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Resistant pigweeds in Roundup Ready soybeans

I wish every soybean field in Arkansas which had Palmer pigweed as a problem weed could be planted to an alternate crop such as corn or rice or that an alternative weed control technology such as LibertyLink could be used in the system.

However, the rotation does not fit for every grower, the rotation is not always fool-proof (you sure see a lot more pigweeds on rice levees), and soybean seed supply alone will dictate that Roundup Ready soybeans will be planted in a lot of Palmer pigweed fields in 2010.

If you sprayed two timely applications of upper end rates of glyphosate on pigweeds and missed some this year, chances are good that there are resistance weeds in the population.

As you continue to plant those fields to Roundup Ready soybeans and continue to depend primarily upon glyphosate for control, you will continue to shift the population toward more resistance.

My philosophy also is to manage the fields where glyphosate may still be controlling pigweeds just as though you have a resistance problem. The sooner you implement a resistance program, the more valuable you can keep the Roundup Ready technology on your farm.

In fields where you cannot rotate to a crop like corn that has excellent pigweed management options or plant LibertyLink soybeans, you must make a pigweed control program in Roundup Ready soybeans work. The only way to do that is with conventional herbicides.

To summarize some of the options I discussed in conventional soybeans, I like the early preplant applications (around 14 days prior to planting) of residual herbicides such as Valor or a Valor-containing product such as Envive. There are a lot of choices and doing something is often more important than which product you choose.

If pigweeds have emerged at planting, consider Ignite or paraquat as the burn down treatment instead of glyphosate.

There are several choices for pre-emergence treatments. Prefix has been an excellent treatment in University of Arkansas research trials. This choice may be influenced by the amount of Flexstar you think may be needed postemergence as the label limits the amount of total Reflex (in the Prefix) and Flexstar that can be used.

If you know there is a resistance problem in the field, you can assume you will need one application of Flexstar and perhaps two.

Other choices for the pre-emergence treatment, such as Authority MTZ, Sencor, Dual or combinations of these, can be used. Again, doing something may be more important than what you do.

As in conventional soybeans, the key to the system will be the postemergence program. You always have to assume the residual program is not going to work or at least will provide less than 100 percent control. If you have not had escapes in the past, perhaps glyphosate alone is all that will be needed.

If you know you have resistant weeds, getting a full rate of Flexstar on them when the “ground turns red” is the key. If a second flush emerges, a repeat application will be needed.

A program such as the one outlined above (or variations of it) can work. It is a great program to prevent resistance if you do not already have it and can be a good program in fields with low levels of resistance. However, in fields where a very obvious glyphosate resistance problem exists, the program has all the same pitfalls outlined in my articles on conventional soybeans.

Residual herbicides are erratic in nature. If the residual fails, you are in a firefight. There is hope with the Flexstar program to get things back under control. However, if you miss the timing by a few days things are going downhill in a hurry.

If you leave the Flexstar out of the first application to “see what happens” and the glyphosate fails, it is pretty much over. I am a huge Roundup Ready fan, but glyphosate is no longer a Palmer pigweed herbicide and you must manage accordingly.


TAGS: Soybean
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