Do Water Rates Really Affect Weed Control Efficacy?

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“The take-home message is to identify growing conditions and select a water rate and nozzle type suited for the condition”

Growers have questioned water rates for many years considering some sprayers and aerial applications have been using less than 56 L/ ha (5 gpa). Industry generally believes high water rates provide the proper coverage and penetration needed for optimum weed control.

“But farmers would like to use lower water rates for economic and practical reasons,” says Brian Storozynsky, sprayer technology specialist with the AgTech Centre in Lethbridge.

Current water rates used by southern Alberta growers to apply foliar fungicide, for example, range from 10 to 20 gpa (112 to 224 L/ha) and to apply herbicides, rates range from five to 10 gpa (56 to 112 L/ha). In some desiccation applications rates up to 40 gpa (448 L/ha) are recommended.

High water rates certainly reduce spray drift and provide extra coverage especially for grassy-type weeds, where extra coverage is more beneficial than in broadleaf weeds, considering the low number of spray droplets that typically end up on vertically growing weed leaves and stems.


Overall, different water application rates of 10, 20 and 30 gpa (112, 224 and 336 L/ha) in the 2005 and 2006 fungicide study in dry beans did not significantly affect disease incidence. This trend was similar to studies done between 1999 and 2001, with other foliar fungicides on dry beans using water rates of five and 15 gpa (56 and 170 L/ha).

Over four years, water rates of five and 20 gpa (56 and 224 L/ ha) used to desiccate potato crops prior to harvesting were studied. Again, no significant difference resulted from using less water as both treatments still resulted in requiring a second application to completely desiccate the potato vines.

Five years of testing using herbicides in cereal crops and field peas showed similar results in water rates. At recommended chemical rates, results were similar for 10 gpa (112 L/ha) and five gpa (56 L/ ha) with the nozzle types studied. All nozzles used in these studies were operated at manufacturers’ recommendations. Only one year of a five-year study using a contact herbicide in a canola crop showed applying five gpa (56 L/ha) had less weed control than at 10 gpa (112 L/ha). “This may be attributed to moisture conditions and weed populations being abnormally high that spring,” says Storozynsky .

Some years, in adverse growing conditions such as advanced weed stage and high weed populations, plots sprayed at 10 gpa (112 L/ha) showed weed control was usually more consistent and scored just slightly higher than at five gpa (56 L/ ha), although results at both rates were always commercially acceptable “This indicates that identifying growing conditions is as important as selecting water rates,” says Storozynsky.


Most spraying technologies performed similarly at low and high water rates when appropriate nozzles were used at recommended pressures and heights. Herbicide efficacy depended more on herbicide type and rate, weed growth stage, population and growing conditions rather than water rates.

“The take-home message is to identify growing conditions and select a water rate and nozzle type suited for the condition. If in doubt, follow the chemical manufacturer’s recommendations for water rates and nozzle type,” says Storozynsky.

During extremely wet growing conditions or heavy weed populations, using recommended chemical rates, spraying at early stages and using higher nozzle pressures improved efficacy, especially for venturi-type (air-induced) nozzles. Not all air induction nozzles produced the same coverage and spray droplet sizes. Some research has shown that using high-pressure air-induction nozzles required higher water volumes or higher operating pressures to achieve commercially acceptable control for grassy-type weeds.

In terms of the sprayer type used, air assist systems showed more potential in fungicide applications, while electrostatic sprayers were more effective in pre-burn applications.


Spraying technologies performed similarly when appropriate nozzles were selected and used at recommended pressures and spraying heights, he adds. Producers can access assistance by checking the AgTech Nozzle Selector. That independent tool is being developed by the AgTech Centre to assist producers with knowledge on various nozzle types.

More information on the AgTech Nozzle Selector is available in a Special Report available on the independent website Canada Sprayer Guide at

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