30 January 1999


Timely tips for effective crop spraying came thick and fast at the first of a series of application workshops being staged by Crops and Novartis. Peter Hill reports.

TOO many disappointing results from crop spraying treatments are caused by basic errors in spraying technique. That was the underlying premise for a quick-fire update from Novartis applications specialist Tom Robinson on the key things to get right.

At a sell-out Crops/Novartis spraying workshop in Oxford, he identified the principal factors as:

&#8226 understanding the target (be it disease on the upper portion of mature plants or weeds hidden away beneath oilseed rape leaves).

&#8226 selecting the type of spray that will most effectively reach and cover the target.

&#8226 using the nozzle and pressure that will achieve that appropriate spray quality.

&#8226 selecting the water volume, boom height and (if appropriate) adjuvant combination that will get the largest amount of active chemical on to the target.

"Although 50% of the likely effectiveness of any spray treatment is down to the product, and perhaps a further 20% to spray timing, basic sprayer maintenance and performance accounts for another 20%, and fine-tuning – getting the spray quality, boom height and other factors just right – influences the last 10%," he says.

And that final 10% can be significant for critical weeds such as blackgrass and diseases like potato blight that demand near 100% control.

"Blackgrass is the perfect example: at 500 plants/sq m, a 1% increase in control from 98% to 99% means half as many plants are left in the field," Mr Robinson points out. "It is also the case that the more effective a product is, the more sensitive it is to application conditions; so getting the application technique just right really can win a bit more control."

Nozzle flow rate is a case in point. Where pesticides are applied at very low rates, slight variations in flow rate can have a big impact on the rate of chemical applied.

"At 125ml/ha, a 20% plus or minus variation in flow from nozzles across the spray boom makes a difference of 100ml/ha in chemical application," says Tom Robinson. "Too high a flow rate from one or more nozzles simply means wasted chemical; too low a rate means weed control will definitely suffer."

The answer is regular flow rate checks from individual nozzles and tip changes as necessary – preferably a complete set each time – to ensure even output across the boom.

Methods for getting chemical to the target and keeping it there depend greatly on the target, its situation and the way spray droplets behave in different crops.

"Blackgrass is quite well exposed because there is not much crop foliage at the time of spraying," says Mr Robinson. "But plants are thin on the ground, even with quite high populations, and individual plants present a very small target."

That is particularly evident when young blackgrass plants are viewed from above; seen from the side, they present a bigger leaf surface area. The aim, therefore, must be to get some sideways movement into the spray to increase the chance of droplets colliding with the leaves, and also to use a lot of droplets so that as much chemical as possible ends up on the leaves.

A fine spray is the answer, with the boom set as low as possible to minimise drift.

"In our own trials comparing herbicide application at 30cm and 60cm boom heights, we recovered 30% more chemical from weeds where the lower setting was used," notes Mr Robinson. "That may not be practical in a field situation but it illustrates the need to go as low as possible."

In more mature crops, spray penetration is likely to be an issue. In cereals and other crops where plants are thin and upright, larger droplets will get down into the bottom and so are useful for controlling weeds and stem base diseases; medium-size droplets tend to reach the middle of the plants, so are most suitable for mid-season foliar fungicide sprays; while fine droplets tend to land on the upper parts of the plant and suit late season fungicide and aphicide sprays.

In crops with bigger, spreading leaves (potatoes, sugar beet and oilseed rape), the opposite is the case. Larger droplets tend to stay on the upper surfaces of the plant, while fine droplets can swirl about and penetrate the canopy to reach lower leaves. Angled nozzles are helpful to get sprays beneath the canopy while air assistance does the same but with extra droplet movement to get them to the underside of leaves where chemical is absorbed more readily.

Mr Robinson advises: "Water-sensitive paper is an invaluable tool for seeing whether the spray is reaching the target and giving sufficient leaf cover."

Water volumes also have a role here. In general, where there is a large mass of foliage to spray, high volumes are helpful. But where the target area is small relative to the non-target area, then a more concentrated spray gets more chemical where it is needed.

"Again, blackgrass is a good illustration because the small leaves are surrounded by a lot of bare ground and crop foliage," he notes. "We have had consistently better control in trials from using low volumes, and on-farm the increased output low water volumes bring can improve spray timeliness significantly."

Whatever the crop and target situation, choosing the nozzle and pressure combination producing the most appropriate spray is the key decision – tempered by any need to minimise drift because of spraying conditions or anti-pollution restrictions.

"It is a particular concern that low-drift nozzles, particularly the air induction type, are frequently used when they are an inappropriate choice," Mr Robinson warns. "They do a marvellous job of controlling drift and work well in situations where crop penetration is important. But because they produce a relatively coarse spray, they are not suitable for all spraying tasks."

Complaints about poor control from blackgrass herbicides mostly result from using this type of nozzle.