Get the lowdown on herbicide breakdown

Written by Dr. Jason Weirich on .

Now that you are likely done with harvesting, take a moment to reflect on your fields from this past growing season. When you were spending countless hours in the combine, did you notice areas that had better weed control than other parts of the field? Did you have fields with better weed control than others, yet you had the same weed control program across all of them? This is a common theme. While I would like to tell you that every field will respond the same or that each program will work every time, that’s just not the case.  

Herbicides break down in many ways. Microbes, water and sunlight are the main environmental factors that influence herbicide breakdown. Each herbicide family—and sometimes products within the family—respond differently to each process.  

Microbial degradation is the dominant factor that breaks down herbicides. Certain bacteria, fungi and algae use herbicides as a food source. Microbes are herbicide-specific, and populations are dependent on the rate of the herbicide application. Repeated use of the same herbicide year after year can cause more rapid degradation of the specific herbicide, resulting in shorter efficacy windows from that herbicide.  

Several factors can influence this process, such as soil composition, soil pH and climatic conditions. Soil organic matter influences microbial activity and provides habitat for the microbes to exist. When we look at pH, each microbe favors a certain level, but we see very little microbial activity in the extremes.

We all remember the drought of 2012 and concerns about herbicide carryover into the 2013 growing season. Length of herbicide activity is very dependent on soil temperature, soil moisture and rainfall, just to name a few influential conditions. You’re likely aware that microbes are not very active when soil temps drop below 50 degrees. That’s why a lot of fall-applied herbicides provide weed control well into spring. Very little herbicide breakdown occurs in the fall and winter from microbial degradation.

Water also has a negative effect on herbicide activity. Areas where water pools on the field and low spots are typically the first areas to break down herbicides when we have moisture or excess moisture. This chemical breakdown is a process called hydrolysis.  

Finally, sunlight is a factor in breaking down herbicides, but this photosynthetic decomposition is not as prevalent as it was 20 to 30 years ago. You probably remember having yellow-stained boots, pants, shirts and hands. The Treflan and Prowls of the world would break down relatively fast by light. That’s why I remember, as a kid, following the sprayer with the field cultivator to incorporate Treflan into the soil.   

These factors emphasize why we have to continue overlapping residuals. I can’t predict how long these residuals will provide weed control because it varies from year to year and field to field. This past year, depending on rain, the two-pass weed control program was the cleanest.  

I hope you will take time to evaluate this past year’s successes and failures to make weed-control plans for the upcoming year. These insights will provide us with some of the knowledge we need to make proper recommendations. If you have questions or want to develop a weed-control program, contact your nearest MFA or AGChoice location.


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