Postemergence Herbicide Applications in Hot, Droughty Conditions (06/10/21)
Some areas have received rainfall this week and that’s a brief relief from the above 90° temperatures, but the weather forecasts still predict some hot and dry weather as we enter peak postemergence spray season. There are several things one needs to consider when making postemergence applications in these conditions.
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, and lambsquarters in many fields. Also 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.
Weeds will also be tougher to control if they are not actively growing due to drought stress. Herbicide performance is optimized when weeds are actively growing. Weeds that have endured hot, droughty conditions may have already developed a thicker cuticle than normal in an attempt to slow their rate of transpiration or water loss. Even after a field receives rainfall the weed’s cuticle will not “shrink” back down immediately. This means any herbicide-containing droplet will have a tougher pathway to enter the plant. The use of oil adjuvants, and specifically MSO along with nitrogen fertilizers (AMS or UAN), can improve the herbicide uptake. 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 drought 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; however, the low humidity situation may reduce their efficacy. 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.
Fate of spray droplets
To even begin controlling weeds, we need to get spray droplets to the leaf surface. Hot and dry conditions also lead to increased evaporation of spray droplets. One tool to measure evaporation rate is Delta T. All NDAWN stations will currently report Delta T values, which can be helpful in determining the risk of spray droplet evaporation during application. A thorough explanation of Delta T can be found in this year’s online Weed Guide supplemental pages. In brief, the higher the Delta T value, the higher the evaporation rate of spray droplets. When we have hot temperatures with low relative humidity, many of the smaller spray droplets will evaporate before reaching the leaf surface, and more will evaporate off the leaf surface prior to absorption into the leaf. This is of particular concern with contact herbicides when we are often using smaller droplets to increase spray coverage. Spraying in the morning and evening when Delta T is more favorable can help decrease losses due to evaporation, but we must also avoid spraying during a temperature inversion to avoid losses due to off-target movement. Temperature inversions typically set up in the evening several hours prior to sunset and dissipate soon after sunrise.
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 soybeans this year requires the use of a volatility reduction agent (VRA) to help decrease the risk of volatility in those applications. However, applications of dicamba in corn are currently taking place, and those labels do not require the use of a VRA. There will be an increased risk of volatility from these 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 save dicamba applications in corn for more favorable weather.
Key points to consider for improving herbicide performance in hot and dry weather:
- Use recommended adjuvants at labeled rate to help spray droplets better-absorb into leaf surfaces by dissolving cuticles and slowing the evaporation rate
- Increase the spray volume to improve coverage
- If appropriate, use coarser droplets to minimize evaporation
- 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 occurs before sunrise or after sunset.
- Pay close attention to weed size. Weeds may continue rapid growth despite dry conditions. Target small weeds when herbicides are most effective. The target weed size for any technology in soybean should be 4 inches or shorter.
- Scout fields 7 to 10 days after postemergence applications to judge herbicide performance. If a respray is warranted, then 14 days after the first application is a good target to shoot for. The longer we allow weeds to regrow from a failed application, the more difficult complete control will become.
Extension Weed Specialist
Assistant Professor/Extension Weed Scientist
University of Minnesota
Extension Educator – Crops, University of Minnesota
Morris Regional Extension Office
Extension Educator – Crops, University of Minnesota
Farmington Regional Extension Office