Weeds that pop through the soybean canopy late in the season make for ragged-looking fields, but how much do they hurt yields?

“Not that much, if you've controlled weeds earlier in the growing season,” says Bill Johnson, University of Missouri weed scientist. “Those late escapes — and we saw a lot of them in 2000 — aren't likely to cause much yield reduction if soybeans have established a good canopy and are growing well.”

If fields are clean for the first eight weeks after beans are planted, weeds that come on after that don't cause much yield damage, Johnson explains.

“At any rate, there isn't much you can do about three- or four-feet tall weeds that grow through the crop canopy,” he adds. “Spend your time and money on controlling weeds early.”

Timing is critical, especially with postemergence weed control.

“With the rapid adoption of Roundup Ready technology, it's easy to give in to the impulse to let all weeds come up before you make a treatment,” says Johnson. “This can be costly for a couple of reasons. First, those early weeds are the real yield robbers. Second, even Roundup can let some weeds get through when you delay spraying.”

With a total post program, you need to act four to five weeks after planting; certainly by the time weeds are 6” or so tall. Get weeds under control by the time soybeans are at the V3 stage of growth (when the plant forms three leaf branches).

“When weeds are not controlled early, it's not uncommon to see a 30-bu yield loss,” Johnson adds. “But the scary thing is, a grower may not know that he has already lost five or 10 bu. If he is growing 40 or 45 bu/acre, that's a pretty good yield. But weeds may be costing him several bushels he doesn't know about. Especially with LDP (loan deficiency payments) tied to bushels, a farmer should shoot for 100% maximum yield.”

In Missouri studies, weeds that start earliest cause up to two-thirds reduction in yield. Late weeds that show up after soybeans are already off to a good start cause little yield loss. (See “Be a Weed Wizard,” p. 55.)

Johnson suggests these weed-control tactics:

  • Plant beans in narrow rows. Soybeans themselves are an important line of defense against weeds. The sooner beans canopy, the better they stifle weed competition, and soybeans in narrow rows shade the middles quicker.

  • Use a residual herbicide that controls broadleafs.

  • Target postemergence sprays. Plan to hit weeds with a post spray before they are 8-9" tall.

“I'd do everything I could to minimize the early season competition from weeds,” he says. “If you're going Roundup Ready, consider making two treatments: one when weeds are small, the second to pick up late-germinating weeds. It's those early weeds that do the damage.”

Those weeds that break through eight or nine weeks after soybean planting may rob you of peace of mind, but they don't steal a lot of yield.

“They're ugly, but they're tough to do much about,” says Johnson. “Revenge can be expensive.”