In 2012, Greg Kerber attacked the weeds in his no-till soybean fields with six herbicide modes of action – including three effective pre-emergence herbicides. An early April burndown included full rates of 2, 4-D and glyphosate plus soil-residual products Prowl and Sonic, followed by an early postemergence application of Liberty. Sites of action: 2, 3, 4, 9, 10 and 14.

It was the second year that Kerber used pre-plant residual herbicides on every soybean acre “to try to get more modes of action out there. I’m worried, like everybody, about resistant weeds, especially waterhemp,” says the Gibson City, Ill., corn and soybean producer. Growers to his south are battling multi-herbicide-resistant waterhemp, and to his east, glyphosate-resistant marestail. Fortunately, “Glyphosate is still effective here,” he says, “and if we mix it up, we can keep it effective.”

Kerber spent about $35/acre on his soybean weed program this year and had fairly clean fields, although the drought hindered residual herbicide performance.

As glyphosate-resistant weeds multiply, Midwest growers are relearning the strengths – and shortcomings – of pre-emergence (pre) soil-applied herbicides, says Jeff Gunsolus, University of Minnesota Extension weed scientist.

For two decades, Corn Belt farmers have relied on broad-spectrum, postemergence (post) weed control, especially in soybeans: first Scepter, Pursuit and Classic in the early 1990s, then Roundup. Consequently, “People have lost the knack of using pres,” Gunsolus says. Today, farmers who came of age in the total post era “have little experience with this chemistry.” Simply putting down a pre-plant herbicide doesn’t guarantee good weed control, Gunsolus says. The success of a residual herbicide program depends not only on environmental factors, but also on weed biology and densities, herbicide selection, application rates, and application timing. “We need the right pre for the right weeds, at the right rate, and the right time,” Gunsolus says.

He and other Midwest weed management experts offer tips for improving soil-residual herbicide effectiveness in soybeans:

Target the dominant weed for pre-emergence control. Select the best residual herbicide for the primary weed species in each field, says Aaron Hager, University of Illinois Extension weed scientist.

And keep in mind that the dominant weed may vary from field to field.

Waterhemp drives Craig Herickhoff’s residual herbicide choices. He and his father Mark grow corn, soybeans, sugar beets and wheat near Belgrade, Minn. Like many Midwest farmers, the Herickhoffs were having trouble controlling waterhemp with glyphosate, especially in soybeans. And they wanted to preserve the effectiveness of glyphosate for sugar beets, a crop for which there are few cost-effective POST herbicides. So in 2011 and 2012, they applied Valor to all their soybean acres before planting.

The PPO-inhibitor provided good control of waterhemp, as well as lambsquarters, another problem weed in their fields. “We got about five weeks of weed suppression” for $10/acre more than two glyphosate passes, says Craig Herickhoff. “We wouldn’t go back to a total post program again for soybeans.”

If you select a premix, think about how much of each active ingredient will be applied at the full labeled rate, Hager says. For example, sulfentrazone, an active ingredient in premixes such as Authority XL, Authority MTZ and Authority First, is effective on waterhemp. But the amount of sulfentrazone – and consequently the duration of waterhemp control – varies by product, Hager says. So if waterhemp is the dominant weed in a field, select the premix that supplies the most sulfentrazone.

Apply full rates. Apply the full labeled rate for the soil texture and pH, organic matter content and time of application, says Andrew Schmidt, regional agronomist for Winfield Solutions, Columbia, Mo. The common practice of applying reduced, or “set-up,” rates is not recommended these days, he says. In the past, reduced rates were an option when growers knew they could clean up the field later with glyphosate. But with more weeds resistant to one or more post herbicides, that’s no longer smart, Schmidt says.

Balance application time with duration and weather risk. To maximizeweed suppression after crops emerge, it’s critical to apply a soil-residual herbicide as close to planting as possible. That’s especially important when battling weeds with a long germination period, such as waterhemp.

For no-tillers, “the timing of the burndown can be tricky,” says Kerber, the Illinois farmer. “You don’t want the residual to run out before the canopy forms.” In 2012, weeds in his fields started emerging earlier than usual because of warm weather, but Kerber held off on his early preplant application until April in order to get as much residual control as possible while soybeans were growing.

If you’re battling waterhemp, which has a long germination period, “you’ll want to apply the pre as close to planting as you can,” Schmidt says. Products such as Valor, Gangster or Authority must be applied no later than three days after soybean planting, Gunsolus notes, so have a backup plan in case you get rained out. “You have the option of putting on a post residual like Prefix or Warrant.”

Because there are no herbicides that last all season, parts of the Corn Belt infested with resistant waterhemp may have to resort to an approach called “overlapping residuals,” says Kevin Bradley, University of Missouri weed scientist. Growers apply a pre herbicide at planting, then follow up with a post residual herbicide 14 to 21 days later to suppress later-emerging waterhemp. Waterhemp seed is fairly short-lived in the soil, so if you prevent plants from producing seeds for three or four years, “you can virtually eliminate this weed as a problem,” he says.

Understand crop injury potential. Crop injury from soil-applied herbicides is often related to application timing or environmental conditions, Hager says. Applications made right before or after planting result in a high concentration of soil-applied herbicide near the emerging seedlings. Stressful growing conditions hinder plants’ ability to metabolize the herbicide.

It’s also important to understand herbicide crop rotation restrictions, Gunsolus says, especially for growers who raise sugar beets, dry beans, peas, sweet corn or canola in the rotation. “These are a big deal.”

Craig and Mark Herickhoff, the Minnesota farmers, grow sugar beets, which narrows their corn and soybean herbicide options considerably, says Darrol Ike, a Delano, Minn., crop consultant, who’s helping them diversify their weed control chemistry. For example, Authority products are a good pre-emergence complement to glyphosate for soybeans. But the long crop rotation intervals for sugar beets – roughly 40 months – takes them off the table in a beet rotation.

Incorporate, if possible

All soil-applied residual herbicides have the same Achilles heel, Hager says: they have to be dissolved in the soil through mechanical incorporation or a rain within seven to 10 days.

“We dig everything in,” says Craig Herickhoff. “Yes, it’s a hassle in the spring, but if you don’t get a timely rain, it’s a lot better to have it incorporated.” In 2012, though, seedbeds were dry and cloddy, so he set the planter’s row cleaners deeper than usual. That affected pre placement. “It was perfectly clean between the rows, but in the rows, there was more weed pressure. I think it was because we went deep with the row cleaners, and when we pushed the dirt aside, we pushed some of the herbicide aside, too.”

If incorporation isn’t an option, “you can try to hedge rainfall risk by putting the herbicide out a little sooner,” Hager says. In 2012, though, many parts of the Corn Belt did not receive enough rainfall to move the pre into the soil solution, so weed control suffered.

Nevertheless, “I still believe it was beneficial,” Missouri’s Bradley says. “Our own evaluations showed us there was still some residual weed control, just shorter than normal.” Weed suppression “probably lasted two weeks, where typically it would be about twice as long.”

If it stays dry into 2013, should growers apply a pre-plant herbicide next spring? Yes, Gunsolus says. “We can’t predict the weather. The bottom line is, it’s a risk management tool.”