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When Herbicides Get Hot
By Emily Unglesbee
Friday, June 11, 2021 2:32PM CDT

ROCKVILLE, Md. (DTN) -- Few farmers enjoy working outside when temperatures soar into the 90s or higher -- and they aren't alone.

Herbicides, weeds and crops all perform differently in high heat -- generally not in the applicator's favor. With hot, dry conditions roasting parts of the country this week, particularly the northern Midwest and Plains, weed scientists are asking farmers and applicators to keep these five things in mind before they roll the sprayer into their fields.

1. WATCH THE MERCURY

Droughty conditions expanded across the northern Midwest and Plains this past week, and as often happens, scorching hot temperatures played a role, explained DTN Ag Meteorologist John Baranick.

"For the most part, where drought currently is located is where the heat has been the most intense so far," he said. "Since June 2, temperatures have been in the 90s and sometimes eclipsed 100 degrees from the Dakotas to Wisconsin. One-hundred-degree temperatures have also been a regular occurrence for West Texas."

Cooler temperatures are in the forecast for some of country next week, but not for long and not for everyone, Baranick added. "A front coming through the region Friday is knocking temperatures back several degrees, but we'll see temperatures rise back up into the 90s across the Plains, and possibly the western Midwest -- Iowa and Minnesota -- next week, while temperatures will be much more seasonable elsewhere."

2. TRY TO AVOID CROP INJURY

When heat and dryness stress a crop, its ability to rapidly metabolize and escape herbicide injury lessens. North Dakota State University weed scientist Joe Ikley said this scenario sometimes plays out when systemic herbicides such as growth regulators like dicamba are sprayed on heat-stressed corn plants. "It's just an additional stress on the plant," he told DTN.

Moreover, contact herbicides tend to become more potent in high heat, cautioned Iowa State University weed scientist Prashant Jha.

"Most contact herbicides (carfentrazone, atrazine, fomesafen, lactofen, fluthiacet, acifluorfen) become more active at high temperatures (90 degrees or above)," Jha wrote in a university news alert. "Increased activity of these herbicides under hot weather conditions may improve weed control, but can also increase risk of crop injury."

Growers who are spraying contact herbicides on a hot day and are worried about crop injury can safely use the lower labeled rates of the herbicide and its recommended adjuvant, given that weeds will be more susceptible to them in these conditions, especially if humidity is high as well, Jha noted.

Avoiding applications in the hottest midday hours can also give crops a break, especially corn plants, which tend to "take any stress a little harder on the chin," as Ikley puts it. "If your sole concern is crop stress, then evening spraying would be better than morning spraying, as that gives the crop all night to recover before the heat cranks up again the next day," he said.

3. EXPECT WEEDS TO SHAKE OFF SOME APPLICATIONS

It's not just crops that get weary and sluggish in hot conditions. Weeds will also slow their rate of growth, which is bad news for herbicide uptake, said Ikley. It can be especially problematic for systemic herbicides, which rely on a weed's healthy growth and uptake to do their damage. Aiming for labeled weed sizes will be especially important -- the smaller, the better, Ikley said.

"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," Ikley listed in a university newsletter this week. "Glyphosate and Group 4 (auxin mimics -- e.g. dicamba and 2,4-D) will also have reduced efficacy in these conditions."

Even worse, some weeds actually pack on a tougher, thicker coat in the face of droughty conditions, which will limit the efficacy of any kind of herbicide, Ikley warned. "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," Ikley wrote. "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."

When labels permit, Ikley recommends adding oil adjuvants or nitrogen-based spray additives like AMS or UAN to increase weed response. Keep in mind that this will also increase potential crop injury, he noted. "But many of our broadleaf crops, and specifically soybean, can recover from this type of injury," Ikley concluded. "In most cases, the yield loss due to weed competition would be worse than any crop response from the adjuvant."

Keep in mind that weeds' attempts to protect themselves from high heat and sun can thwart herbicides, as well. Grassy weeds will roll up like a cigar and broadleaf weeds will droop downward, both of which decrease spray coverage. Consider spraying in the morning and evening (while keeping an eye on temperature inversions) to avoid this, Ikley said.

4. BRACE FOR HERBICIDE EVAPORATION

Hot, dry air will result in more evaporation of fine spray droplets, often before they can reach their target weed leaves. Knowing the percentage of driftable fines that your nozzle produces is a good first step to avoiding this, Ikley said.

"If you know that percentage, it might be worth it to see if you can switch to a nozzle with similar droplet sizes, but lower driftable fines," he said. Specifically, you'll want to reduce the number of droplets smaller than 100 microns, added Ohio State University pesticide specialist Erdal Ozkan, in a newsletter this week.

Larger droplets are much more likely to survive the hot, dry fall to the crop canopy, Ozkan explained. For example, whereas a 70-micron droplet will completely evaporate after traveling 13 feet at 86 degrees, a 150-micron droplet will lose only 3% of its size in those conditions.

Another way to reduce those tiny, evaporation-prone droplets is to use drift-reducing agents, or DRAs, in your spray mixture, Ozkan wrote.

Finally, consider lowering your spray pressure, while slowing your sprayer speed. "What reducing spray pressure does is make larger droplets," Ikley explained. "The lower the pressure, the larger the droplets."

5. GUARD AGAINST VOLATILITY

As temperatures rise, so does the risk for volatility of herbicides, Ikley said. "Dicamba and 2,4-D are the primary ones we're concerned about," he said.

If you're spraying Xtend crops with dicamba this year, remember that volatility-reducing agents (VRAs) are required in the tank this year -- don't leave them out, Ikley warned.

But there is no such requirement for dicamba applications in corn, he added. "So volatility from dicamba use in corn could be a bigger issue for those spraying in heat," he said. "And we know that dicamba will volatilize more readily off leaf surfaces than bare soil, so given how much leaf area you have in a cornfield, that's going to be a risk."

Keep in mind that one state -- Illinois -- also bans spraying dicamba on days that the temperature climbs beyond 85 degrees, and a host of other new federal and state guidelines exist for dicamba applications this year. See them here: https://www.dtnpf.com/… and here: https://www.dtnpf.com/….

See more from Ikley on spraying in hot conditions here: https://www.ag.ndsu.edu/….

See Jha's article for Iowa State here: https://crops.extension.iastate.edu/…. You can find Ozkan's explanation of evaporation of herbicides in the heat here: https://agcrops.osu.edu/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee


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