Herbicide carryover concerns—-Challenges from the drought will keep on coming

Herbicide carryover concerns—-Challenges from the drought will keep on coming

Vince M. Davis, Department of Agronomy, UW-Madison/Extension

Is herbicide carryover a concern for fall 2012 and spring 2013 crops?———Yes

Are there great remedies and advice to alleviate carryover concerns?——–Few, and some are arguable

It has been a challenging year where rainfall was scarce through much of the early part of the crop growing season. Unfortunately, the challenges associated with herbicide carryover may keep on coming into the fall and spring if crop rotations aren’t carefully matched behind residual herbicide products.

In July I put an article in the WCM titled: “Forage harvest and re-crop considerations following these drought conditions”: http://ipcm.wisc.edu/blog/2012/07/forage-harvest-and-re-crop-considerations-following-these-drought-conditions/  In that article I made the simple suggestion to make sure you review your herbicide application records and review the recrop restriction tables in A3646 and on herbicide labels to insure you won’t have a failure in a succeeding crop establishment. I also provided the re-crop interval table and you can still download that from the article.  I know that suggestion was more subtle than it should have been, and perhaps more vague in advice than some wanted, but honestly I think that was the extent of the advice I felt could be delivered with confidence.  I still think the best suggestion is for you to study and interpret those re-crop intervals as much can be learned about what may or may not be a concern.  Other recommendations to alter management practices like bioassays or increasing tillage are a guessing game, in my opinion.

When issues are important, like this issue, I really strive to provide information in these newsletter articles that I am confident can be backed up by scientific data and manuscripts from which I form an opinion.  i.e., I like to be confident in the advice I give.  Unfortunately, there is little science and few data that truly outlines the extent of the problems we may face following 2012, and how we should address those problems are equally questionable. Yes, it is true that carryover is not a new phenomenon and much was learned in the drought years of the late 80’s.  So, we do have some precedence, but in many ways things have changed since then like the crop genetics we plant, the herbicide chemistries we are, and in some cases the amount of herbicides.  So, there are lots of opinions, and we can make lots of guesses, but rest assured we are on un-charted territory with what we will see moving forward.

The biggest challenge for making broad spectrum predictions and sweeping management recommendations is that I expect herbicide persistence in fields will be extremely variable.  Early anecdotal reports of corn yields in some southern counties of Wisconsin range from 0 to 180 bushels per acre, and that’s in the same field.  So, that should indicate exactly how variable growing conditions were in 2012, and that’s exactly how variable I expect, even within a field, herbicide persistence will be. Why? because what we do know is there are several factors that are important for herbicide persistence.  Hartzler and Owen did a nice job of presenting the factors that determine the risk of carryover injury in a recent Iowa State Extension article. They presented seven factors:

1)      chemical half-life

2)      rate of herbicide application

3)      application date

4)      soil characteristics (texture, organic matter, pH)

5)      rainfall (total amount and distribution throughout year)

6)      sensitivity of rotational crop

7)      growing conditions following planting next spring

As you can see from that list of factors, there are several factors that can’t be changed ‘after the fact’.  What I mean is you obviously can’t go back in time and change the herbicide rate or timing of what you already applied and you are stuck with the soil and weather parameters you’re dealt. So, the only factor you can influence is number six; sensitivity of rotational crop.  Thus, I feel confident that the only good advice is to do exactly as I said in July, review your herbicide labels and plan your crop rotation accordingly.

More specifically, I would recommend interpreting the re-crop tables and intervals——and READ THE FOOTNOTES—–on the cautious side.  For instance, it is acceptable to rotate to soybean the year following a Lumax® application, however, it is not acceptable to rotate to soybean in the year following Lumax IF the application was made after June 1.  That restriction applies to a ‘normal’ year.  I bet many Lumax applications were made after May 10 this year. So, my question to you would be this…., how different were weather conditions between May 10 and June 1 in 2012 compared to a normal year? Or worded differently, was any rainfall received in that time frame and were weather conditions conducive for the early stages of herbicide degradation prior to June 1 this year?  If the answers to those questions were ‘not really’, then rotating to soybean might ‘not really’ be a good idea.  Other products with important footnotes to consider are on the Hornet® label that states: “When annual rainfall and/or irrigation is less than 15 inches on soils with less than 2% organic matter, alfalfa, dry beans, lima beans, peas, snap beans, and soybeans should not be planted until 18 months after treatment.”  Again, did your fields receive close to 15” rainfall? Are those soils close to 2% organic matter? These are important details in a year with unknown outcomes.

I am only mentioning these two products as EXAMPLES, and I do not mean to single these particular products out in the marketplace. But, my point is there may be very important clues in the ‘fine print’ of the labels that need to have a lot of consideration this year.  May other herbicide labels have the same type of ‘fine print’ in the directions for use that revolve around other important field conditions like soil pH and texture. Reading these labels closely and interpreting them with caution should help avoid many of the carryover problems that will be encountered, and yes, in many cases this may very well mean that a cautious interpretation will mean planting the same crop back into that field!

If you want to study this issue in even more detail, it is important to note that herbicide persistence is most often a function of herbicide family, and herbicide family does not always directly correlate to herbicide mode-of-action.  A good example is there is a big difference in the way sulfonylureas and imidazolinones degrade in the soil even though they are both amino acid synthesis inhibitors (ALS inhibitors). Imidazolinones are more reliant on rainfall for degradation than sulfonylureas. So, in addition to reading the herbicide label for re-crop intervals and/or using the chart in A3646, I advise you to use a combination of Dr. Colquhoun’s Herbicide persistence and carryover publication [http://learningstore.uwex.edu/Assets/pdfs/A3819.pdf] and the Corn and Soybean Herbicide Chart [http://ipcm.wisc.edu/download/pubsPM/Herbicide_MOA_CornSoy_02_2012.pdf] to determine which corn and soybean herbicides have a likelihood of long persistence.

Wouldn’t it be better to just recommend bioassays to help make re-cropping decisions?

I’m personally not convinced that bioassays are a tremendous help in decision making for the amount of work and time they require in order to conduct them correctly, and the amount of error that they can sometimes still present.  I admit, this is my opinion and others have been promoting bioassays more than I have.  I do think bioassays can help, if done correctly, but how much it improves your decision making skills above a 50/50 chance after reading labels and crop rotation guidelines is still a question in my mind. In Illinois Dr. Hager has conducted greenhouse bioassays this year and has shown lacking herbicide degradation may be an issue this year: http://bulletin.ipm.illinois.edu/article.php?id=1712. So, if you are interested in bioassays, there are good recommendations for conducting bioassays in the UW-EXT Herbicide persistence and carryover publication, and also in a 2008 Extension article from Nebraska: A quick test for herbicide carry-over in the soil.  Just note, that I said ‘if done correctly’ they can help. My point is this, if you’re going to conduct a bioassay, give adequate effort toward doing a good job in order to make a calculated conclusion about the extent of the problem you may be facing. First, carefully read the publications I just referred to and follow the directions. Second, make certain you carefully sample several different areas of a field targeting areas you expect should be the worst case scenarios (end rows, boom overlap areas, low OM areas, high pH areas), target different soil drainage and topography areas (get on top the hills and in the valleys), target contrasting weedy and less weedy areas (this may indicate the amount of herbicide that was metabolized by foliage), and target contrasting yield areas of the field.  And then, replicate the sampling in these areas.

Will tillage help? 

This is the most common question I have received. It is another difficult question to answer because the only correct answer is: “it depends”. In most cases, however, I don’t think tillage is going to help. In some cases, it might. I polled a few of my colleagues to get their opinions. One colleague said “Tillage, if it is deep enough will dilute the herbicide residues, particularly since they weren’t moved into the soil profile by rain this year.  The other thing to keep in mind is for anything that is broken down microbial, tillage will stimulate microbial activity for a short time and specific herbicides will be broken down more quickly.” Another colleague said “I believe the risks of tillage outweigh any benefits that are expected.  Experience from many years ago with the imidazolinone and sulfonylurea herbicides suggested that you may or may not get carryover response after tillage and similarly, you may or may not get responses from doing nothing.  The risks of added expense, time, and potential for erosion are 100%.”  The logic of physical dilution seems to side with the first response, but data in the literature supports the second opinion.  Which one is correct?, I think they both are depending on the situation and herbicide.  Research conducted in Missouri in 1988, 1989, and 1990 by Walsh et al. (1993) found no evidence that tillage reduced carryover potential of chlorimuron, clomazone, imazaquin, imazethpyr, or metribuzin in succeeding crops when applied at normal and twice the normal use rates.  On the other hand, there are plenty of data sources to show herbicide breakdown will increase with warmer soils, and you can get warmer soils in the spring with tillage and potentially increased microbial activity with burial of crop residues. Additionally, it makes perfect sense that tillage would help dilute the activity of seedling root and shoot inhibitors (i.e. Prowl, Treflan, Dual II Magnum, Harness) because the seedlings need to come into contact with a high concentration of the herbicide during the germination and emergence process.  However, dilution of ALS inhibitors, HPPD inhibitors, or PPO inhibitors may not matter as much.  So, I’ll leave the recommendation of whether tillage will help as ‘undecided’ or ‘depends’ on the situation.  I wish it were more clear cut, but it’s not. However, if you’re in a cropping system and topography where tillage is not too detrimental from an erosion standpoint and tillage would help you sleep better at night, then it at least it will help you sleep better.  – Vince M. Davis, Extension Weed Scientist

Below are links and citations to more additional information:

Colquhoun. University of Wisconsin-Extension. 2006. Herbicide persistence and carryover: http://learningstore.uwex.edu/Assets/pdfs/A3819.pdf

Corn and Soybean Herbicide Chart: http://ipcm.wisc.edu/download/pubsPM/Herbicide_MOA_CornSoy_02_2012.pdf

Hager, University of Illinois. September 7, 2012. Considerations for Fall-Seeded Small Grains: http://bulletin.ipm.illinois.edu/article.php?id=1712

Hartzler and Owen, Iowa State University. August 8, 2012. Carryover concerns for 2013: http://www.extension.iastate.edu/CropNews/2012/0807hartzlerowen.htm

Hornet®, product of Dow AgroSciences: http://www.dowagro.com/usag/prod/015.htm

Klein, Bernards, and Shea. University of Nebraska. September 2008. A quick test for herbicide carry-over in the soil: http://elkhorn.unl.edu/epublic/pages/publicationD.jsp?publicationId=1052

Legleiter. Purdue University. August 6, 2012. Herbicide carryover in dry conditions: http://www.ppdl.purdue.edu/ppdl/weeklypics/8-6-12.html

Lumax®, product of Syngenta Crop Protection: http://www.syngentacropprotection-us.com/pdf/labels/SCP1152AL1D0106.pdf

Walsh, J.D., M.S. Defelice, and B.D. Sims. 1993. Influence of tillage on soybean (Glycine max) herbicide carryover to grass and legume forage crops in Missouri. Weed Science. 41:144-149

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