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Agricultural News


Drought Could Cause Herbicide Carryover in Fall Planting

Thu, 04 Aug 2011 08:59:28 CDT

Drought Could Cause Herbicide Carryover in Fall Planting Among the long list of concerns that has developed over the summer, many producers are wondering about the effect of dry weather on herbicide carryover as they prepare for fall planting. While the lack of rainfall will certainly slow the breakdown of many herbicides and increase the potential for carryover, there are several other factors to consider when deciding if it will be safe to plant this fall.


1. What herbicides have you used this year? When were they applied?

Despite the dry weather, not all herbicides will persist in the soil from one crop to the next. Commonly used herbicides such as glyphosate, Ignite®, Cobra®, or Dual Magnum®/Cinch® should not present any concerns for rotational crops. Other herbicides, such as atrazine, Pursuit®, or Classic®, may remain active in the soil for a longer time due to the dry conditions. It is very important to read each product's label to determine the rotation restriction to other crops and the effect of dry weather on the herbicide's persistence.

Application date can also influence your ability to safely plant a rotational crop this fall. Herbicides that were applied later in the year still require the appropriate rotation restriction, which may not allow for planting this fall.


2. How much precipitation have you received since the herbicide was applied?

Moisture is often the driving force behind herbicide breakdown in the soil. Many herbicides are degraded by soil microorganisms that use the herbicide as a source of food. However, these microorganisms also need moisture to remain active in the soil. When dry conditions are coupled with extreme soil temperatures, the soil microorganisms are nearly inactive and the herbicide will remain active in the soil. If we get rain between now and fall planting, these microorganisms will become active again and resume breaking down the herbicide.

In addition to microbial degradation, some herbicides, particularly atrazine and ALS-inhibitor herbicides (such as Classic®, Pursuit®, Permit®, Accent®, and Ally®), are broken down by a chemical reaction called hydrolysis. As the name suggests, hydrolysis requires water to successfully complete the reaction and degrade the herbicide. Therefore, if you used atrazine or an ALS-inhibitor herbicide this year, be sure to double check the label for the appropriate rotation restrictions or precipitation requirements.


3. What rate of the herbicide did you use this year?

For several products, the more herbicide that is applied, the longer it takes to break the herbicide down to a safe level to plant rotational crops ("the rate is the residual"). For herbicides that allow a wide range of application rates, be sure to check the label for differences among rotation restrictions for the various rates.


4. What is the soil pH where the herbicide was applied?

Soil pH can also have a large impact on the speed of herbicide degradation in soil. For example, atrazine and most of the ALS inhibitor herbicides have much shorter persistence in soils with a low pH (6.5 and below). Conversely, these herbicides will remain active in the soil for extended periods in soils with higher pH levels While not all herbicides are affected by soil pH, it is very important to know the soil pH of each field to better understand how long the herbicide may remain active in the soil.


The factors that slow down or speed up herbicide degradation can also work in combination. For example, if you applied 2 pounds of atrazine to a field this spring but have received little rainfall this year and the soil pH on your field is above 7.0, it is more likely that there may be enough atrazine in the soil to prevent rotation to wheat or canola this fall.


Unfortunately, it is very difficult to predict herbicide carryover during a drought year. Besides reading the herbicide label, the best way to determine if it will be safe to plant a winter crop is to conduct a field bioassay. To conduct a bioassay, plant strips or a small area of the field to wheat or canola and evaluate plant health after it germinates and emerges. Field bioassays can provide valuable information regarding herbicide carryover without the risk of planting an entire field; however, they may not always indicate the potential crop injury that could appear after a rainfall event or emergence and early crop growth.


Our thanks to Joe Armstrong, Extension Weeds Specialist from Oklahoma State University, for this article from the latest edition of the Extension News Newsletter.



   

 

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