The point of weed control is to limit the competitive pressure of weeds in order to reduce crop damage, as maize is a plant that is sensitive to competition. Weed control is essentially aimed at maintaining the crop’s potential (both in terms of yield and quality) and preventing any future infestation of the field, as well as any potential increase in the weed seed stock that can affect the following crops and neighbouring fields.
The advantage of plants sown at wide interrow spaces such as maize is that weed control is easier. The disadvantage is that weeds are allowed enough room to develop until the space between the rows gets covered by the crop canopy. However, the benefits of this sowing method are clear, since the need to limit the use of phytosanitary products is ever so important. Maize weeds can therefor be controlled either by chemical or mechanical means – or both.
Weed control has several objectives. Yet before choosing one’s strategy, one should do a risk-assessment (to determine the presence and impact of potential weeds) and develop their methods of action based on the areas requiring attention, the available chemicals, and necessary logistics (however, taking measures right away is always preferable).
Maize Treatments and the Damaging Impact of Weeds
Maize is a short-cycle crop, sown at wide interrow spaces, and highly-sensitive to weed pressure. Knowing exactly how the damaging mechanisms of weeds work (depending on their stage and type) allows one to better manage the timing and level of weed control, be it by chemical or by mechanical means.
Some Important Reminders:
- Peak weed competition occurs between the 3 and 10 maize leaves and it is proportionate with the length of the presence x number of weeds x weed species.
- Growing weeds are highly-competitive for water and nitrogen and have a significant impact upon yield.
- Losses are proportionate with the length of the competition: they can average 0,75 q/hectare/day and can be similar, for example, to daily losses incurred when planting was done beyond the optimum date. In cases of restrictive water and/or nitrogen supplies, the weed impact increases very fast. The losses are low if the water supply is not limited.
- Pest and disease development may be encouraged by the microclimate that is created by the invading weeds or by their acting as relays for viruses, bacteria, fungi, spider mites, or insect pests.
Weed Control Strategies
To the grower, weed control strategies must first of all be effective and sufficiently flexible to adapt to the unforeseen factors related to the work schedule and the climate. Depending on the options available, one can control weeds before planting, between planting and emergence (using root herbicides), after the emergence of both maize and weeds (using foliar and/or systemic/selective maize herbicides), or through mechanical cultivation.
Yet the first criteria for a good weed control strategy is to adapt it to the target weeds (looking at the species and their growth stage). Thus, one may have to tackle annual or perennial plants, be they grass or dicotyledons. The flowering succession and its development in time depend on the methods used.
WEED CONTROL ON PRE-EMERGENT MAIZE
Some Recommendations for Optimum Effectiveness
- Root-active herbicides work on germinating seeds and require minimum moisture. They should be applied on soils with a good level of moisture, which are well-pressed and not very bouldery.
- Post-planting, pre-emergence applications should be carried out as soon as possible after planting, in order to take advantage of favourable weather conditions during the seedbed preparation.
- Herbicide doses shall vary depending on the clay and organic matter contents, as to become effective, the products need to attach themselves to the absorbing soil complex.
- For selectiveness reasons, it is best to avoid treatments on emerging maize.
- To reduce the risks of product leaching or runoff into the environment, one should delay the treatment by a few hours if an important rainfall is expected, if at all possible.
- The amount of herbicide mush may be reduced to fit the limits allowed by the equipment, whilst observing the minimal required pressure for the type of nozzles that have been selected (nozzles with drift limitation should be used above all).
WEED CONTROL ON POST-EMERGENT MAIZE
Some Recommendations for Optimum Effectiveness
- By definition, foliar herbicides need favourable humidity in order to be effective (more than 70 percent air humidity), with guaranteed moderate temperatures of 10 to 25°C during the first 48 hours after the treatment, to prevent any toxic effects on the maize plants. One must avoid treatments on stressed maize plants or in windy conditions (risk of herbicide drift).
- Although they help reduce the number of equipment passages, active ingredient mixtures (premixes) applied after emergence – which are increasingly used nowadays – raise the risk of phytotoxicity and may result in a poor control over part of the target weeds.
- Herbicide doses should therefore be adapted to the flora present, based on the stage and species that are most difficult to control. Nevertheless, depending on the herbicide effectiveness and especially with complex flora structures, one should apply the treatments on young weed plants (below 3-4 leaves), in order to secure the highest success rate.
- As to the growth stage of the crop, treatments should be applied before 8-10 maize leaves, in order to avoid the “umbrella” effect. With less selective products, one should avoid broadcasting after 6-8 maize leaves.
- The recommended rate of herbicide mash is 100-400 l/ha, with an optimum level standing somewhere between 150 and 200 l/ha. It is possible, however, to reduce that rate – without going below 50 l/ha, with systemic products and 80 l/ha, with contact products –, as long as nozzles are set at the minimum required pressure (depending on the nozzle type) and especially if treatments are applied on very young weed plants (maximum two leaves) – and in ideal temperature and humidity conditions, to ensure satisfactory effectiveness.
Mechanical weed control
Mechanical weed control (cultivation) is practiced to a great extent in many countries. The method suffices on simple and not very dense weed populations, such as in rotated maize crops and in regions where the climate favours a quick establishment of the crops – which is often the case in Eastern Europe.
With maize crops, there are three distinct weed control methods possible, involving no herbicides. Firstly, post-emergence cultivation can be done after a pre-emergent chemical treatment (either by broadcasting or by “spot/in-row placement”). Secondly, cultivation may be done after a first post-emergent chemical treatment. Thirdly, one can cultivate mechanically very early, before emergence, with a rotary hoe or a comb harrow. Subsequent interventions are done chemically.
- Perennial plants: particular attention should be given to fields that are highly-infested with perennial weeds, especially bindweed. Basically, what every mechanical operation actually does is to cut the perennial plants’ rhizomes; as a result, each cut-out portion will produce a new plant. Thus, mechanical cultivation will only accelerate the vegetative reproduction of perennial plants, which defeats the very purpose of weed control.
- Combining techniques: results of various trials indicate that cultivating after a pre-emergent/post-emergent chemical treatment is more effective than early cultivation by harrow or hoe. The cultivation is done in that case at 3 to 7 leaves of maize, depending on what the crop stage was during the previous chemical treatment.