Spraying in a Drought: Adapting to Dry Spraying Conditions for Better Weed Control

By Mitchell Japp, Research & Extension Manager

Heat, drought, dust, smoke and wind – spraying in adverse conditions means a change in approach should be considered.

Why Dry Spraying Conditions Require a Different Strategy

There are a lot of factors affecting herbicide efficacy or sprayer effectiveness that become more challenging in the current conditions. Hot and dry conditions can lead to increased drift potential, reduced herbicide efficacy, vapour drift and reduced crop tolerance. As if wind wasn’t enough of a challenge on the prairies at spraying time. And, that combination of environmental factors will lead to more dust – which can increase herbicide binding, meaning that it won’t get to the target surface.

Understanding the Impact of Smoke and Drought on Herbicide Effectiveness

The smoke from northern wildfires may have further impacts. There is no research available on the effect of smoke on herbicide application. But Tom Wolf at Sprayers101 suggests that smoke influences are likely limited – it’s more important to spray in a timely manner, as effectively as possible. The impacts that may occur from smoke would include: influence herbicide uptake due to reduced growth in plants, affect thermal turbulence and decrease relative humidity – increasing droplet evaporation.

Check Herbicide Labels for Drought-Specific Guidelines

Spraying restrictions for specific growing conditions are noted in the Guide to Crop Protection for each product under the section heading “Effects of Growing Conditions.”

Best Practices for Spraying in Drought and Dry Conditions

• Avoid the hottest parts of the day. Shut the sprayer down when it is warmer than 27 or even as low as 25 °C for some herbicides.
• Increase water volume to the maximum recommended on the label.
• Use adjuvants and surfactants as appropriate to improve droplet retention on the leaf, and dissolve wax on the leaf surface for increased absorption.
• Spraying systemic herbicides in the morning when plants are more hydrated can help.
• Avoid low rates with systemic herbicides.
• Contact herbicides can be sprayed in the evening to minimize crop injury; use lower label rates for these.
• Dust has limited impact on most herbicides, but glyphosate and reglone are exceptions. Dust on plants near roads can impact spray contact with the leaf. Higher rates may compensate in those zones.
• Spray early to reduce the moisture and yield loss potential from weeds.

Plan Ahead: Residual Herbicide Risks in Drought Conditions

Many herbicides require adequate water to break down in the soil over the summer. When soils are too dry, residual herbicides remain in the soil longer than normal, potentially affecting next year’s crop. Take this into account when planning your crop rotation and herbicide strategy for the coming season.

Want to Learn More About Spraying in Dry Conditions?

• Herbicide Spraying in Hot, Dry Conditions, Manitoba Agriculture
• Spraying in Dusty Conditions, Sprayers 101
• Dry conditions. Should I spray weeds?, Canola Council
• Weed Control Advice for Dry Conditions – Frequently Asked Questions, Manitoba Agriculture
• Dry Weather Weed Control, Manitoba Agriculture
• Guide to Crop Protection, Saskatchewan Agriculture

Additional reading material on weed control:

• Weed Control Strategies with Dry Conditions, NDSU
• Spraying Weather, Sprayers 101