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 problem with in soybean 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 a 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 – in this case soybean. According to the U.S. Drought Monitor map for Missouri (Figure 1), much of our state is classified as abnormally dry or in a moderate drought. Portions of our state are in a severe drought. 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.

Figure 1 Drought monitor map for Missouri (updated March 14).

The last factor that I will discuss related to the likelihood of herbicide carryover 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. What few studies I could find with mesotrione showed that degradation became slower as soil pH decreased, which is consistent with some soybean samples I received several years ago from a field that had a very low pH (~5) and were laden with symptoms of mesotrione injury. As mentioned previously, clopyralid degradation is almost completely microbial in nature, so soil pH has even less of an impact on the carryover of this herbicide in the soil.

Unlike soil pH, however, the soil organic matter content is a characteristic that can affect the breakdown of group 27, HPPD-inhibiting herbicides or clopyralid. In general, the higher the organic matter content of the soil, the greater the microbial degradation and the lower the organic matter content, the less microbial degradation.

So what all of this means to me is that fields that you intend to plant to soybean this season that are at greatest risk of herbicide carryover are those that: 1) have not received much rainfall between last summer and this spring and are in a drought area of the state, 2) contain sandier soils and/or lower organic matter content, and 3) have had any of these herbicides sprayed last year in a manner that could lead to more carryover (i.e., late or "double-up" applications).

If you are concerned about the possibility of herbicide carryover in any of your fields this season, you still have time to act. One way to check for the possibility of herbicide residues is to conduct a soil bioassay now to ensure you don't run into a lot of problems once you plant your entire field. If you aren't familiar with the process, I found the University of Nebraska guide A Quick Test for Herbicide Carry-over in the Soil to be an excellent publication that provides step-by-step instructions on how to conduct a proper soil bioassay.