Tips for Spraying In Dry Weather

Efficacy of postemergence herbicides, particularly those that are translocated within the target weed, is highly dependent upon active plant growth. Typically, the better the growing conditions, the better the performance of postemergence herbicides. Good soil moisture, moderate temperatures, and high relative humidity are conditions favorable for maximum growth and herbicide activity.

Why do weeds escape in dry weather? Many weeds, especially wild oat, are highly efficient at extracting even limited amounts of available soil water. The combined effects of drought and weed competition can severely decrease spring wheat and barley yields.

Further, weeds do not grow as quickly in dry conditions and metabolism is slowed. Many weeds are shallow rooted and stop growing in dry soil. Perennial weeds may handle drought better because underground roots can tap into moisture deeper in the soil profile.

This does not mean perennials will not be affected by drought. Look at leaf composition: plants grown in dry conditions have smaller leaves and develop a thicker cuticle which amounts into less droplets intercepting leaves and less herbicide absorbed.

Superior adjuvants can help overcome leaf barriers like thick cuticle. Most POST herbicides used are translocated, so movement through the cuticle and then through the plant to growing points are critical for adequate control.

Where product labels permit, addition of crop oil concentrate rather than nonionic surfactant usually results in greater herbicide activity. Some products also allow for N-based spray additives which tend to improve efficacy of certain products during periods of slight stress.

Dust is another problem and can inactivate many herbicides. That is why poor control can be seen behind tractor tires and along gravel roads. Do not cut herbicide rates. We have been spoiled the last several years with ideal weed killing conditions. Many have found, especially with wild oat herbicides, excellent weed control with reduced rates. However, this is not a good idea during prolonged dry weather.  For best results, use the highest recommended rate and use superior adjuvants if allowed on the label.

 Consider spot spraying—wild oat patches, for example—if crop quality and quantity is poor and in jeopardy and may not justify complete field spraying. 

Applying herbicides in the early morning (except glyphosate) may improve weed control in dry conditions. During the night, the plant has recovered some from the heat and stress of the previous day. The leaves may not be wilted as much, increasing the leaf’s surface area. The dew may help to keep spray droplets hydrated longer allowing better absorption.

Outside of proper calibration, good working components, and quality nozzles, there is not much to improve performance from your sprayer in dry conditions.

Puma and Discover have greater efficacy in cool, moist conditions and Assert and Everest have greater efficacy in hot, dry conditions.

Essentially all postemergence herbicides have a statement on the label regarding weed growth and environmental conditions. A typical example is “Do not apply to grasses or crops under stress such as stress due to lack of moisture...as unsatisfactory control may result.” To attain adequate control, weeds must be actively growing and must not exceed the maximum growth stage.

For some weeds, delaying postemergence control efforts until moisture stress is relieved is not feasible. Many postemergence treatments are effective only on small weeds, and small weeds are always easier to control than larger ones. For weeds under drought stress, use the highest labeled rates for the herbicides and possibly alter adjuvants if label allows.

The decision of delaying a herbicide application and waiting for a rain or spraying drought-stressed weeds is debatable. Control is generally unsatisfactory when weeds are drought-stressed, but delaying an application often allows the weeds to get too big, making control even more difficult. Making this decision must be done on a field-by-field basis.

If rainfall is in the immediate forecast, post-emergence herbicide activity will be maximized by waiting until after rain to apply, but large drought-stressed weeds can be extremely difficult to control. Where there appears to be no rain coming, apply when weeds are small for best activity. A later application may be required if late rain stimulates a new flush of weeds.

Follow label directions and remember every product is different.

—Bev Durgan, U of M extension weed specialist, Richard Zollinger, NDSU extension weed specialist

Weed Info Online

North Dakota State University www.ag.ndsu.nodak.edu/weeds
(with link to weed I.D.)

South Dakota State University  http://plantsci.sdstate.edu/weeds

University of Minnesota http://appliedweeds.coafes.umn.edu
(see link “MN Weed Research Reports” for product performance results for various crops)

Weed Science Society of America  www.wssa.net
(includes link to weed photos and I.D.)