With the advent of Roundup Ready technology, many Midsouth soybean growers are battling an increase in annual grass infestations.

Grasses such as barnyardgrass, Southwestern cupgrass and browntop millet are going to seed after harvest. This increases the level of grass seed in the soil seed bank, making grasses more difficult to control in soybeans and crops that are rotated into soybean fields. Pigweeds and nutsedge are also on the increase.

Dan Poston, a weed scientist at the Delta Research and Extension Center in Stoneville, MS, is working to better control annual grasses.

“Pre-emergence herbicides applied to March- and April-planted soybeans are often dissipated by the time summer annual grasses emerge in May and June,” he says.

Bobby Maxey of Indianola, MS, says the idea of controlling annual grasses with a residual herbicide during the growing season sounds promising.

Maxey, a soybean grower and owner of Rebel Auction Company, tankmixes Dual with his pre-emergence application of Roundup to provide early season residual herbicide activity.

“If I could spray a residual herbicide over-the-top with my second glyphosate application, I may not have to go back with a third shot of glyphosate to control break-through grasses,” he says. “That tankmix could also save us some money if we could avoid using Gramoxone or chlorate to dry out the soybeans just before harvest.”

Mixing residual herbicides in with postemergence glyphosate applications when the grasses are emerging may work to control mid- to late-season annual grasses, he says.

In his studies, Poston has used an in-season application of several residual herbicides in combination with glyphosate in a Roundup Ready soybean production system. Included in the field tests are Dual by Syngenta, Outlook by BASF, Prowl H2O by BASF and Detail by Bayer.

They have all significantly reduced annual grass pressure at harvest and eliminated the need for harvest aids, as compared to two applications of glyphosate alone.

Currently, the only tested herbicides labeled for post-emergence use in soybeans are Sequence and Outlook.

Outlook herbicide has been commercially available for several years, but Poston says its in-season use has been cost-prohibitive for most growers.

Sequence is a premix of glyphosate and s-metolachlor, the active ingredient in Dual. “Dual has no post-emergence activity. It's only there for soil residual activity after you apply your Roundup,” Poston says. “A treatment of Sequence reduces trips across the field and eliminates the need for harvest aids, which together offset its added cost.”

Poston has also tested the Bayer herbicide Detail in his studies, but it's not yet labeled for post-emergence use in soybeans. However, he says, his in-season applications of the product tankmixed with glyphosate resulted in noticeable crop injury and slight yield reductions.

On average, the residual herbicides used in Poston's research added $10-17/acre to weed control treatment costs. In comparison, the cost of Prowl H20 would likely be about half the cost of the other residual herbicide treatments if it ever were to become labeled for over-the-top use.

Poston has found an over-the-top treatment of a residual herbicide controls the annual grasses that grow into harvesttime.

“One application of any of these mixtures is usually as good as two applications of glyphosate where annual grasses are your primary weed problem,” he says. “In the glyphosate-only program, you may still have to use a harvest aid product.”

These over-the-top residual herbicide systems will likely not work as well as the two-shot glyphosate system with larger seeded weeds such as morningglories, says Poston. On the other hand, tankmixing a residual herbicide with glyphosate may help reduce the need for a harvest aid, he adds.

“Grass pressure at this level will really slow down a combine and increase soybean loss at harvest,” he says.