Postemergence Herbicide Applications in Hot Weather

By Joe Ikley and Tom Peters et.al

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.

Rapidly growing giant ragweed in corn.
Photo: Lisa Behnken

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 postemergence spray season. There are several details one needs to consider when making postemergence applications in these conditions.

Weeds

Depending on local rainfall patterns (or the 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.

Herbicide performance

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.

Fate of spray droplets

In 2021, we experienced hot, dry weather during spray season. Herbicide performance may have been related to small spray droplets evaporating before reaching their intended target. The current forecast keeps our humidity levels higher in comparison. This means that overall, we can expect less loss due to droplet evaporation. We can of course still get the occasional day with hot, dry air masses, and Delta T will be very important during those days. But in general, expect less droplet evaporation, leading to better weed control and potentially more crop injury compared to 2021.

Herbicide volatility and dicamba application restrictions

The volatility of herbicides is also increased when temperatures are high. Dicamba is often the first herbicide that comes to mind when we discuss volatility due to the off-target movement concerns over the last several years. A reminder that any dicamba application in Xtend or Xtendflex soybeans requires the use of a volatility reduction agent (VRA) to help decrease the risk of volatility in those applications. 

In contrast, applications of dicamba in corn are currently taking place in some areas and those labels do not require the use of a VRA. There will be an increased risk of volatility from dicamba applications during hot weather. There is also a risk of increased corn injury from dicamba in hot and dry conditions. In general, it would be best to target dicamba applications in corn for more favorable weather.

Dicamba restrictions

The U.S. Environmental Protection Agency (EPA) approved Minnesota Department of Agriculture’s (MDA) state-specific use restrictions for three dicamba herbicide products during the 2022 growing season in Minnesota. The restrictions (including temperature) are aimed at curbing off-site movement of the products.

• DATE CUTOFF: No application shall be made south of Interstate 94 after June 12, 2022. North of Interstate 94, use is prohibited after June 30, 2022. For North Dakota dicamba products users, dicamba products use is prohibited after June 30, 2022.
• TEMPERATURE CUTOFF STATEWIDE: No application shall be made if the air temperature of the field at the time of application is over 85 degrees Fahrenheit or if the National Weather Service’s forecasted high temperature for the nearest available location for the day exceeds 85 degrees Fahrenheit. Forecasted temperature must be recorded at the start of the application. While there is no temperature cutoff for North Dakota dicamba product users, they should continue to use best practices to reduce herbicide volatility.
• CROP GROWTH STAGE: In addition to the cutoff date, Xtendimax and Tavium have crop growth stage requirements.

Key points

Consider these points for improving herbicide performance in hot and dry weather:

• Adjuvants - Use recommended adjuvants at labeled rate to help spray droplets better-absorb into leaf surfaces by dissolving cuticles and slowing the evaporation rate.
• Spray volume - Increase the spray volume to improve coverage.
• Droplet size - If appropriate, use coarser droplets to minimize evaporation.
• Application timing - Make applications in the morning when plants have recovered from the heat and the leaves are oriented to intercept more droplets, but pay close attention to temperature inversions, which typically occur until just after sunrise (and set up several hours prior to sunset).
• Weed size - Pay close attention to weed size. Weeds will rapidly grow in hot weather with adequate soil moisture. Target small weeds when herbicides are most effective. The target weed size for any technology in soybean should be 4 inches or shorter.
• Don't spray and walk away - Scout fields 7 to 10 days after postemergence applications to determine herbicide performance. If a respray is warranted, then 14 days after the first application is a good interval to target. The longer we allow weeds to regrow from a failed application, the more difficult complete control will become.