- Author: Bradley Hanson
Press release (April 28, 2021) from the Weed Science Society of America
Have you heard about HRAC's Mode of Action updates?
The Herbicide Resistance Action Committee (HRAC) has updated its Herbicide Mode of Action Classification System, which is a vital tool in developing sustainable weed control programs.
Explore WSSA's recent fact sheet to find out why updates were needed and what changes are being made.
Good info at the link above including:
- What is the HRAC Mode of Action Classification System and why is it important?
- Why was the herbicide mode of action classification system revised?
- What changes have been made to the classification system?
- 5 new or reclassified herbicide modes of action
- 15 new active ingredients added
- New numerical classification system (harmonizes the WSSA system with the global system and is more consistent with the approaches used by other pesticide classes)
- Updated chemical family names
Where can I get additional information?
HRAC offers a variety of tools and resources to support the recent classification system updates, including:
▪ A summary of the mode of action changes, as well as an in-depth technical description
▪ An updated herbicide mode of action poster
▪ A mode of action classification look-up app and webtool
- Author: Brad Hanson
Glyphosate is one of the most widely used herbicides in the world and is extremely important in many of our orchard, vineyard, and annual crops as well as in non-crop and home situations. However, it can be confusing to understand some of the differences among various formulations of glyphosate herbicides.
I'll paraphrase a recurring extension question as “I'm trying to compare the rates and cost-effectiveness of two glyphosate herbicides. One lists the active ingredient as ‘41% glyphosate as the isopropylamine salt' and the other as 48.7% glyphosate as the potassium salt'. How do I compare these two herbicides?”
First important point, glyphosate is a weak acid herbicide. The various salt formulations have major impacts on how the herbicide behaves in the jug, and to some degree on how it gets into the plant. But once in the plant, it is the glyphosate acid that binds to the target enzyme in susceptible plants and causes the herbicidal effect.
What is a salt? From a chemistry perspective, a salt is simply a compound formed by ionic bonding of two ions of opposite charge. Glyphosate acid has a weak negative charge and the salt is formed when the glyphosate acid is bound to a base that has a positive charge. In the cartoon below, this is illustrated a little incorrectly - you can see the negatively charged glyphosate acid on the left (C3H8NO5P); however, the "salt+" tagged on the right should really be labeled "base+" (the combined molecule is actually the "salt").
What are some common glyphosate salts? There are several glyphosate salts currently available in the market and others have been in the past but are less common now. Four examples (below) include: isopropylamine, dimethalamine, trimesium, and potassium salts. (images mostly lifted and modified from www.chemsrc.com).
In case it's been a while since your last organic chemistry class, here's a quick refresher on nomenclature. In the isopropylamine salt, that simply means it as a 3-carbon chain (that's “propyl”) and amine group (NH), and the chain is connected in the middle (the “iso” position) rather than on one end. Dimethylamine indicates two (di) methyl groups and an amine. Trimesium is a shortened name for trimethylsulfonium which means three methyl groups and a sulfur. Potassium means, well that one's pretty straightforward and means potassium (K).
How does the salt formulation affect the herbicide? The salt formulations are those of us managing weeds for a couple of important reasons.
- First, the different glyphosate salts have different solubility in water (or other solutions). This doesn't have much, if any, effect when we have the herbicide diluted in water to make a spray application (eg. quarts of product in 10 or more gallons of water). However, it has a big impact on how concentrated the herbicide can be in the formulated product. (It's useful here to note that pure glyphosate is actually a solid crystal that is dissolved in liquid to make the products we use in the field.) In the example I started with, that is the big driver behind why one product has 41% active ingredient and the other has 48.7%. From a packaging, shipping, storing, and handling perspective, it's far more efficient to have more concentrated materials (eg less water). So, we can assume the potassium salt is quite a bit more soluble than the isopropylamine salt form of this herbicide.
- Second, the different glyphosate salts have different weights. Remember the periodic table of the elements? The atoms that make up the salts (and any other chemical structure for that matter) have markedly different weights on an atomic scale. For our purposes, we can think of this as a carbon (C, atomic number 6) weighing six times a hydrogen atom (H, atomic number 1) and a potassium (K, atomic number 19) weighs about three times what carbon does. Of course, the glyphosate acid weight is the same C3H8NO5P in each formulation – only the salt is different. So, it is important to remember that in these herbicides the “active ingredient” (AI) is the salt formulation (eg glyphosate isopropylamine) and that each of the different active ingredients has a slightly different weight.
How can I use the percent active ingredient to compare products? I'll be honest here, I find it very confusing to think about the percent AI list on the label of a glyphosate herbicide. As I indicated above, this does not allow a straight across comparison for two different salts (eg a potassium salt glyphosate is not the same weight as a dimethylamine salt). More importantly, the percentage listed on the label is actually on a weight basis (weight of glyphosate salt per weight of formulated herbicide). I don't know about any of you, but I typically measure glyphosate herbicides by volume (fluid ounces, quarts, milliliters) rather than by weight (ounces, pounds, kilos). So knowing the amount of glyphosate per pound of Roundup Powermax isn't very helpful to me when I don't weigh the liquid.
Instead of focusing on the percent active ingredient, look on the label just below where the percentages are listed for the active ingredient and acid equivalent information. (I'm looking at a Roundup PowerMax label and a Credit41 label, but there are dozens of examples).
In these two herbicides, you can see:
- Credit41 has 4 lbs per gallon of glyphosate as the isopropylamine salt, which is equivalent to 3 lbs per gallon of glyphosate acid.
- Roundup PowerMax has 5.5 lbs per gallon of the glyphosate potassium salt, which equates to 4.5 lb/gal of the acid.
So, once we're comparing on an acid equivalent (AE) basis, we can see that that one product is 50% more concentrated than the other (3 lbs vs 4.5 lb ae). If you wanted to make an equivalent rate (AE per acre) you'd have to apply 50% more of the 3 lb product to have the same amount of glyphosate acid. Similarly, in making cost comparisons, you should be thinking about it in terms of cost per unit of glyphosate acid. In the above example, if making an equivalent AE rate per acres, the breakeven point would be if the 3 lb material costs 2/3 of what the 4.5 lb material costs. Don't make the mistake of assuming a quart of one glyphosate is the same as another (or that a 2% solution of two different products is equivalent!
Check out this publication we did a couple years ago On the second page, there's a chart that illustrates this concept – if you wanted to apply a rate of ¾ lb ae per acre, you'd need to apply 32 fl oz of a 3.0 lb ae/gal material (eg Credit41, Roundup Original) but only 22 fl oz of a 4.5 lb ae/gal material (eg Roundup PowerMax).
What else is different among the formulations? This is a really good question without a really clear answer.
Besides the glyphosate-salt that makes up the active ingredient in each formulation, there are other components of the herbicide product. Some of these influence physical properties that affect handling, storage stability, etc. Others are surfactants and other adjuvants that impact how the herbicide penetrates the leaf surface. The surfactants (types and loading) are proprietary information and are not reported in the same way as the active components are so this can be difficult to ascertain. However, one way that price points can be reduced in this competitive market (lots of generic glyphosate herbicide due to loss of patent protection) is to reduce or change the surfactant packages. Thus, even if the active ingredient is the same between two products, they could have substantially different surfactant loading which can impact weed control efficacy (amount of herbicide getting to the target enzyme in the plants). If you use a low-surfactant-load glyphosate product, you should consider adding appropriate surfactants to make up for it.
I'll wrap this up by saying that I think a weed manager can get similar levels of weed control performance with the wide range of glyphosate herbicides currently available on the market. However, it is really important to make sure you're comparing apples-to-apples in determining rates needed from the various salt formulations and concentrated products. In my experience, at equivalent AE rates, a 3 lb glyphosate plus a good surfactant can perform similarly to a higher AE and higher surfactant load formulations; however, be sure to sharpen your pencils to make sure the savings from the cheaper AI isn't offset by higher required rates and surfactant additions.
12/21/17 - lightly edited to clarify that the "salt" is the combined molecule that includes the glyphosate acid and the "base" (it's been a while since my last O-chem class too!). BH
- Author: Brad Hanson
A quick note to share a link to our (semi) regularly updated "Herbicide Registration on California Tree and Vine Crops".
Click here for a link to the PDF version on the Weed Research and Information Center page (this is probably your best bet for the most up-to-date version). I've also attached the pdf version at the bottom of this posting as well as the low-resolution image right here. --->
There were quite a few updates to this version compared to the last one I did in early 2016. This reflects both real activity in label expansion as well as updates I may have overlooked last time.
A partial list of changes includes:
- PindarGT - added most stonefruit, pomegranate, olive.
- Treevix - added pomegranate and olive.
- Mission - added walnut and most citrus.
- Zeus - added pecan.
- glufosinate (Rely 280 and others) - added olive, most stonefruit, pear.
- I added an "Organic" section to this version of the chart to include several OMRI approved herbicides.
- I also removed Greenmatch and Matratec from this version as I could not find a current registration for those materials.
Disclaimer: this summary document paints with very broad brush strokes so be sure to read and follow current label instructions for each specific crop and use pattern.
Also, this is an every-evolving chart so if you notice errors or omissions, please let me know and I'll correct or incorporate into future updates.
Take care,
Brad
edit 03/28/17. Corrected saflufenacil (Treevix) to include pomegranate and olive. thanks LH!
Tree and Vine crop herbicide registration 032817
- Author: Brad Hanson
I found out today that I previously misread the label of a new herbicide registered in several California orchard crops.
Broadworks Herbicide (from Syngenta) was registered in fall 2015 on several citrus, stone fruit, and tree nut crops (but not every crop in those groups).
I'm not entirely sure if I misread the label or if I simply made a typo when I last updated the T&V Herbicide Registration chart (which is totally possible after you've read several dozens of labels). At any rate, I erroneously listed mesotrione as registered on peach in my November update of that chart; however, Broadworks is not actually currently registered on that crop.
My apologies for any confusion this may have caused.
Here are the crops Broadworks is registered on as well as a current label.
- Citrus fruit (citrus hybrids, grapefruit, lemon, lime, sour orange, tangelo, tangerine (Manderin), cultivars, varieties, and/or hybrids of these)
- Stone fruit (nectarine, plum, cultivars, varieties, and/or hybrids of these)
- Tree nuts (almond, hazelnut (filbert), pecan, pistachio, black walnut, English walnut, cultivars, varieties, and/or hybrids of these).
I made an editor's correction to a previous post on this blog about mesotrione herbicide and have attached an updated version of that T&V herbicide registration chart below as well as at its semi-permanent address at the Weed Research and Information Center (click HERE for a direct URL to the chart at WRIC)
Please remember to refer to a current label before applying any pesticide as labels to change (and can be misread or misunderstood on occasion).
Brad
/span>Tree and Vine crop herbicide registration 031616
Broadworks Label
- Author: Brad Hanson
I thought I'd share a link to a YouTube channel from the Ohio State University Weed Science program.
There is a series of short time-lapse videos of several herbicides acting on different weed species. Includes paraquat (a PSI inhibitor), fomesafen (a PPO inhibitor), atrazine (a PSII inhibitor), glyphosate (an EPSPS inhbitor), quizalofop (an ACCase inhibitor), cloransulam (an ALS inhbitor), 2,4-D (a synthetic auxin) and glufosinate (a glutamine synthase inhibitor.
Cool videos (assuming you like watching plants die agonizing deaths, of course...)
Click here: Herbicide Injury - Time-lapse Mode of Action
or directly here: https://www.youtube.com/playlist?list=PLrQLElJHkjuh65RjQZ8nWDBh0Da_MekB0