Jan. 5, 2012 - The direct effects of the 2011 drought have been clear for months, but some experts warn that growers are about to encounter indirect issues as a result of last year's abnormally dry weather. Dry soils may still be holding residue from last year's herbicide applications, creating problems for those switching crops this year.
"Normally every herbicide label that has soil activity it can be that it has pre-emergent activity on weeds or what not," explained James Locke soils and crops consultant for the Samuel L. Roberts Noble Foundation. "It will have what they call a 'plant back interval' for different types of crops. It varies with the different classes of herbicides and what they are acting on."
Typically the number one method of natural herbicide breakdown, which the drought would affect, is by microbial activity. Microbes naturally occur by the billions in the soil are negatively impacted by drought. Normally, these microbes are able to break down herbicide residue and reduce the likelihood of harm to future crops.
"In a drought situation obviously the soils are very, very dry, so those microbes are not active," Locke said. "There are not as many of them as there would be under normal conditions. So, they don't break down the herbicide as quickly and you have the opportunity for carry over to last longer than what it is actually listed on that herbicide label."
What is recommended in many herbicide labels is a bioassay test. There are two different ways to perform the test. The first way is to take small samples of soil out of the field from different areas the grower is concerned about and plant a herbicide susceptible plant, usually whatever they think they will be planting in the next season.
"You will plant that in those little test flowerpots or wherever you put that soil," Locke explained. "You take the plant that you want to grow and watch if it shows herbicide injury. If it does, then you know you have a carry-over issue and you need to make other plans or at least know that residue is there. Of course, if you see no symptoms then you feel like you have got a fairly safe opportunity to plant back into that location without having a herbicide carry-over problem."
The other method that most herbicide manufactures recommend is a field bioassay -- where the grower goes out into that field and plants test strips of the intended crop across the field and monitors those test strips for injury.
"Of course that can be somewhat difficult," Locke said. "Either the time of year for the crop to grow may not be correct, or if you don't have irrigation, as the drought continues, your test crop may not come up. Then, most guys don't want to get out there and hook up a planter and do all that kind of stuff just to go out there and do a few test strips. But, those are the two methods that are typically recommended to tell whether or not you are going to have a herbicide carry-over problem."
Other methods of herbicide breakdown are by sunlight or oxidation, both are chemical reactions the drought really doesn't affect, ensuring some breakdown from last season.
If the bioassay test indicates a herbicide carry-over potential, the upcoming growing season isn't necessarily a lost cause.
"It really depends on which herbicide it is that is carrying over," Locke explained. "Typically, what will happen is that if you test it and know that you have an issue, most people will change what they are going to go back and plant with. All of those herbicides have different levels of activity.
"For instance, if you are looking at a particular herbicide that carried over and it had a lot of activity on clovers or legumes, and you had been planning to plant alfalfa, instead of planting alfalfa, you would choose to come back with wheat, because wheat was tolerant of those herbicides," he continued. "That would be your usual way to go about mediating the affect of that herbicide carry-over."
Locke said the most important way to prevent being affected by problems like herbicide carry-over is to be proactive against possible drought-related growing problems. But he mentioned that while the drought has made way for weed issues, herbicide carry-over and other potential growing problems in 2012, there is a silver lining.
"At least one good potential out of it is that we could possibly have more clovers and legumes come into pastures in those open spaces, because of that bare ground that was out there," Locke said. "It is the same thing that kind of happened after the drought of 2006. We saw a lot of that, which is about the only good thing you can say about a drought."
