Should you put your weed control program on “residual power?” Adding a pre-emergence residual herbicide to your glyphosate program has a lot of advantages, says Dawn Refsell, formerly an Extension specialist at the University of Illinois, now with Valent in Kansas City, MO. A pre-emergence residual:

RESISTANCE THREAT

But more important, Refsell says, adding a residual to your glyphosate program discourages glyphosate-resistant weeds in your fields. Soil-applied herbicides provide “another site of action for weed control, helping to decrease the opportunity for herbicide resistance.”

The widespread adoption of Roundup Ready crops “has led us to heavy dependence on post-only herbicide programs with no residual activity,” Refsell says. Growers love the safety, convenience and ease of a total post glyphosate system.

But too often, she says, growers delay their first glyphosate pass until early season weed competition has already caused yield losses. Corn yields may lose 4 bu./acre for every inch of weed height over 4 in., she says.

OTHER BENEFITS, TOO

Likewise, soybean yields lost to early season weed growth are “often greater than producers realize,” says Lowell Sandell, a University of Nebraska Extension weed expert. Nebraska researchers found that soybean yield loss from weed competition occurs by the V3 growth stage or sooner, depending on row spacing, and can occur as early as V1 in 30-in. rows.

But the potential for weed resistance is an even bigger threat to growers, Refsell says. Little more than a decade after glyphosate-tolerant crops were first introduced, major corn and soybean-producing states — including Ohio, Indiana, Illinois, Minnesota and Missouri — have reported glyphosate-resistant common waterhemp, giant ragweed and common ragweed.

In Missouri, for example, a 2007 University of Missouri survey found that glyphosate-resistant waterhemp occurs on about 200,000 acres of soybeans. A random survey of weedy soybean fields in the northern half of Missouri in 2008 found 45 separate populations of gyphosate-resistant waterhemp in 28 counties.

The future value of glyphosate “can be preserved only by proper management,” Sandell says. For waterhemp, giant ragweed, lambsquarters and other hard-to-control weeds, a pre-emergence residual may be your bestoption, Refsell and Sandell say.

Adding a residual herbicide to your glyphosate program has other benefits, too. It extends the window for timely post application by seven to 14 days, Refsell says. It also improves the effectiveness of your post-glyphosate application. Total weed density is lower and weeds are smaller and more uniform in size, so there is better herbicide coverage.

Of course, there are also risks in using a pre-emergence herbicide. These products need rainfall to be activated. They are selective for certain weed species. And they may cause crop injury, although that's usually minor, Sandell says. There's also the additional expense and labor at a time when “growers would rather be planting,” Refsell adds.

Refsell recommends a residual herbicide — either applied pre-emergence or included in your post application of glyphosate — in fields:

Keep in mind that higher crop prices lower the economic threshold (ET) for herbicide applications, Sandell says. For example, the ET on a $23/acre herbicide application drops from 3.8 bu./acre for $6 soybeans, to 1.6 bu./acre for $14 beans.

In 2008, high crop prices and high glyphosate prices made residual herbicides very competitive with glyphosate, Sandell notes. Since then, prices for both generic and brand-name glyphosate have dropped sharply, falling to $5-13/acre by summer, 2009, according to the University of Nebraska's annual herbicide survey. The estimated cost for a pre-emergence herbicide in soybeans ranges from $10 to $28/acre, depending on the herbicide and rate, he says.

Although low glyphosate prices may result in many growers using glyphosate-only weed control, there are good reasons that “producers should still consider a pre-emergence herbicide,” Sandell says.