There has a lot been written about the use of residual herbicides in soybeans and it is time for some of them to be going out.
I recommend the use of a residual program whether you are planting Roundup Ready, LibertyLink or conventional soybeans.
The weed that has everyone’s attention right now is Palmer pigweed, barnyardgrass should not be lost in the shuffle. Glyphosate-resistant Palmer pigweed is the greatest threat to the soybean crop itself. However, the barnyardgrass going to seed in far too many soybean fields is compounding our weed control problems in rice.
For Palmer pigweed control where you already have a glyphosate-resistance problem and plan to plant the field back to Roundup Ready soybeans, consider overlapping some residual treatments. I do not think you can afford to count on just one.
In Roundup Ready soybeans, postemergence grass control can initially be accomplished with the glyphosate, so more for the early residual should be targeted to the pigweeds.
One popular herbicide has been Valor or some of the Valor-containing products such as Envive and Enlite. I recommend any Valor-containing product as an early preplant treatment as opposed to a pre-emergence treatment. You have a good chance of getting it activated before planting and I believe the injury risk is less when applied early preplant.
Do not use Envive on high pH soils if rice follows in the rotation.
One of the problems with using Valor is the labels recommend against tank-mixing it with Dual or even using Dual sooner than 30 days after a Valor application. Therefore, if you use Valor at 14 days prior to planting, Dual (or generic metolachlor) can not be used at planting.
Later on in the season you would still have the option of adding Dual or a metolachlor product tank-mixed with a postemergence treatment.
One product you could follow Valor with at planting is Authority MTX to give you a one-two punch. You could also just use Reflex or Cheminova’s generic Dawn at planting. This would give you the fomesafen component in Prefix without the metolachlor.
Another option is to leave the Valor off and apply Prefix as a pre-emergence treatment. If it gets activated it is an excellent treatment. Keep in mind, however, if your residual treatment fails and you have resistant pigweed, then Flexstar or Cheminova’s Rhythm or perhaps Blazer are the only postemergence options. I like the option of being able to use those twice if you have to.
If you use a fomesafen product — Prefix, Reflex or Dawn — as a pre-emergence treatment, you are limited on the amount of Flexstar or Rhythm you can use without carryover issues.
Perhaps the program that gives you the most flexibility is to use a Valor-containing product prior to planting, be very timely (first 10 days after emergence) with a Flexstar, or Rhythm plus glyphosate mixture and do it again if you have to.p> If you do not think the Valor treatment was properly activated, you could use Authority MTZ at planting.
In fields where barnyardgrass is also an issue, consider adding Dual or a metolachlor product to the last glyphosate application for late-season residual grass control.
The above recommendations are for where you know you have resistance problems and are prepared for a fight. If you have not yet seen any escapes in your normal program, then back off some on stacking the residuals. Get a good residual like Prefix up front and see what happens.
In LibertyLink soybeans I think you can also back off the amount of residual you need to use some. Since you should not need Flexstar postemergence in the LibertyLink program, Prefix makes a nice treatment at planting. You can also use the Valor preplant treatment. However in the LibertyLink program, I target the residual herbicide program more for grass control.