Ideal weed control occurs when plants are actively growing, when weather conditions are warm and there is no lack of moisture.
The 1998 experience of dry spring conditions and resulting failed weed control programs have caused many farmers to re-evaluate their herbicide program. With present (May 4) dry soil conditions and forecast rains never materializing - to date- in Southwestern Ontario, many pre emerge herbicide programs could fail either from lack of moisture for activation or from deep germinating weeds. The option is to shift to post emerge weed control programs that are based more on contact and less on rainfall activation. However, in post emerge weed control programs, farmers must also deal with plants that are under drought stress.
When crops are growing in drought stressed conditions, farmers have one of three choices. The first is to wait for moisture, for a return to ideal growing conditions. The second is to apply herbicides at the proper stage. The third choice is to switch to a mechanical weed control such as inter row cultivation.
Shallow cultivation, or in some situations rotary hoeing, should be considered as an option. This option will be most effective where soil surfaces are dry, when air temperatures are warm with a strong breeze. Many farmers worry about additional moisture loss from cultivation, however, this is usually minimal compared to potential moisture loss from unchecked weeds.
Where herbicide application is the weed control method of choice, most experts agree that timely application will give the best weed control for annual weeds. Waiting for rainfall carries the risk that drought stressed weeds will continue to grow and become more difficult to control regardless of soil moisture.
A weed under drought stress will "harden off", or will acclimatize to dry conditions by creating a more waxy cuticle to prevent moisture loss. Many weeds will appear stunted so that going by herbicide label height charts will not be as accurate as determining leaf stage.
With contact herbicides good coverage is essential. Proper water volumes and nozzle selection combined with use of full recommended rate adjuvants are essential. Whether an oil concentrate that reduces evaporation of the spray droplet and improves penetration, or a non-ionic surfactant that improves leaf surface coverage, these products are fundamental companions for some herbicides in drought conditions.
When applying herbicides to drought stressed crops many farmers worry about crop injury. Crop injury is most serious when temperatures are cold and soil conditions wet and herbicides can't be metabolized quickly enough. Under drought conditions late evening or early morning application is preferred since plants are under the least stress at that time. Stomas are open, and herbicide uptake will be at its best. Some weeds such as velvetleaf fold their leaves downward in the dark, resulting in poor contact of herbicides with weed.
Scouting fields is important. Know what weeds need to be controlled, how heavy the weed pressure, what stage they are at and, what unique characteristics they present.