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Post-emergence weed control

December 2015

Post-emergence weed control

Weeds compete with maize plants for sunlight, nutrients and water particularly during the first 3 - 5 weeks after the plants have emerged so it is important to control the weeds before they get too tall and vigorous.

Late season weed infestations do not negatively impact the yield nearly as much as early weed competition does. Weed control in your maize fields is a very important management practice. All primary and secondary tillage operations help to prepare a weed-free seedbed however this article is turning the spotlight on post-emergence weed control.

Once a field has been planted, it is important to attend to the weeds which are likely to compete with the young seedlings. Fortunately there are several options available for post-emergence weed control in maize but there are a few important details which must be clear before any spray program is embarked upon. It is always a good idea to consult the chemical company’s representatives or other experts for their advice as a great deal of science lies behind the use and application of herbicides. Before consulting the experts however, the farm manager must know a few details.

Know your seed cultivar

It is important to know which variety of maize was planted in the field, for example if the seed is Roundup Ready® or not. If the seed is not a special herbicide resistant/tolerant cultivar, then you have to be careful not to use Roundup or a similar product containing the active ingredient glyphosate because that will obliterate your crop overnight.

Know the problem weeds

It is necessary to be well informed on the particular types of weeds growing as this informs the selection of herbicide and helps the experts to advise you. It also helps to determine the timing of the post-emergence spraying. Most herbicide control needs to take place before the weeds are too tall but there are some herbicides which will provide control for much taller weeds as well.

Know the stage of development of the maize plants

Another factor to take into consideration is the height of the maize plants. Many herbicides have limits to maximum height, maximum leaf stages or developmental stage of the maize plant listed as herbicides can cause the plants damage which in turn will affect the end yields.

Know your soil

When herbicides are applied, it is important to understand that soil status can influence their effectiveness. Once the herbicide has been sprayed it is in effect suspended in a “soil solution” and the properties of the soil including soil texture, levels of organic matter and pH will affect the availability and activity of the herbicides. The rates for soil-applied herbicides in the chemical weed control tables on the labels are calculated for medium textures (loamy) soils with organic matter levels of 3% - 4%.

Soil texture refers to the percentage of sand, silt and clay in the oil. Clay particles are negatively charged and have a large surface area. Soils high in clay (heavy soils) have the capacity to adsorb or “hold hostage” the herbicides applied, so generally it is necessary to apply higher herbicide rates than for loamy or sandy soils to be sure the weeds take up enough herbicide.

Know your organic matter status

Organic matter also affects the adsorptive capacity of soils. Plant and animal residues which have not decomposed well will limit the performance of the herbicide while well-decayed organic humus is of great value. Herbicide rates of application need to be adjusted according to the soil organic matter.

Know the soil pH

Soil pH can also affect the availability of the herbicides. Soils with low pH levels can also hold the herbicide hostage so that it is unavailable for uptake by the weeds. Soil pH in no-till fields must be carefully monitored and it is often a good idea to check the pH in the top inch of the soil profile in fields that have been no-tilled for some time. A standard soil sample analysis may give a different result to the surface soil which is where the herbicide must do its work.

Know your water and its pH levels

Water quality and water pH has a significant effect on the effectiveness of your herbicides. The water used in your spray tank must be clean with no dirt particles in it, for example, water drawn from a running stream is not good.

Silt and organic matter suspended in the water reduces the activity of the herbicides. Hard water,i.e. the water has excess magnesium and calcium (alkaline) ions, can cause the glyphosate and other ingredients in the herbicide to form a chemical reaction which forms insoluble salts. This is difficult to identify but will negatively impact the efficiency of the herbicide. The ideal solution is to have your water tested for quality and pH and then to seek expert advice on how to manage the process.

Another recipe for disaster would be to first add the chemical mixes into the tank before the water! There must ALWAYS be at least half the mix of water in the tank before any glyphosate or 24D is added. Add each ingredient into the water and mix thoroughly before adding the next one. You want to make sure that you get an evenly mixed solution with no lumps or gelling which could occur if an incorrect mixing procedure is followed.

There is no doubt that post-emergence herbicides are a very useful tool for controlling weeds and ensuring the best yield possible but it must be a well-informed process guided by experts and managed very carefully on the farm.

Article submitted by Jenny Mathews, Pula Imvula contributer.
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Publication: December 2015

Section: Pula/Imvula