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Dry Weather & Herbicide Carryover Concerns

Posted by Albert Adjesiwor | July 2, 2021
The memories of 2020 would remain with us for years to come. Most of us couldn’t wait for 2020 to come to an end. We were very hopeful that 2021 would be a better year. For the most part, we would say things are getting back to “normal”. But are things really getting back to normal? If you ask a lot of farmers, I presume the answer would be “NO”. Well, it turns 2021 came with its troubles. For a lot of farmers and agricultural workers, the high temperatures and dry weather has caught our attention (Figure 1). We know drought is bad! It slows down plant growth, reduces yields (sometimes resulting in total crop failure), reduces herbicide efficacy, just to mention a few. Aside from these notable effects of drought on crop production, there are concerns about herbicide carryover.

This spring and early summer, I have fielded several calls regarding herbicide injury, mostly on sugar beets. While a few of these cases were due to drift from neighboring fields, the majority of the sugar beet damage I have seen this year was due to carryover from the previous crop season. The risk of herbicide carryover to the next crop is determined by a myriad of factors including but not limited to:

  1. the type of herbicide and innate persistence of the herbicide
  2. herbicide application rate
  3. amount of moisture (irrigation and precipitation)
  4. soil type and characteristics
  5. length of time between herbicide application and the crop of interest, and
  6. cultural practices (e.g., tillage).

I will mainly focus on moisture in this post.

Why is moisture so important when it comes to herbicide carryover? Well, most herbicides are broken down by soil microbes and various chemical processes. These processes all require soil moisture. Dry weather means all these processes would either be slowed down or brought to a screeching halt. For example, imazamox (Beyond®), a Group 2 herbicide used in Clearfield wheat could have up to 36 months of the plant back restriction to barley if there is less than 16 inches of moisture (precipitation + irrigation) following application. However, if there is more moisture (more than 16 inches), the plant back restriction to barley could be cut back to just 9 months. While soil pH is also a requirement for the shortened plant back, the point is that just a few more inches of moisture can make a huge difference in the persistence of an herbicide. This is why herbicide carryover could be a really big concern in 2022 if we do not get enough moisture in the next couple of months.

Drought Monitor in the West June 2021

What can you do?

If you know which herbicides were applied, the rates, and timing of application the previous season, the first thing to do is consult the herbicide label to see the plant back restrictions to your rotational crops. This will let you know whether it is safe to plant wheat, barley, mustards, canola, etc.

If you are not sure which herbicides were applied or the rates and timing of application, it is recommended that you conduct a bioassay to determine if it will be safe to plant a particular crop. Even if you are sure of the herbicides, rates, and timing of application, a bioassay is still recommended. To conduct a bioassay, collect soil samples from the top 2 inches at the worst parts of the farm (lowest organic matter, eroded, poorly drained, etc). You should collect about 5 pounds of soil (about an ice-cream bucket full) and mix it thoroughly. Collect an equal amount of soil from a field not treated with any herbicides. Your backyard or family garden is often the ideal location. Place the two soil samples (one from the farm that had herbicides and the other from non-herbicide treated soil) in wooden boxes or pots and plant 6 to 10 seeds (of the crop you intend to plant) in each soil. Ideally, space the seeds as they would be spaced in the field; however, if your pots are too small for this, you could plant more seeds and thin after seedling emergence. Place the pots in a sunny location and water as needed to enable the seeds to emerge. As soon as they emerge, remove extra seedlings if too crowded and observe the living plants in the herbicide-treated soil for signs of herbicide damage.

Lab analysis: If you cannot conduct a bioassay, you can collect soil samples from the herbicide-treated field as described above and send them to a nearby laboratory for residue analyses. It is important to note that laboratory analyses can be very expensive, so it is helpful to let the laboratory know what they are analyzing for. In other words, it is important to know the herbicide application history so that the laboratory can conduct specific residue analysis.

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