to appreciate this critical piece of information is the very reason why there is a misconception by some farmers that herbicides affect soil fertility and productivity “Mishonga yesora inouraya ivhu”, ichi hachizi chokwadi what is important is to appreciate the residual period and plan accordingly. Farming is a business and therefore planning remains an essential tool in the equation.
• Metolachlor 1.5 Litres/ha + MCPA 3 -5 Litres/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist. However note that MCPA is both a pre-emergent herbicide as well as a post emergent herbicide. This combination is very suitable to growers who are into rotations for example those growers who rotate maize with soyabeans, tobacco and other broad leaf crops in the following season.
• Metolachlor 1.5 Litres/ha + Terbutryn 2.5 Litres/ha. Mix these two herbicides in 200 litres of water and apply immediately after planting or within three days after planting when the soil is relatively moist.

1. You can alternatively use Acetochlor/Relay/Harness or S-Metolachlor/Dual Magnum or Alachlor/Laso in place of Metolachlor.

Acknowledgements:

John Basera – SeedCo Head of Agronomy Services.

Bongayi Gokoma – Agricura Sales Manager.

Onias Tawananyasha Mlambo – Agricura Technical Agronomist.

Critical weed-free period