14 June 2002

Making most of pesticide


The sprayers all set up and

ready to go; but is it set

up correctly for todays

workload when various

crops and targets present

different challenges?

Peter Hill reports

CHOOSING the best pesticide and most appropriate dose rate is only half the battle when it comes to achieving effective control of weeds, pests and diseases.

The rest comes down to using the application technique that ensures the maximum amount of spray reaches the target. And given that weeds, pests and diseases, and the crops in which they occur, present a diversity of targets, that is no easy task.

To make the most of the pesticides potential, operators need to appreciate:

&#8226 The way sprays of different droplet compositions behave;

&#8226 How behaviour is influenced by different crop shapes and densities;

&#8226 How best to set up the sprayer to hit different targets.

Only then can operators juggle spray characteristics, water volume, spraying speed, nozzle type/size and boom height to find the perfect combination for best possible control on the day.

A visual image of different spraying situations – seen from the spray nozzles perspective – can help understand the challenges involved.

Take blackgrass and similar grass weeds. Viewed from the usual crop-walking perspective in early spring, infestations stand out well enough and appear to present a decent-enough target.

But when viewed from directly above, it becomes apparent that plants are thin on the ground, even when the population is relatively high, while slim leaves and an upright growth habit offer few landing sites for spray droplets travelling earthwards from the spray boom.

At least in cereals the crop presents little or no barrier; in oilseed rape, however, those pernicious weeds hide beneath a canopy of leaves.

Consider also the contrasting challenges presented by more mature cereal crops and broad-leaved types such as potatoes. While the former presents a relatively open, upright canopy, with targets at the bottom, in the middle or at the top of the plant, the latter offers a dense canopy with targets (be it pests or diseases) often hidden beneath.

For a further contrast, consider a crop of onions in need of an effective fungicide spray; looked at from above it becomes clear that spray droplets falling vertically from the nozzle are more likely to end up on the ground than on the shoots.

As with controlling early blackgrass, some horizontal movement of the spray is partly the key to meeting the onion spraying challenge, simply because there is then much more likelihood of droplets intercepting leaf or shoot surfaces.

That calls for a relatively fine spray and the help of an enthusiastic operating speed, light air assistance or even the gentlest of breezes. Using low water volumes should help by ensuring that droplets that do hit the target deliver sufficient fungicide for effective control to be achieved.

Autumn and spring control of broad-leaved weeds represent less of a challenge because their leaves are bigger and tend to lie prostrate. So a coarser spray, more likely to move straight to the target without drifting off, should do the trick.

The same applies to late weed control as targets are increasingly shielded by the crop. In this situation, getting droplets past the foliage barrier on to the weeds is the primary aim.

Things get more difficult when it comes to hitting targets on the crop itself, since the aim then is to concentrate sprays on a particular part of the plant.

Typical examples include the base of cereal plants in the case of eyespot, or the mid-plant leaves when septoria and rust need to be stopped in their tracks. If sooty moulds are the target, fungicide applied much lower than the ears is wasted.

The key to setting up the sprayer to cope with these situations is to consider the way droplets of different sizes behave and how that influences where they end up on the plant.

As a general rule, larger droplets move from the sprayer towards the ground fastest and with a near vertical trajectory. In cereal crops, therefore, they are more likely to burst through any upper foliage and end up depositing pesticide on the lower leaves and stem – as well as on the ground.

Medium-size droplets, travelling more slowly, are more likely to enter the crop at a bit of an angle because of the forward motion of the sprayer. This, and the lack of velocity and mass, means their progress is likely to be arrested by the crop. So they end up distributed on the upper half of the plant.

Small droplets, light in weight, are lazier. Without some form of propulsion from air assistance, they will gently float into the crop to settle on upper leaves and the ears.

In crops with a dense canopy, such as potatoes and sugar beet, the opposite scenario generally applies because large droplets hit the large leaves, collapse and spread. That is a problem when trying to get herbicide on to weeds beneath the canopy.

Fine droplets, on the other hand, are inclined to swirl about and will be carried by gentle air movement into the canopy and on to stems and the underside of leaves, which if they contain fungicide or insecticide, is where they are needed.

Air assistance can help in propelling spray into the crop and creating the air movement beneath the canopy that gets droplets on to underside surfaces of leaves.

The same effect can be achieved in cereals and other upright crops to some extent and, in both cases, the amount of air used needs to be sufficient to displace air within the crop but not so much as to pump the spray out again.

Penetration into dense canopy crops can also be helped by exploiting the gaps between different layers of foliage – view potatoes from above and from 45í and it is immediately apparent that the crop presents plenty of openings for sprays delivered to the crop at an angle. That is why a combination of backward and forward facing standard nozzles, or nozzles with two angled tips, have proved very effective in this situation. &#42

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