Last spring/early summer, I probably received more calls and complaints about herbicide carryover to soybean than I had received in my entire career up to that time.
Most of these cases could be directly related to the persistent drought that we experienced in many areas of the state. Unfortunately, I haven’t really seen a major shift in our weather patterns from last summer through this spring that alleviates these concerns as we head into this growing season.
The potential for herbicide carryover will be determined by a variety of different factors, but for the purposes of this article I will just discuss a few that I believe are the most important and most relevant to our situation heading into 2024.
First, the likelihood of herbicide carryover is dependent on the persistence (or half-life) of the herbicide in question. In our case, the herbicides that we saw the most problems with in soybeans last year were the group 27, HPPD-inhibiting herbicides like mesotrione (Callisto, etc.) and topramezone (Impact, Armezon, etc.).
While these herbicides haven’t historically been all that much of a concern for carryover to soybean from one year to the next, there are some conditions that are likely to increase their carryover potential.
These include applications of these herbicides that were made much later than usual during the growing season than might typically occur, and/or “double-up” applications of these herbicides in corn (some amount or combination of these products were applied pre-emergence and post-emergence).
Another herbicidal active ingredient worth mentioning here is clopyralid. Along with mesotrione, I would classify clopyralid as an active ingredient that is fairly persistent and has become a more common component in a lot of corn herbicide premixes over the past few years (such as Kyro, Maverick, Resicore, Surestart II, Tripleflex II, etc.).
Let me be clear that I have not received very many calls about clopyralid carryover injury to soybean, but since it is an herbicide that depends exclusively on microbial populations for its degradation, I thought it was worth noting.
A second factor that can increase the likelihood of herbicide carryover is the environmental conditions that have occurred since the herbicide in question was applied and the time you intend to plant the sensitive crop.
Herbicides will persist much longer (and degrade much slower) in dryer soils due to decreased microbial and chemical degradation. This is more than likely what led to the situation we experienced with herbicide carryover last year, and it is what I am concerned about this year.
The other piece of the herbicide degradation puzzle, however, is that soil temperature also plays a role in herbicide degradation. Herbicide degradation will slow down considerably in cool soils and completely cease when soils are frozen. Herbicide degradation will be faster in warm soils. So, while our soils haven’t been all that cool this winter, they have been fairly dry.
The last factor is the soil characteristics of the field in question. In most cases, the pH of the soil is an important consideration in the degradation of herbicides. For instance, degradation of atrazine (group 5) and certain sulfonylurea (group 2) herbicides is likely to be much slower in soils with a higher pH. However, soil pH is likely not as important of an issue when it comes to the degradation of the group 27, HPPD-inhibiting herbicides or clopyralid.
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