The wet weather has made it difficult for timely herbicide applications, but it can also affect herbicide residual activity and potential crop injury issues.
Excessive Rainfall and Herbicides
In those areas with regionalized heavy rainfall and flooded fields, if residual herbicides are already applied and water (and soil) is moving off the field, some of the herbicide will likely be taken with it, or it will simply degrade and lose activity. However, each herbicide will be affected differently depending on its water solubility and soil adsorption characteristics.
Under heavy rainfall events (>4 inches), herbicides can be leached out of the zone and down into the soil profile, which will negatively impact herbicide efficacy. For example, low solubility herbicides such as atrazine, Balance Flexx, simazine, Prowl, Valor, and Zidua-containing products will be more “resilient” under excessive rainfall totals. While medium and highly soluble products such as Callisto, s-metolachlor, acetochlor, metribuzin, Outlook, Pursuit, Reflex, Spartan/Authority, and Stinger will be negatively impacted and could experience more weed escapes sooner. Furthermore, herbicide longevity is also related to how strongly they adhere to soil components and organic matter.
There will likely be more significant herbicide movement and loss in sandy, shaley, or coarse soils with low organic matter compared to those that have medium or heavy textures with higher organic matter content. Another factor at play is microbial and chemical degradation. The majority of herbicides are primarily broken down in the soil by microbes such as bacteria, fungi, and actinomycetes. This typically occurs mainly in the top 12 inches of soil since there is better aeration. However, optimal conditions required for adequate microbial activity include – favorable moisture (>50% field capacity); good soil aeration; mild temperatures (80 to 90°F); optimum pH (6.5-8); and adequate organic matter content.
Certain herbicide classes, such as the sulfonylureas and triazines, are also degraded by chemical reactions in the soil. For a more in-depth discussion of these various factors, refer to an article written by my Extension colleague at the University of Nebraska.
Also, due to this excessive rainfall, weed escapes may be more prevalent sooner. Over the next couple of weeks, monitor those fields for a lack of weed control or weed escapes. Post herbicide applications may be necessary for appropriate weed control. If a field needs to be replanted to corn or another crop, remember to adhere to the crop rotation guidelines of the most restrictive herbicide that was applied.
Stressed Crops and Herbicide Injury
And finally, due to the rainy and cloudy weather, the crops have been struggling to grow and likely will be more sensitive to postemergence applications. Certain formulations such as EC (emulsifiable concentrate) formulations as compared to encapsulated formulations (CS and ME) can cause more injury. For example, Warrant/Enversa and Prowl H2O are generally safer on emerged corn as compared to their counterparts, Harness and Prowl 3.3EC, respectively. Also, there is an increased crop injury potential when combining residual herbicides with loaded glyphosate products or adding additional adjuvants to the spray mix.
Regarding application timing after environmental stress, the general rule of thumb is to allow a few sunny days to pass after coming out of a rainy, cool period before applying herbicides (but we may not have that luxury this season). Since the plants are stressed, this allows them time to build up a thicker leaf surface and to get their metabolic processes functioning at a faster pace to detoxify the herbicide. Also, with all the moisture, warmer temperatures, and eventual sunlight, the plants will be growing very quickly and are succulent, so consider using nonionic surfactant (NIS) instead of crop oil concentrate (COC) or methylated seed oil (MSO) as the spray additive if the label allows.
