Are You Ready For The Weeds?

By Kevin Bradley, Ph.D., state extension weed scientist, University of Missouri

Five-step plan for getting prevented-plant acreage back under control

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 — if we don’t plan accordingly, the problems experienced this year could lead to even greater problems next year.

This year, a large amount of cropland was never planted as a result of the continually wet conditions experienced in various parts of the soybean belt, particularly my home state of Missouri. Some 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), ragweed and grasses like giant foxtail and fall panicum — all of which are also capable of high seed production.

In short, the amount 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.
Where to Start

How can you be ready to tackle the potential problems that exist in your fields right now? 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 spacing, 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.

Let’s consider five steps:

1. Consider 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, especially 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 AutumnTM Super and any of the chlorimuron-containing products, like Canopy®, Canopy® EX, CloakTM, CloakTM 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 factors (especially herbicide cost) to consider.

2. 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 if they have emerged by the time of planting. We now have weeds resistant to multiple herbicides, including Palmer amaranth, marestail, giant ragweed and waterhemp. This essentially means that there are few to no post-emergence herbicide options for the control of these weeds in Roundup Ready® soybeans.

3. Make sure to apply a full rate of a pre-emergence, residual herbicide that targets your most problematic weed species.

For farmers in my state, 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. For farmers in other states who have other weeds that prove to be problematic, select a product that targets that weed species. 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 the label that contains information about reduced rates. 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 toward more resistance problems in the future.

4. Scout fields regularly after emergence and make timely applications of post-emergence herbicides to control 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®, MarvelTM, PhoenixTM, 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 LibertyLink® soybean. If you apply one of these herbicides to a waterhemp plant that is greater than 4 inches in height and aren’t happy with the outcome, it isn’t necessarily because the weed is resistant; it’s probably because 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. The spray-application parameters that may be ideal for glyphosate aren’t usually the best for these contact herbicides. So higher volume per acre and nozzles that provide good coverage will be critical.

5. “Layered” or “overlapping-residual” herbicide programs are insurance against late-season flushes of certain species 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.