Minnesota Heat! Tips for applying herbicides in hot weather

Spring planting in many areas of Minnesota and North Dakota was delayed due to unseasonably cool and wet conditions which have resulted in delayed crop emergence and crop growth. However, crop and weed growth has greatly accelerated with recent above average air temperatures. As a result, postemergence herbicides have been applied in less than desirable hot temperatures and windy conditions thus limiting the desired application window for area row crops.

Some areas have received rainfall this month and while that’s a brief respite from the above average temperatures, the weather forecasts still predict some hot (90 – 100 degree) and dry weather as we continue the post emergence spray season. There are several details one needs to consider when making postemergence applications in these conditions.

Depending on local rainfall (lack of rainfall) or supplemental irrigation, weed height may be highly variable across North Dakota and Minnesota fields. For example, there are already reports of larger kochia, waterhemp, ragweed and lambsquarters in many fields. Crops planted in minimal tillage situations may have allowed weeds a head start in germination and emergence, especially in fields where no burndown application was applied. Thus, strict adherence to labeled weed sizes on many postemergence herbicides is strongly encouraged.

Last year, we spent a lot of time discussing the control of drought-stressed weeds during our hottest days. The contrast this year is that most folks have adequate moisture, and we are dealing with weeds that are growing fast, but will likely shut down growth in the peak heat of the day.

We also tend to have a southeast wind that is pumping in humid air from the Gulf of Mexico; contrast that to last year’s southwest winds bring in more arid air masses over dry soils. So a general rule of thumb is to expect better weed control due to adequate moisture (and yes, also expect more crop injury for the same reasons).

The use of oil adjuvants, and specifically MSO along with nitrogen fertilizers (AMS or UAN), can improve the herbicide uptake if one needs to spray during the hottest parts of the day. Some may be wary of using oil adjuvants due to increased crop response, but many of our broadleaf crops, and specifically soybean, can recover from this type of injury. In most cases, the yield
loss due to weed competition would be worse than any crop response from the adjuvant.

Herbicides that will have the largest drop in performance during hot conditions are usually systemic herbicides like Group 1 (ACCase inhibitor – e.g., Select Max, Assure II, and Puma, etc.) and Group 2 (ALS inhibitor – e.g., Raptor, and Pursuit, etc.) herbicides. Glyphosate and Group 4 (auxin mimics – e.g. dicamba and 2,4-D) will also have reduced efficacy in these conditions.

On the other hand, contact herbicides, such as Group 14 (PPO inhibitors – e.g., Flexstar, and Cobra, etc.) and Group 10 (glutamine synthetase inhibitor – Liberty) herbicides become more active under higher temperature. Contact herbicides including Cobra, Liberty, Reflex applied in hot, humid conditions will likely result in greater foliar injury to crops, but also greater weed control.

Finally, consider the leaf angle of weeds throughout the day. Like our grass crops, grass weeds will roll their leaves during the peak heat of the day to conserve moisture. Broadleaf weeds will usually be droopy. Both scenarios will lead to decreased spray coverage simply due to leaf architecture. Thus, spraying in the morning or evening will also help with coverage on weeds in
hot dry conditions. In general, applying systemic herbicides early in the morning, after plants have had a chance to recover from heat stress, will give the best chance for the herbicide to reach the active site and effectively kill weeds.

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