Are you ready for the weeds?

Published: September 9, 2015

Are you ready for the weeds? Prevented plant acreage from this season will most likely mean high weed pressure in 2016.

Benjamin Franklin once said, “By failing to prepare, you are preparing to fail.” I think that statement is appropriate for where we are with weed management right now in Missouri—if we don’t plan accordingly, the problems experienced this year could lead to even greater problems next year. This year more than 1.5 million acres of cropland were never planted as a result of the continually wet conditions experienced from April through July. Most of this acreage was left fallow without any sort of weed management program and these fields grew up into a weedy mess. These weeds have matured and produced viable seed that, in most cases, have already been deposited back into the soil seedbank. The primary weed I have seen in most of these fields is waterhemp, which produces about 300,000 to 500,000 seed per plant. I have also seen plenty of fields infested with marestail (a.k.a. horseweed, Conyza canadensis), ragweed species, and grasses like giant foxtail and fall panicum—all of which are also capable of high seed production. In short, the number of weed seed sitting in the soil seedbank waiting to germinate and wreak havoc next year may be unlike anything we’ve ever experienced before. And as Mr. Franklin put it, we must have a plan or we might suffer the consequences.

So where do you start? How can you be ready to tackle the potential problems that exist in your fields right now? In this article, I suggest a stepwise approach for selecting your soybean herbicide program for the 2016 season. But remember that herbicides shouldn’t be the only component of your weed management program – we have to think beyond herbicides for weed management, and this includes cultural control methods like narrow row spacings, optimum planting populations, crop rotation, cover crops, and tillage where appropriate. These cultural control practices need to be combined with an effective herbicide program to achieve the best weed control possible.

Marestail/horseweed was especially difficult to control this past spring in Missouri soybean fields. Fall herbicide applications are one component of an effective strategy to manage this weed.

First, consider whether your management system and predominant weed species would benefit from a fall herbicide application. Fall herbicide applications aren’t the answer to all our weed problems but they are an effective tool for winter annual weeds and especially for marestail. If marestail is one of your “driver weeds”, a fall herbicide application can save you from having much bigger problems next spring. Some of the more effective fall residual herbicides for the control of marestail in soybean include Autumn Super and any of the chlorimuron-containing products like Canopy, Canopy EX, Cloak, Cloak EX, Valor XLT, Authority XL, and others. These herbicides should be combined with a base program of glyphosate plus 2,4-D and/or dicamba to control any seedlings and rosettes present at the time of application. Another option is to leave the residual out of the fall application and wait to apply a full rate of a residual herbicide in the spring. Both approaches can be effective and there are many of factors (especially herbicide cost) to consider.

Second, plan to start weed-free next season with an effective tillage operation or burndown herbicide application. We cannot afford to plant into weeds that have not been adequately controlled or that are already emerged at the time of planting. This will put you behind the eight ball before you even begin, and it’s likely you will never catch up. Another reason to start weed-free is that there are too many resistant weed species that will not be controlled in-crop if they have emerged by the time of planting. We now have multiple herbicide resistances within numerous populations of marestail, giant ragweed, and especially waterhemp. This essentially means that there are little to no post-emergence herbicide options for the control of these weeds in Roundup Ready soybean.

Third, make sure to apply a full use rate of a pre-emergence, residual herbicide that targets your most problematic weed species. For most Missouri producers, this means you need an effective herbicide for waterhemp. For waterhemp, some of the most effective pre-emergence residual herbicides include the group 14 herbicides such as those that contain flumioxazin and sulfentrazone (Authority products, Envive, Fierce products, Sonic, Valor products, etc.) and the group 15 herbicides such as those that contain metolachlor (Dual II Magnum, in Boundary, in Prefix, etc.), dimethenamid (Outlook, OpTill PRO), pyroxasulfone (Zidua, in Fierce, etc.), and acetochlor (Warrant, in Warrant Ultra). Trifluralin (Treflan, group 3) and metribuzin (Sencor, Tricor, etc., group 5) can also provide good waterhemp control. A key point here is that you should apply a full use rate of whichever product you have chosen. Unfortunately, some pre-emergence herbicides still have a section on their label that contains information about reduced rates for use in Roundup Ready soybean systems. Usually this reduced rate is half that of the full use rate listed in a different section of the label. This makes no sense to me and is likely going to drive us towards more resistance problems in the future. If you have glyphosate-resistant waterhemp in your fields and know that glyphosate isn’t going to control this weed no matter what rate you use, what’s the rationale for applying a lower rate of a pre-emergence residual product that actually does control waterhemp? The bottom line is this – you need to make sure you do the best job you can up front with pre-emergence herbicides, if for no other reason than our inability to successfully control waterhemp post-emergence across in the majority of fields throughout the state.

Fourth, scout fields regularly after emergence and make timely applications of post-emergence herbicides to weed escapes. Simply put, there was a time when many farmers forgot about the importance of weed size because glyphosate controlled the weeds no matter the size. Now glyphosate does not control most of our driver weeds (waterhemp, marestail, giant ragweed, etc.) and I think everyone is in the process of “re-learning” that lesson. Group 14 (or PPO) herbicides like Cobra, Flexstar, Marvel, Phoenix, Ultra Blazer, and others just simply will not provide adequate control of waterhemp that is greater than 4 inches in height at the time of application. The same is true of glufosinate (Liberty, group 10) in Liberty Link soybean. If you apply one of these herbicides to waterhemp that is greater than 4 inches in height and aren’t happy with the outcome, it isn’t necessarily because the weed is resistant; the products aren’t labeled for waterhemp that size in the first place. Another thing to think about when it comes to post-emergence applications of the group 14 herbicides and also Liberty is that coverage is critical, and that the spray application parameters that may be ideal for glyphosate aren’t usually the best for these contact herbicides. So higher gallonage per acre and nozzles that provide good coverage will be critical.

Fallow fields like this were a common site across Missouri in 2015 due to the unusually wet conditions experienced throughout the season. Many of the weeds in these fields produced viable seed that were deposited back into the soil for farmers to contend with next season.

Fifth, “layered” or “overlapping residual” herbicide programs are insurance against late-season flushes of certain and have proven to be an effective strategy for the management of waterhemp. If you aren’t familiar with this type of weed management strategy, it involves a pre-emergence residual herbicide before planting, followed by an in-crop application of another residual herbicide (Anthem, Cinch, Dual II Magnum, Outlook, Prefix, Warrant, Zidua, etc.), usually made at the same time as a glyphosate application in Roundup Ready soybean, or Liberty application in LibertyLink soybean. This herbicide strategy won’t work on every weed species, but it does provide effective residual control of waterhemp and a variety of other small-seeded broadleaf weeds and grasses. More Missouri growers have adopted this herbicide program in recent years as a result of its effectiveness on waterhemp.

As mentioned previously, the most important thing is for you to have a plan. If you follow the 5 steps above, I believe you will be able to offset many of the consequences of this past season. For more information on this topic or other weed-related issues facing Missouri producers, visit us on the web at www.weedscience.missouri.edu or follow us on Facebook and Twitter at Mizzou Weed Science.