Over time, Mid-South soybean farmers exclusively relying on glyphosate for weed control has resulted in an ever-increasing problem: later-emerging annual grasses.

But research done by soybean agronomists and weed scientists at the USDA experiment station, Stoneville, Miss., and Mississippi State University, has found an effective, cogent solution.

What’s more, says Trey Koger, USDA soybean agronomist, the adjustment can make wise financial sense.

Koger said farmers’ elective to follow up the customary one or two applications of glyphosate on early soybean productions systems with applications of a residual herbicide works effectively and more than pays for its extra financial cost.

“The No. 1 weed problem in soybeans in the Mid-South is annual grass, mainly because we’ve removed all the residual herbicides from our Roundup Ready production systems; and because we are harvesting our soybean crop earlier in the year, we are leaving ample time in the fall for grasses to produce a seed crop before a killing frost,” Koger said.

In years past, when the majority of farmers were planting soybeans in late May and June, they were killing many of the grasses with tillage and herbicides before soybean crops were planted.

But now that most farmers are planting in April and are having to control grasses with glyphosate in the crop, there remains sufficient time for the grasses to produce seed in the fall after harvesting.

Consequently, Koger said that farmers and scientists have observed obtrusive annual grasses emerging in late season.

“The vast majority of our soybean acres are only sprayed with glyphosate. Soybeans planted in narrow rows (less than 30 inches wide) are sprayed usually with one to two glyphosate applications, while wide-row soybeans (30 inches or wider) are sprayed with two to sometimes three to four applications,” he said.

The options for residual control vary, depending on row spacing, according to Koger. “We have more options in wide rows. However, overall very little of our acreage is planted to wide rows. We can apply Prowl or metolachlor-based herbicides with glyphosate with a layby bar underneath wide-row soybeans, but with narrow-row soybeans, it is a different story.”

Economically, Koger said, Sequence is the practical option for residual control, and a good fit for early soybean production systems.

Based on joint research conducted by Koger and MSU weed scientist Dan Poston, Koger said, “We typically get a big flush of annual grasses that emerge in late April to early May. This is the same time in which most of our soybean crops planted in early April are in the V2 to V3 growth stage. If we apply Sequence at V2 to V3, we can control the grasses that emerged in late April to early May with the glyphosate component of Sequence and get the most out of the residual component of Sequence, which is metolachlor.”

Koger cautioned that metolachlor does not provide residual control of browntop millet. “If a grower has this annual grass species, the glyphosate will control what is up, but the metolachlor is not going to provide residual control,” he said. “Sequence can be applied over-the-top of soybeans before the V4 growth stage. By applying before V4, we get most of the herbicide down to the weeds and soil, and not on soybean plants.

“Applying Sequence at 2.5 to 3 pints per acre — especially at 3 pints per acre — in early to mid-May does a good job of providing residual control of annual grasses and allows us to harvest a soybean crop pretty much clean of annual grasses.”

Koger said while Roundup applications remain relatively inexpensive, an increase in applications from one or two sprays to three or four sprays will take its toll on herbicide and production costs to farmers, especially in light of increasing fuel costs.

“We are eroding the profitability of the Roundup systems. The higher the number of applications, the higher the overall costs,” he said.

However, he said that by simply adding an application of residual herbicide translated into a $8 to $10 net return per acre on wide-row soybeans and either paid for itself or added up to a $1 to $2 net return per acre in narrow-row soybeans.

The economic advantages were based on extensive trial studies conducted on dryland soybean production systems, with planting dates ranging from early April through mid-May. Results evidenced a yield increase of 5 bushels per acre (from 39 bushels per acre to 44 bushels per acre) in wide-row crops, and a yield increase of 2 bushels per acre (from 45 bushels per acre to 47 bushels per acre) in narrow-row crops when a residual herbicide (Sequence at 3 pints per acre applied to soybeans in the V3 growth stage) was applied.

“I truly understand that spending more money on a crop is tough — especially with costs increasing everywhere else,” he said. “I also understand that a residual herbicide may not be warranted every year — especially in narrow-row soybeans because they canopy so much quicker — but a residual herbicide applied every other year or one out of three years in fields with lots of grasses will pay off.

“Some fields have built up such a grass seed bank, that a residual herbicide may be needed every year until we get the grass pressure in check.”

Koger said that adding the residual in wide-row and narrow-row systems translates into favorable results that extend beyond yield measurements to include seed quality, harvest efficiency, and reducing the grass seed bank, which can be especially important from a soybean/rice rotation standpoint.

Improving grass control in soybeans with glyphosate and a residual herbicide will help reduce the pressure and number of grass weeds that must be controlled in the following rice crop.

“Things that are harder to put a number on with residual herbicides like Sequence or Prowl plus glyphosate include the fact that weeds slow harvest and we must throw some beans out the back of the combine that are included in the grasses we are discarding,” he said.

“Another potential benefit to consider — but again extremely difficult to measure — is the fact that in years when we do get a lot of annual grasses and we get a rain right before harvest, it’s harder for those soybeans to dry; and if we have a prolonged wet period we tend to have more seed decay and damage than we would if those beans were clean.”

Koger noted that the research was conducted during atypical dryland conditions. “So we think that it should more than pay for itself in an irrigated type situation.”

e-mail: abell@farmpress.com