There are several residual herbicide premixes available now for use in soybeans. Most of these are listed in the 2015 edition of the Weed Control Guide, but it seems like there’s always one or two that we don’t know about.

Many new premixes are essentially generic versions of products that had already been available. For example — Boundary, Ledger and Tailwind are premixes of metribuzin and metolachlor from different companies that are essentially identical in formulation and label. Once there are more than a couple of trade names for the same herbicide or herbicide premix, we usually list them in the guide by active ingredient, with a list of trade names underneath. So we now have a listing for “flumioxazin” instead of Valor, but show the trade names for flumioxazin underneath — Valor, Encompass, Outflank and Panther. 

Many of the available premixes are broad-spectrum and take care of a lot of the basic broadleaf weeds. This is evident in the effectiveness table in the Weed Control Guide, where there are a plethora of "8s" and "9s" on lambsquarters, smartweed, pigweed, velvetleaf, and even nightshade with a few exceptions. We have generally emphasized the need for residual broadleaf control more than grass control, but a number of the premixes now also contain a grass herbicide such as metolachlor or pyroxasulfone, which provides early-season control. 

So when sorting through options, it’s possible to ignore these easier weeds somewhat and make sure that the herbicide handles the weeds that have developed herbicide resistance and are generally tougher to control — giant ragweed, marestail, common ragweed, and also waterhemp and Palmer amaranth, if you are unlucky enough to have one of the few infestations of these in Ohio. It makes sense to key in on these weeds when using resources such the OSU weed control guide or in conversations with your supplier. 

Be aware that with the current emphasis by manufacturers on developing herbicides for control of Palmer amaranth, some new premixes may not fit well even if they are broad spectrum, due to deficiencies on marestail and ragweeds. The basic principles on residual herbicides for the tougher weeds mentioned below:

Marestail

Many populations are resistant to both glyphosate and ALS inhibitors, and so the chlorimuron, cloransulam, or imazethapyr in many premixes fails to contribute residual marestail control. The non-ALS active ingredients that have residual activity are flumioxazin, sulfentrazone, metribuzin, and saflufenacil (primarily at rates higher than the base rates that are used for burndown). While we rate the premixes containing some of these as an “8” on marestail, this can be optimistic, especially when applied a week or more ahead of planting. 

Our research increasingly shows that combinations of two non-ALS actives provide more consistent residual control than just increasing the rate of a single active. For examples — using a mixture of Sonic + metribuzin rather than just Sonic; using a mixture of Canopy DF + metribuzin + Sharpen rather than just the Canopy DF + metribuzin. For the latter example, the higher rate of Sharpen would have to be applied 2 weeks ahead of planting to use 1.5 ounces of Sharpen and maximize residual.  

Something else that’s important to keep in mind for marestail — where something has been applied the previous fall or early spring to control emerged marestail, it can be adequate to apply glyphosate+2,4-D closer to planting. Any residual herbicide can be included with glyphosate and 2,4-D. Where nothing was applied the previous fall or early spring, there can be a need for a more aggressive burndown such as glyphosate + 2,4-D + Sharpen, and this can limit the ability to use flumioxazin and sulfentrazone or force earlier application. 

The bottom line here is that if it appears that there will be a need for an application fairly close to planting that includes Sharpen, stick with products that contain metribuzin — Canopy DF, Matador, Boundary, etc. — and add metribuzin as appropriate to bring the rate up to 8-12 ounces where soil type allows.

Giant Ragweed

Residual control of giant ragweed comes primarily from two ALS inhibitors that are components of many premixes — chlorimuron (Classic) and cloransulam (FirstRate). We rate these as “7” in the guide but they can be better or worse depending upon density of the population, rainfall, etc. The imazethapyr (Pursuit) in some premixes also provides some control, which we rate as a "6." None of these work on ALS-resistant giant ragweed, and nothing else provides much control either — maybe a little from fomesafen or the higher rates of Sharpen. 

Populations resistant to ALS inhibitors are most effectively controlled by two post applications of glyphosate or Liberty, or Flexstar followed by Cobra in non-GMO soybeans.

A similar story to giant ragweed but a few more choices. In addition to the ALS inhibitors mentioned above, the PPO inhibitors flumioxazin (Valor, etc.) and fomesafen (Prefix, etc.) have fair to good activity. Pryroxasulfone (Zidua, Anthem, etc) has at least some activity, as does metribuzin. In ALS-resistant common ragweed populations, premix products containing flumioxazin have the edge over those containing sulfentrazone, because the latter has no activity on ragweeds. 

There are some populations of common ragweed with resistance to both ALS and PPO inhibitors (sites 2 and 14), and it’s difficult to obtain even fair residual control of these with pyroxasulfone and metribuzin. LibertyLink soybeans can be a good option for these multiple-resistant populations.

Waterhemp and Palmer Amaranth

Both of these weeds are challenging because they can emerge well into the growing season. ALS inhibitors are largely ineffective due to a high frequency of ALS resistance, but the following active ingredients provide fair to good residual control for the early part of the growing season (combinations or premixes that contains two of these can be more effective then single actives, especially for Palmer): acetochlor, metolachlor, pyroxasulfone, dimethenamid, pendimethalin, flumioxazin, sulfentrazone and metribuzin. 

Some of these can be applied post in combination with herbicides that control emerged Palmer amaranth and waterhemp, in order to extend the residual control and minimize the need for additional post treatments. This strategy, known as “overlapping residual”, does not replace the need for residual herbicides at or before planting. It should therefore be implemented primarily where these two weeds are already a problem, since it is added cost and does not work for marestail or giant ragweed. An exception might be if other fields in the area already have the problem, and the goal is to prevent the spread of waterhemp and Palmer amaranth.