STONEVILLE, Miss. -- It’s no secret why farmers took to Roundup Ready soybeans when the technology was first introduced in the mid-1990s. Growers could make one or two applications of glyphosate after planting and turn their focus to other things.

But as soybean producers have moved to earlier planting and earlier-maturing varieties, they’ve begun to notice that the Roundup Ready system may not provide the season-long weed control they need to produce the higher yields to which they’re becoming accustomed.

“My No. 1 weed problem in soybeans these days is annual grasses at harvest,” said Dan Poston, weed scientist and Northwest District Extension soybean specialist at the Delta Research and Extension Center at Stoneville, Miss.

“Annual grasses were not a problem at harvest when everyone harvested in October. But with more and more farmers planting early, we’re harvesting in August when conditions are much more conducive for annual grass germination and development.”

Growers have increased yields to such levels with early plantings that “it’s clear we’re not going back to later planting,” said Poston. “We’re going to continue to plant soybeans early because of the advantages we see with early soybean planting systems or ESPS.”

So what can soybean farmers do to avoid situations where they have to spray a desiccant to eliminate grassy areas or experience yield losses at harvest?

Poston and other researchers at Stoneville have been testing tank mixtures of glyphosate and residual grass herbicides applied in early to mid-May before summer annual weeds typically begin emerging in soybeans.

“Under the old planting system, residual herbicides usually went out at planting in May and June during a period when many troublesome annual weeds are emerging,” said Poston. “Now we’re planting in March or early April. Residual herbicides applied this early would have to lie in the field for four to six weeks before the emergence of many problem weeds. Most of the residual herbicide would have dissipated prior to when it was most needed.”

Farmers often make their first glyphosate application on Roundup Ready beans in early May so adding a residual herbicide to glyphosate at this time during the growing season will work well because it coincides with weed emergence. Adding a residual herbicide to the spray will.

Poston is working with several residual herbicides — some not yet registered for postemergence use in soybeans — to try to determine what can provide farmers with the most control for their money. Among those are metolachlor (Dual II Magnum), dimethenamid-p (Outlook), pendimethalin (Prowl H2O), flufenacet (Define) and Sequence, a mixture of glyphosate and metolachlor.

“Sequence is now labeled for postemergence soybeans and works well in this situation,” said Poston. “Outlook is also labeled and can be mixed with the glyphosate of your choice. Prowl H2O is relatively inexpensive and looks promising, but it is not labeled for this use in soybeans at this time.”

Poston said the third trifoliate leaf stage makes a good target for the tank mix of the glyphosate and residual herbicide combination.

That usually occurs in early May for soybeans planted in late March or early April. “This year it’s been cold and wet, and soybeans haven’t been growing,” he said. “It may be more like mid-May before we get to that stage this year.”

Adding a residual grass control component to a glyphosate only postemergence program using currently labeled products will add $10 or more per acre, depending on the product selected. However, one application with glyphosate and a residual grass herbicide is as good as two applications of glyphosate alone, especially where annual grasses and pigweeds are the primary targets.

“You pay the technology fee for the Roundup Ready system so you might ask why not use Roundup only,” said Poston. “But that doesn’t help solve all the weed control problems a grower can encounter.”

When most growers were harvesting Group 5 and Group 6 soybeans in October, it was generally too cool for grasses to germinate and the soybean plants “shaded out” the grasses and other weeds until they were ready to harvest.

But as harvest moved to September and then to August, soybeans began dropping their leaves when warm temperatures provided nearly ideal conditions for grasses to germinate and become large enough to interfere with harvest.

“Our work has shown that we frequently get the largest flushes of grasses from May 9 to June 9,” said Poston. “As long as the glyphosate controls the weeds that are present, the residual herbicide can prevent those flushes. It doesn’t have to be that long-lived as long as you have the herbicide in place when the grasses are germinating.”

It also depends on row width, he said. “Residual herbicides can be much more important in wide rows. In narrow rows, it’s possible that with one shot you’re going to be done most of the time.”

The added expense of the residual herbicide has to be a consideration when soybeans are selling for $6 a bushel. “But you also have to ask what’s the value of not having to make another trip or to desiccate the grasses prior to harvest? Time-wise it could be worth $8 to $10 an acre not to have to go across the field again.”

Poston said researchers are hopeful Prowl H2O will eventually become labeled for postemergence applications in soybeans. More tests are needed to ensure crop safety under various environmental conditions and on different soils types. In addition, manufacturers will have to be willing to pursue a label. “It appears to be a safe product – yields have been as good as with other herbicides – but we have to make sure we have a safe product.”

Poston and Trey Koger, soybean agronomist with the USDA’s Agricultural Research Service at Stoneville, are also conducting studies on seeding rates and narrower row spacings under Delta conditions.

“The Delta in general is probably not the best place for narrow rows — or true narrow rows — because of the drainage problems we experience,” said Poston. “It may be better if growers use a system of planting twin rows on traditional 38 to 40-inch beds or narrow rows on wider beds to improve drainage, especially with extremely early-planted soybeans.”

He believes growers will see more twin-row systems in the future but that they won’t negate the need for residual herbicides. “Even though they canopy well in the 7.5-inch and 15-inch row spacings, you still may need a residual herbicide because of the gaps between the beds.”

Researchers also are working with combinations of traditional burndown herbicides and residual herbicides to help growers achieve cleaner fields at planting. And they’re finding that sooner is better than later for applications of the mixtures.

“An earlier burndown — March 1 or earlier — appears to provide better control because you’re spraying smaller weeds,” he said. “But you need to add a residual herbicide to control subsequent flushes of weeds. Our work shows the residual herbicide generally will pay for itself.

“We’re recommending that you start scouting your fields to see what’s out there on Valentine’s Day and spray the herbicides you decide on by March 1,” he said. “Some producers made spring burndown applications as early as January and with the addition of a residual herbicide were able to plant in late March and did not make a glyphosate application until May.

“A lot of folks in the south Delta did that this year and it worked. It makes it so easy to drop your planter into a clean field in early April and plant.”

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