Most of the area has received significant, beneficial rains over the last ten days. While it won’t “make” the soybean and milo crops, it certainly helps. The rains are also longer lasting with the higher humidities and lower temperatures. Things have improved dramatically but we still need timely rains, warm temperatures, and an average hard frost date. Now on to today’s topic.
The last two columns focused on the story of glyphosate tolerant soybeans and other crops, what went wrong, why it went wrong, how it could have been prevented, and some of what is occurring to fight this. Today will focus on one of the technologies developed to deal with the development of glyphosate resistant weeds.
In a nutshell, grass control was possible in soybeans without glyphosate (Roundup), resistant broadleaf weeds, remember soybeans are a broadleaf species, like Palmer amaranth presented a huge problem because the chemistries controlling them would also damage or kill soybean plants. The two prominent chemicals are 2,4-D and dicamba, commonly named Banvel. Soybeans were engineered to be tolerant to both these chemistries but we will focus on dicamba. The soybeans were developed along with particular formulations of dicamba. This was approved for sale and sales started. However, there was a problem lurking. A problem that weed scientists were concerned about.
There are two types of herbicide drift from the application target that can cause offsite damage – physical drift and vapor drift. Physical drift is just what it sounds like. The spray is carried by wind and deposited off the field where it lands and causes damage. This is controlled through droplet size, adding products termed drift reduction adjuvants to the tanks, and not spraying above certain wind speeds. This is obvious. Vapor drift is a different matter.
In vapor drift, the chemical is sprayed and deposited where you want it. However, under the right atmospheric conditions, the chemical deposited on the field becomes a vapor and reenters the atmosphere. If it is moderately windy and the temperature at the surface is warmer than the air above, the vapor will likely disperse and not cause much of a problem. But, if there is a temperature inversion where the air temperature increases with height instead of decreasing which is normally the case, even if only for a brief period of time, the vapor will stay near the ground. Combine this with light winds and the herbicide, in this case dicamba, will move along the ground and not disperse. If the dicamba lands on a grass plant it probably is no big deal but if it lands on plants like soybean, sunflower, and cotton that are not dicamba tolerant, the plant will be damaged or even killed.
Not at herbicides are susceptible to vapor drift, however, both dicamba and 2,4-D are. Temperature inversions are more common early in the morning and later in the evening. Logically then you would delay spraying until the inversion is gone, say mid-afternoon. The problem is spraying during those times is common for one major reason – it’s typically when wilds are the lightest and you can avoid physical drift. And you can spray when conditions won’t favor vapor drift and several hours later conditions will change. Even doing everything right may lead to problems.
To wrap up, a significant acreage in states like Missouri and Arkansas where they grow a lot of crop acreage of tolerant and not tolerant to dicamba soybeans, this year has been a mess. And to make it worse, the damage may not be evident immediately but noticed later. And this is true everywhere the tolerant and non-tolerant crops are grown. The result is involving lawyers, producer groups, and state ag agencies trying to deal with the mess and minimizing it in the future.

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