It is essential to achieve effective weed control early in the crop's development. To avoid an irreversible effect on yield all weeds should be removed by the four-leaf stage of the crop at the very latest, and before the weeds reach 100mm in height.
A range of herbicides are available for use in maize for broadleaved weed and grass weed control. They can be divided into two categories depending on their recommended timing of application - either pre-emergence or post-emergence.
Arguably the most reliable strategy since the herbicides are broad spectrum and the timing is easier to get right. By applying the herbicide before weeds germinate their establishment is almost entirely prevented which is extremely beneficial given the importance of early weed competition. Crop-scorch is often associated with post-emergence treatments, but obviously, when the chemical is applied pre-emergence this is not an issue and there is usually little or no crop-effect.
Extremely good results can be achieved from post-emergence treatments when they are timed correctly. However, the window for application may be missed due to poor weather at a critical time, leading to inadequate weed control and dirty crops.
As previously mentioned, crop-scorch can be a significant feature of post-emergence applications. The check to growth is often temporary and the plants recover and grow away, but it would be better if there was no check at all!
Herbicide Choice and Strategy
There are many considerations relating to strategy and choice of product. Obviously, the weed spectrum drives the decision-making process and factors such as soil type can also be very important.
Predict likely weed problems
Apply residual, pre-emergence chemistry
Assess weed species at germination during very early crop establishment
Aim to remove all weeds before the crop reaches four leaves
Tailor post-emergence herbicides to the weed spectrum present
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