Article taken from the Agronomist & Arable Farmer Vegetable Guide
Early weed control is essential to ensure the crop suffers as little competition as possible and gets off to the best possible start.
Optimum efficacy of pre-emergence herbicides is achieved through good timing and ensuring applications are carried out in favourable conditions.
Applying pre-emergence herbicides before any emergence of weeds is vital. This means in an ideal world the sprayer should be into the crop soon after drilling and after first irrigation or rain.
Applying to a damp seed-bed optimises efficacy. Soil will also settle around seeds and transplant roots after rain or irrigation, reducing the risk of herbicide being washed down the soil profile.
A firm, fine seed-bed should be the aim. This means careful planning of cultivations to minimise clod formation and to ensure those that do occur can be dealt with effectively. However, care must be taken on poorly structured soils to ensure these do not cap after drilling/rolling.
Seed-beds that contain larger clods are difficult to treat effectively with pre-ems. The even coverage that is required to ensure optimum performance in terms of optimum weed kill with minimum crop damage is a real challenge, due to the uneven surface that can result in shading some areas and over deposition in others.
However, research work carried out by Agrovista has shown that fitting forward-angled and vertical nozzles alternately along the sprayer boom can make a significant improvement to the evenness of deposition. This, though, should not be relied upon as a get-out-of-jail card – the primary aim must still be to achieve the best possible seed bed in the available cultivation and drilling window.
Nozzle Height
Check out the recommended height for the nozzles you are using as this will vary between nozzles and their spacing on the boom. The operator also needs to keep up a reasonable speed to ensure decent work rates, but not at the expense of efficacy – boom yaw and roll can affect coverage significantly.
Drift is a real concern with pre-em sprays at this stage caused by a combination of excessive boom height and forward speed. This can also greatly increase turbulence behind the sprayer. In both cases a high percentage of the active may never reach the soil.
Overlapping must also avoided to reduce the risk of crop damage and to avoid unnecessary expense. Various marker systems can be employed to help, but GPS and individual nozzle shutoff is not being more widely used, consigning overlaps to history between bouts and, importantly at the end and beginning of each bout, especially on short, angled work.
Alternative early weed control
While pre-emergence herbicides form the backbone of the early weed control programme in vegetable cropping, other techniques can be used to help boost their performance or even replace them in certain circumstances.
The use of stale seed-beds should not be overlooked. Drilling or planting into a seed bed that has already grown a flush of weeds which has been knocked out with diquat or glyphosate gets the crop off to a cleaner start, and reduces pressure from early-germinating weeds in the seed-bed/young crop.
The success of this technique obviously depends on weed populations expected, soil type and the range of tools available to control weeds pre-emergence and in the crop, as well as the length of window available between the preceding new crop.
In favourable circumstances it may be possible to get one or even two strikes before drilling takes place.
In drilled crops, particularly those that take longer to emerge, it is worth considering well-timed applications of diquat to assist in the early control of weeds. This approach is crop safe, provided that no plants are emerging and extremely cost effective compared to post-emergence herbicides, mechanical hoeing or, worse still, hand weeding.
Early weed control is essential to ensure the crop suffers as little competition as possible and gets off to the best possible start.
Optimum efficacy of pre-emergence herbicides is achieved through good timing and ensuring applications are carried out in favourable conditions.
Applying pre-emergence herbicides before any emergence of weeds is vital. This means in an ideal world the sprayer should be into the crop soon after drilling and after first irrigation or rain.
Applying to a damp seed-bed optimises efficacy. Soil will also settle around seeds and transplant roots after rain or irrigation, reducing the risk of herbicide being washed down the soil profile.
A firm, fine seed-bed should be the aim. This means careful planning of cultivations to minimise clod formation and to ensure those that do occur can be dealt with effectively. However, care must be taken on poorly structured soils to ensure these do not cap after drilling/rolling.
Seed-beds that contain larger clods are difficult to treat effectively with pre-ems. The even coverage that is required to ensure optimum performance in terms of optimum weed kill with minimum crop damage is a real challenge, due to the uneven surface that can result in shading some areas and over deposition in others.
However, research work carried out by Agrovista has shown that fitting forward-angled and vertical nozzles alternately along the sprayer boom can make a significant improvement to the evenness of deposition. This, though, should not be relied upon as a get-out-of-jail card – the primary aim must still be to achieve the best possible seed bed in the available cultivation and drilling window.
Nozzle Height
Check out the recommended height for the nozzles you are using as this will vary between nozzles and their spacing on the boom. The operator also needs to keep up a reasonable speed to ensure decent work rates, but not at the expense of efficacy – boom yaw and roll can affect coverage significantly.
Drift is a real concern with pre-em sprays at this stage caused by a combination of excessive boom height and forward speed. This can also greatly increase turbulence behind the sprayer. In both cases a high percentage of the active may never reach the soil.
Overlapping must also avoided to reduce the risk of crop damage and to avoid unnecessary expense. Various marker systems can be employed to help, but GPS and individual nozzle shutoff is not being more widely used, consigning overlaps to history between bouts and, importantly at the end and beginning of each bout, especially on short, angled work.
Alternative early weed control
While pre-emergence herbicides form the backbone of the early weed control programme in vegetable cropping, other techniques can be used to help boost their performance or even replace them in certain circumstances.
The use of stale seed-beds should not be overlooked. Drilling or planting into a seed bed that has already grown a flush of weeds which has been knocked out with diquat or glyphosate gets the crop off to a cleaner start, and reduces pressure from early-germinating weeds in the seed-bed/young crop.
The success of this technique obviously depends on weed populations expected, soil type and the range of tools available to control weeds pre-emergence and in the crop, as well as the length of window available between the preceding new crop.
In favourable circumstances it may be possible to get one or even two strikes before drilling takes place.
In drilled crops, particularly those that take longer to emerge, it is worth considering well-timed applications of diquat to assist in the early control of weeds. This approach is crop safe, provided that no plants are emerging and extremely cost effective compared to post-emergence herbicides, mechanical hoeing or, worse still, hand weeding.