Avoiding spray slips

15 February 2002




Avoiding spray slips

EACH spring millions of hectares of cereals are sprayed with herbicide. The effectiveness of the operation is constantly improving, but basic mistakes are still being made which compromise weed control, and waste both chemical and money, says Syngentas applications specialist Tom Robinson.

Key spring targets are grass weeds and cleavers. If either gains a foothold crop potential can be severely compromised, warns Mr Robinson.

"With an up-to-date sprayer there is every chance that effective weed control can be achieved if the right nozzle is used and the job is done at the right time."

Spraying is essentially simple, he stresses. High-tech gadgets, such as spray monitors, should help not confuse. But they are not vital for good results and even a basic sprayer, in trained hands, can do an effective job.

"However, it is all too easy to make a simple mistake." &#42

and spoil the result. With modern herbicides applied at just a few g/ha of active ingredient there is a lot of pressure on operators. The penalty for a mistake could be costly disappointment.

"With proper maintenance and avoidance of these common mistakes spraying efficiency will be improved and weeds less trouble," says Mr Robinson.

Common spraying errors

Poorly timed applications For best results, particularly on wild oats, apply herbicide at GS31 rather than GS39. For broad-leaved weeds, it is important to spray before they get too big and become harder to kill.

Wrong dose If treatments are late, weeds need to be hit hard with full doses and good quality adjuvant. Early treatments need less chemical, but a frequent mistake is to use a low rate late.

Wrong nozzles The wide range can be confusing and it is easy to select the wrong type for the target and prevailing conditions. Air induction nozzles are often chosen when fan jets would do a better job. Air types are attractive because of their lower drift risk, but they produce fewer and heavier droplets that fail to stick to weeds as well as those from a fan jet.

Too much water Best grass weed control from Topik (clodinafop-propargyl) comes from a water volume of 100 litres/ha. But often four times as much is applied. Excessive water dilutes the adjuvant, reducing its wetting properties.

Too fast Optimum forward speed is 8kph, but the trend is to go faster and many operators now drive at 16kph. As speed increases there is less chance of spray penetrating the crop.

Wrong time of day The best time to spray is in early morning and evening when the air is still. But there is less difference between mid-day and early morning/evening conditions in spring than in the height of summer.

Boom too high Ideally it should be 40cm (16in) above the top of the crop, but often it is set much higher. This reduces penetration and control because droplets slow down before reaching the crop.

Whippy boom Causes variable height and reduces herbicide effectiveness. Easily avoided by proper maintenance. In pre-season checks there should be no more than 12cm (5in) height difference between nozzles across a 24m boom.

Boom isolating system inoperative If isolators are not working chemical can be lost and there is the risk of overlapping or double-dosing in corners and around obstructions.

Worn nozzles Regular checks must be made to ensure nozzles deliver the same amount of liquid across the boom. These are best done using a jug and a watch.

Poorly timed applications For best results, particularly on wild oats, apply herbicide at GS31 rather than GS39. For broad-leaved weeds, it is important to spray before they get too big and become harder to kill.

Wrong dose If treatments are late, weeds need to be hit hard with full doses and good quality adjuvant. Early treatments need less chemical, but a frequent mistake is to use a low rate late.

Wrong nozzles The wide range can be confusing and it is easy to select the wrong type for the target and prevailing conditions. Air induction nozzles are often chosen when fan jets would do a better job. Air types are attractive because of their lower drift risk, but they produce fewer and heavier droplets that fail to stick to weeds as well as those from a fan jet.

Too much water Best grass weed control from Topik (clodinafop-propargyl) comes from a water volume of 100 litres/ha. But often four times as much is applied. Excessive water dilutes the adjuvant, reducing its wetting properties.

Too fast Optimum forward speed is 8kph, but the trend is to go faster and many operators now drive at 16kph. As speed increases there is less chance of spray penetrating the crop.

Wrong time of day The best time to spray is in early morning and evening when the air is still. But there is less difference between mid-day and early morning/evening conditions in spring than in the height of summer.

Boom too high Ideally it should be 40cm (16in) above the top of the crop, but often it is set much higher. This reduces penetration and control because droplets slow down before reaching the crop.

Whippy boom Causes variable height and reduces herbicide effectiveness. Easily avoided by proper maintenance. In pre-season checks there should be no more than 12cm (5in) height difference between nozzles across a 24m boom.

Boom isolating system inoperative If isolators are not working chemical can be lost and there is the risk of overlapping or double-dosing in corners and around obstructions.

Worn nozzles Regular checks must be made to ensure that nozzles deliver the same amount of liquid across the boom. These are best done using a jug and a watch.


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