Each year, from about the beginning of July through the middle of August, there isn't a lot of variety to the calls I get related to weed management in soybeans. The questions usually have to do with essentially one issue, "How do I control big weeds in soybean?" Obviously "big" is a relative term but my definition is usually way different from everyone else's. This is primarily due to the fact that if you have weeds over 6-inches or so in height, it is highly likely that you are already losing soybean yield. But that is a topic for another time. Most of the calls I get don't have to do with 6-inch weeds; more commonly the questions have to do with weeds double or even triple that height. Either these weeds never got sprayed because of the wet season we've had, or the weeds have already been sprayed one or more times with glyphosate (sold as Roundup, Touchdown, or a variety of other trade names) and they are still living. In recent years, the vast majority of the questions have to do with this latter scenario. I believe we have now moved into an era where glyphosate-resistant weeds like waterhemp, palmer pigweed, marestail, and giant ragweed have taken over as the predominant weed problems in Missouri soybean production fields. Based on all of the research I have conducted or seen over the years on both resistant and non-resistant weed populations, I have to divide my answer to the question of controlling "big" weeds in Roundup Ready soybean into the following two categories:
1.) If you don't think you have glyphosate-resistant weeds present.
If you have a field where the weeds (including waterhemp) have gotten tall and you DON'T suspect you have any glyphosate-resistant weeds present, then our research shows that increasing the rate of glyphosate will generally provide as good or better weed control than adding a tank-mix partner to glyphosate in Roundup Ready soybeans. There may be some exceptions to this statement if you are dealing with weeds that have some natural tolerance to glyphosate. For example, a tank-mix of flumiclorac (Resource) can sometimes provide better morningglory control than even a higher-than-normal rate of glyphosate. Also, there are some weeds like Asiatic dayflower and field horsetail that we are probably never going to kill with glyphosate and a tank-mix can often help with these kinds of weed species. For the most part, however, our research has shown that if there are no resistant weeds present, our "normal" spectrum of weeds in Missouri will usually be controlled as good or better by a higher rate of glyphosate when compared to a standard application rate of glyphosate plus some other tank-mix partner. Another way of saying it is that if you fall into this category (no glyphosate-resistant weeds), it is often better for you to take the money you were going to spend on the tank-mix partner and put that money towards a higher rate of glyphosate per acre.
2.) If you suspect you have a glyphosate-resistant weed present.
The other side of the coin is that if you suspect you have a glyphosate-resistant weed like waterhemp, palmer pigweed, marestail, or giant ragweed present, then a tank-mix partner is almost a requirement and is essentially our last line of defense for the control of these species. As I mentioned previously, unfortunately I'm afraid that a large percentage of the soybean acreage in Missouri now falls into this second category. In these fields, increasing the rate of glyphosate when you have glyphosate-resistant weeds present will rarely provide better weed control and will obviously just cost you more money.
Based on all of the research I have seen or conducted on glyphosate-resistant waterhemp and palmer pigweed, we have found that the addition of acifluorfen (Ultra Blazer), fomesafen (Flexstar, Dawn, etc.), or lactofen (Cobra, Phoenix) to a standard rate of glyphosate provides similar and significantly better control of glyphosate-resistant waterhemp than applications of glyphosate alone (at any labeled rate). However, the larger the waterhemp is in size, the less effective these tank-mix partners will be. So, if you have a field that falls into this category, make sure you apply the tank-mix combination as soon as you can. Obviously there are several other products on the marketplace that are promoted as tank-mix partners with glyphosate that I have not mentioned. Some of these include carfentrazone (Aim), 2, 4-DB (Butyrac), fluthiacet (Cadet), flumiclorac (Resource), and cloransulam (FirstRate). In our research, we have found tank-mix applications of these products to be ineffective on glyphosate-resistant waterhemp. In our research with glyphosate-resistant giant ragweed, we have found that fomesafen (Flexstar, Dawn, etc.) may provide slightly better control than lactofen (Cobra, Phoenix), but like waterhemp the larger the giant ragweed is in size, the less effective the tank-mix partner will be. In most cases, cloransulam (FirstRate) will also provide good control of glyphosate-resistant giant ragweed, assuming it is not too tall, and assuming your giant ragweed is not ALS-resistant.
Finally, as far as glyphosate tank-mix partners are concerned I think one of the biggest things you need to avoid is the temptation to use a tank-mix partner just because it only adds another couple of dollars per acre to the total application cost. Also, we should be aware of the potential for antagonism of some of these products with glyphosate. Just because a product appears to control weeds quicker, that doesn't always mean that the product or tank-mix treatment is better.
As I have mentioned in many previous articles and talks on this subject, a tank-mix partner that is included with your glyphosate application is not the way to solve your glyphosate-resistant weed problem; it is simply our last line of defense during the season. If you have weeds that you know or suspect are glyphosate-resistant and you plan to continue with Roundup Ready soybeans in the future, I would strongly urge you to use a pre-emergence, residual herbicide next year that is effective on your problem weed in question. This will do much to reduce the population of your problem weed over time. An in-crop, post-emergence tank-mix treatment may still be needed, but the number of weeds that ever get treated with this tank-mix will be dramatically less and much more manageable.
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REVISED: October 2, 2015