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Horseweed, pigweed problems in soybeans

Spring rains not only delayed soybean planting, but have also put many growers behind the eight ball in terms of weed control.

I have received many calls this year on larger-than-normal horseweeds that were not been treated preplant or with a burn-down. This is a tough situation to be in.

If tillage is simply out of the question, I have been recommending running about 40 ounces per acre of Ignite and all the water you can afford to get good coverage, go in and plant and then be ready to run 0.3 ounce per acre of FirstRate with both post applications of Roundup.

A good variation on this is to run a different burn-down, maybe Gramoxone + Python or Synchrony XP or both or equivalent, again at high rates and lots of water, and then plant LibertyLink soybeans so that you can make two in-season shots of 22 ounces per acre of Ignite.

Some guys have switched to LibertyLink soybeans in the wake of all this rain and in the face of larger-than-normal horseweed. Ignite is the only post soybean product that can actually kill glyphosate-resistant horseweed.

I know little information is out there on the varieties, but they have done well in my plots so far and I have not received any negative comments from the field. Some crop response in the form of a yellow flash has been reported, but I have not personally seen this yet.

In my work, the varieties we looked at yielded well and were extremely tolerant to Ignite.

Another weed to consider in soybeans is pigweed. We have had many confirmed cases now of glyphosate-resistant Palmer amaranth across the state. If you had pigweeds at harvest last year, you should consider doing something besides straight Roundup this year. I am already getting calls on failures of the first application to control pigweed.

Unfortunately, it is usually too late by then to do something. Flexstar will kill only a pigweed that is 2-3 inches tall or less. So applying it to large pigweeds will only burn them back.

Again, one option is LibertyLink soybeans. Two 22-ounce applications starting at about 3-inch pigweeds has performed well for us at our site in Jackson County the past two years. I am comfortable with this program in drilled soybeans. I suggest adding a residual up-front, such as, Dual, Prefix, Valor (or Valor premix), or Authority MTZ. Many other combinations of residuals look good, too. Just make sure they are good on pigweed and if they help on grasses that is a bonus for LibertyLink system. Adding Dual to the first post Ignite application looked good at our pigweed site last year.

If you are in a Roundup Ready system and are having trouble with pigweeds, you really should put down a residual ahead of planting or at planting. If you get into a post situation, then our only hope is a full rate of Flexstar. Lower rates of Flexstar, like 6-8 ounces per acre, will not control small pigweeds if they are resistant to glyphosate. This should be applied when pigweeds are 2-3 inches tall.

You can also tank-mix Roundup with Dual post or use Sequence if the pigweeds are not up yet.

TAGS: Soybean
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