27 February 1999

On target for maximum effect

The sprayers all set up and ready to go – but is it set up correctly for todays task? Peter Hill asks how to get the spray on target.

GETTING the sprayer in tip-top condition, organising supplies of pesticides and water, judging the best time to spray – all essential elements of a successful crop protection campaign.

But making the most of these resources can only be achieved if the maximum amount of spray pesticide reaches the target. And given that weeds, pests and diseases, and the crops in which they occur, present a diversity of targets, thats no easy task.

The most ham-fisted of spray applications will achieve some sort of result; after all, good timing and the effectiveness of the pesticide itself determines the outcome to a large extent. But to make the most of the pesticides potential, operators need to appreciate:

&#8226 the way sprays of different droplet compositions behave

&#8226 how that behaviour is influenced by differing 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, speed, nozzle configuration and boom height to find the combination for best possible control.

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

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 the picture changes when viewed from directly above. 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 cereals and broad-leaved crops such as potatoes. A relatively open, upright plant on the one hand, with targets at the bottom, in the middle or at the top; a dense canopied crop on the other, with targets (be it pests or diseases) hidden away beneath.

Or 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, inducing the spray to move sideways is partly the key to meeting the latter challenge, simply because there is then much more likelihood of droplets intercepting leaf or shoot surfaces.

That calls for a relatively fine spray – constrained by the need to drift and the uneven application that results – 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 herbicide for effective control to be achieved.

Autumn and spring control of broad-leaved weeds is less of a challenge because their leaves are bigger and tend to lie prostrate. 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 crops increasingly shield the target; getting droplets past the foliage barrier and onto 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. Such as the stem base in the case of eyespot on cereals, or the mid-plant leaves when septoria and rust need to be stopped. If sooty moulds are the target, fungicide applied much lower than the ears is a waste.

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 less quickly, are more likely to enter the crop at an angle because of the forward motion of the sprayer. This, and the lack of velocity and mass, means their progress is more 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 distinctly 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 leaves and spread. But that is not necessarily where the herbicide is needed.

Fine droplets, on the other hand, are inclined to swirl about and be carried by gentle air movement into the canopy – where the pesticide is more often needed.

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

Penetration into dense canopy crops can also be helped by exploiting the gaps between different layers of foliage – view potatoes from above and from a 45 degree angle to get the picture. Nozzles set at an angle (or with angled tips) have proved to be very effective in this situation.

Less drift with equal control

Growers, particularly those with sugar beet crops growing almost inevitably next to water courses, need also to be aware of the risks of drift when using products subject to buffer zone restrictions.

Trials by Libby Powell of Morley Research Centre, Norfolk, show that air entrainment or air induction nozzles – those with small air inlets in the nozzle body – reduce drift without compromising broad-leaved weed control significantly. She warns there might be some loss of control if fine grass weeds such as annual meadowgrass are the main herbicide target when using these nozzles instead of a standard flat fan.

The eight nozzles tested at Morley produce fewer very small droplets than sprays from conventional or low-drift nozzles operating at the same system pressure. Although the sprays were relatively coarser they were still sufficiently fine and gave an even coverage.

Neither drift nor spray deposition were measured in the Morley trials but Libby Powell adds that the Billericay Farm Services Bubblejet nozzle appeared to create least drift. The Turbodrop air inlet nozzle available from Lurmark produced the closest weed control to that from a standard flat fan nozzle but the Turbodrop requires an operating pressure of 5-7bar (72-100psi) to achieve 100 litres/ha spray rate, instead of the more normal 2-3bar (29-44psi).