Drs. Daniel Stephenson and Josh Copes
Herbicide-resistant weeds, especially glyphosate-resistance, is not a new topic. Glyphosate-resistant (GR) Palmer amaranth was documented in Louisiana in 2010. GR waterhemp was documented in 2015. As of today, glyphosate-resistant Palmer amaranth can be found in virtually every row crop parish in Louisiana. I’m not saying it has infested every field in every parish, but those fields having infestations range from a few plants to an extreme number of plants. GR johnsongrass and Italian ryegrass have been documented too. Although we haven’t officially documented GR horseweed (mare’s-tail) in Louisiana, I am certain it infests many acres in northern Louisiana. Therefore, Louisiana producers must implement strategies to mitigate and/or manage this extremely troublesome pest.
In 2015, the LSU AgCenter published an extension publication entitled “Herbicide Programs for Managing Glyphosate-Resistant Palmer Amaranth and Common Waterhemp in Louisiana Corn, Cotton, and Soybean”. It can be found at http://www.lsuagcenter.com/~/media/system/c/7/5/a/c75a63bba3f758d391b8c91871076ba6/pub3522herbicideprogramsformanagingglyphosateresis.pdf. This publication provides suggested programs that can help mitigate and/or manage glyphosate-resistant pigweeds. In addition, if these programs are implemented, they offer control of many if not all of the other grass and broadleaf weeds Louisiana crop producers deal with every year.
I’m not going to discuss the programs in this article in depth, so I ask that you view the document. If you have questions, please call. However, I will highlight the main focus of all programs. The main, primary, essential thing to remember in designing a program to manage glyphosate-resistant weeds is residual herbicides. Let’s break it down.
Step 1: It is crucial for producers to apply a residual herbicide just prior to planting, at planting, or preemergence. Paraquat at 0.5 to 1 lb ai/A (i.e 1 to 2 quarts/A of Gramoxome SL or 0.67 to 1.33 quarts of a generic 3 lb ai/gal paraquat) needs to be tank-mixed with this preemergence residual herbicide to kill any emerged weeds to ensure that the crop emerges in a weed-free seedbed. All the residual herbicides listed in the preemergence section of the publication will provide residual control of pigweed following proper activation. The choice of preemergence herbicide depends upon other weed species found in the field. I won’t go into each different situation, so please call us to discuss if needed.
Step 2: The next crucial step is to apply a residual herbicide tank-mixed with a non-selective herbicide 3 to 4 weeks after planting. Examples of herbicides that offer residual control when applied POST are Dual Magnum (or many generics at proper rates), Prefix, Warrant, or Zidua. Tank-mix one of them with glyphosate in Roundup Ready crops or with Liberty in Liberty Link crops.
Implementing steps 1 and 2 overlays residual herbicides during the early growing season, which protects the crop from early season competition. The best time to kill a pigweed is when it is emerging or when it is very small (less than 3-inches). Residual herbicides will kill the pigweed as it germinates or while it is emerging. In addition, research has shown that maintaining soybean weed-free for the first five weeks after emergence maximizes yield, assuming proper growing conditions and insects/diseases are managed.
Between burndown and planting, pigweed and other weeds could emerge and reach heights too large to kill with an at-planting application of any labeled non-selective herbicide, specifically paraquat. This situation usually occurs in fields that received a burndown application greater than 4 weeks prior to planting or when a burndown application didn’t include a residual herbicide. Remember, Palmer amaranth has the potential to grow one-inch in height per day. Therefore, it is critical that emerged Palmer amaranth or any other weed species be controlled when they are small with either tillage or a non-selective herbicide before planting. Tank-mixing a residual herbicide with this preplant application will help to maintain your field weed-free up to planting. However, do not think that applying a residual herbicide weeks prior to planting will be sufficient for residual control in-crop. A preemergence residual herbicide will still be needed to maintain the crop weed-free until the first postemergence application.
In many states to our north, PPO-resistant Palmer amaranth and waterhemp have been documented. To date, the LSU AgCenter has not documented any PPO-resistant pigweed in Louisiana. However, we are screening some populations, so the potential for this is there. You are probably wondering what are PPO’s? PPO-inhibiting herbicides include Valor, Envive, Enlite, Valor XLT, Rowel, Rowel FX, all the Authority products, BroadAxe, Prefix, Flexstar, Flexstar GT, Reflex, Cobra, Ultra Blazer, Resource, ET, Cadet, and many more. Honestly, this worries me as a weed scientist more than glyphosate resistance! In Louisiana, the most common weed is morningglory. Producers historically rely upon one of these herbicides to control morningglory. It was always a big positive that they controlled pigweed and other weeds such as hemp sesbania, sicklepod, Texasweed, smellmelon, and others too. We all should remember the articles in popular press articles showing the devastating effects of uncontrolled Palmer amaranth on a crop. Imagine spraying a PPO-inhibiting herbicide for morningglory, hemp sesbania, AND Palmer amaranth control and you get little to no control of pigweed. In this situation, I would have no suggestion for a herbicide application to help you. In the presence of glyphosate and PPO-resistance Palmer amaranth, we will still have products that contain Dual Magum and other metolachlor products, metribuzin and products that contain it, Zidua, Warrant, Classic, and Liberty, but use of only these products would severely limit a producers ability to effectively manage herbicide-resistant weeds and all the numerous weed species Louisiana producers struggle with. I’m not trying to be “chicken little” and claim the sky is falling. I just want the reader to understand that this isn’t something to play with and a plan should be developed and implemented to prevent it.
Use of residual herbicides before crop emergence and in the first postemergence application is vital for weed management in Louisiana corn, cotton, and soybean. In cotton, a residual herbicide is most likely needed in the second postemergence application too. To mitigate and/or manage glyphosate-resistance and/or PPO-resistance, we have to use residual herbicides, rotate crops, tank-mix multiple herbicidal modes of action in a single application, don’t use similar herbicidal modes of action every year, and if you see a weed that should have died after application, go pull it up and burn it. Those steps will help in the fight against herbicide resistance. If you have any questions, please call your local county agent. Good luck.