This week will finish our discussion of the effects of weather on pest, specifically weed, control with herbicides. Remember that to combat the Roundup® weed resistance issues that were developing with Roundup Ready® crops new technologies were/are being developed to allow for the use of dicamba and 2,4-D herbicides on soybeans, herbicides that under normal conditions which would severely harm of kill soybean plants. The trouble with this technology, even with new and improved herbicide formulations, is that once applied they can become a vapor under certain environmental conditions and move off target. Sounds simple enough to avoid but it isn’t for the following reasons.
Since producers want to keep these herbicides on the fields they are spraying as they are applied, calm conditions are best. You can spray with “wind” and there are nozzles, sprayer adjustments, and products that can be added to the tank to allow spraying with a decent breeze. However, with these products applicators are particularly sensitive to doing their best to prevent physical drift. When are good wind conditions most likely, night and early morning, normally through about mid-morning. However, these favorable wind conditions lead to a problem.
Under normal conditions, air temperatures decrease with height. As you move from the soil surface and go up, on average, air temperature decreases by approximately 3o F for every 1,000 feet. As air from the surface rises, it is typically moister than the air above it and it cools more slowly so its temperature is higher than the surrounding air so it stays slightly warmer. This makes it buoyant and it continues to rise. A good analogy is the rise of a hot air balloon. When temperature actually increases with height a temperature inversion occurs. When this happens surface air that tries to rise is cooler than the surrounding air. It isn’t buoyant so it can’t rise and stays at the surface. The air “hugs” the ground. Foggy weather is an example here. This condition often occurs during late night through mid-morning, when winds are light. The last two paragraphs are the setup for weather leading to the problem.
There is one more component to add – temperature. Banvel, dicamba, is more likely to volatilize into the atmosphere after application as temperature increases – seventy to eighty degrees is perfect. So applicators trying to minimize the physical drift of the herbicide apply during calm to light wind conditions – early morning. Many late spring/summer mornings this corresponds with a temperature inversion in the atmosphere so surface air isn’t rising. And as the sun rises, heat is provided to the surfaces where Banvel was applied to. The chemical volatilizes from the surfaces it was applied to. If there are decent winds and no temperature inversion, the herbicide is dispersed and no or little damage occurs. If, however, winds are light and there is a temperature inversion, the volatilized Banvel will hug the ground and slowly drift to later be deposited up to several miles from where it was applied. If it lands on soybeans that aren’t dicamba resistant or other broadleaf species, severe plant damage occurs. This is exactly what happened to significant acreage this last growing season.
The public and private sectors of the weed control industry released extensive guidelines regarding ways to minimize this potential problem. Unfortunately millions of acres of soybeans were affected this last growing season, especially in Arkansas and Missouri but in all soybean growing areas. This led to regulatory agencies formulating new guidelines in addition to a significant amount of money paid out in clams.

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.