There was a time when you could barely think of a weed control program for soybeans that did not include starting off with some kind of residual program. When I first got into the ag chemical business, our product of choice was Squadron (Scepter + Prowl). I do not think that one even exists anymore.

Especially in Arkansas, Roundup (glyphosate) was badly needed when it came on the scene. Weeds like common cocklebur and pigweed had become ALS-resistant. Other weeds had undergone species shifts to more difficult weeds like balloonvine, teaweed, morningglories, sesbania and nutsedge.

Roundup Ready saved the day and is still saving the day for many growers. As with any technology that is relied on too much, weed resistance has now caused glyphosate to lose some of its luster, and it cannot be relied on as a standalone program any longer.

Whether it is for weed resistance management or management of a weed problem that is already there, many growers are turning to a few new residual products and some of the old standbys.

We have now documented two important weeds (horseweed and Palmer amaranth or pigweed) and three weeds of lesser agronomic importance (common and giant ragweed and johnsongrass) that are glyphosate-resistant in Arkansas.

Soybean producers have two good options — Valor and Prefix — for residual control of our two most significant glyphosate-resistant weeds.

It should be pointed out that although I am talking about Valor specifically, there are a number of package deals that include Valor and all them have looked good in my trials for pigweed and horseweed residual control. They include, but are not limited to: Envive (Valor + Synchrony), Valor XLT (Valor + Classic), and Gangster FR (Valor + Firstrate).

I focus on Valor because it represents a new mode of action against pigweed. Many populations of pigweed in Arkansas are ALS- and DNA-resistant. Products like Canopy, Scepter and Prowl may not perform if pigweed is the primary target.

There is some injury potential with pre-emerge applications of Valor in soybeans. The danger comes when a rain is received just as the beans are emerging. The herbicide can be “splashed up” onto the emerging cotyledons. Therefore, I prefer to use Valor in a preplant scenario, typically in a tank-mix with glyphosate. If you are willing to take the risk Valor is labeled pre.

We have observed 20 to 30 days of residual pigweed and horseweed control from Valor residual. Also, Valor will control small emerged pigweeds in a burn-down application.

Prefix herbicide is a new premix product that contains Dual and fomesafen, the active ingredient in Flexstar and Reflex. Last year it came as a co-pack, this year it is mixed. It is excellent on a very large spectrum of grass and broadleaf weeds, including Palmer pigweed. On the treatments I have looked at, Prefix is one of the best premixes for soybeans.

There are other good residual options out there. I mentioned the Canopy products. There also are Authority, Authority First, and Authority MTZ, and other grass and broadleaf premixes. I am not dismissing those products, but am focusing on the best options for resistant pigweed in a Roundup Ready program.

So far, we do not have resistance to the ppo inhibitors like Reflex and Valor, so they are the resistance management tools of the day.

It is important to remember that with residual herbicides you must have rainfall for activation. Without it, 100s can become 0s in terms of weed control and a residual that does not get activated is usually a waste of money. This is one reason most weed scientists agree that residual herbicides will never replace Roundup and Roundup Ready Crops.

e-mail: bscott@uaex.edu