Corn+Soybean Digest

Let Late Weeds Linger

Spraying has several drawbacks You've spent all you want on herbicides and think you've put your soybean crop to bed for the year. But then a few late weeds pop their ugly heads through the canopy for all the world to see. Will it pay to go after them with chemicals?

No, say weed specialists who often are asked that question. That even applies to a major troublemaker like waterhemp.

"Late-season herbicide applications have some significant risks," points out University of Illinois weed scientist Aaron Hager. "For one thing, larger weeds are more difficult to control because they're older and because spray coverage can be limited. Additionally, many postemergence soybean herbicides, including some for grass control, have preharvest intervals on their labels."

Crop injury is another potential risk, says Hager. Many post soybean herbicide labels caution against making treatments after the crop has begun to bloom. That's because crop injury at that time could reduce yield.

Farmers also need to consider the interval required between a herbicide application and the planting of a following rotational crop, Hager notes.

Growers seem especially concerned about late waterhemp in their soybeans. But even that does not justify a herbicide application, emphasizes Bob Hartzler, Iowa State University weed scientist.

"It's unlikely that those late waterhemps have a significant impact on soybean yield," he says. "The Iowa Soybean Promotion Board funded a research project investigating the competitive effects of late-emerging waterhemp on soybeans. It showed that waterhemp plants emerging after the V2 stage of soybean development (second node) generally don't affect yield."

A second reason for not treating is that it frequently doesn't work. In many situations, it simply burns off the upper 6-8" of the weed, Hartzler reports. It has little effect on seed production and doesn't help harvest efficiency.

Late waterhemp plants can produce a lot of seed, creating problems in coming years. But a late herbicide treatment probably isn't the answer. "The key to successful waterhemp management is to closely scout fields throughout June and July and treat waterhemp before it can be seen from the road," Hartzler says.

What about a September preharvest herbicide application, especially to get nightshade berries?

"Although herbicides labeled for preharvest application can cause the nightshade leaves to dry and drop, they will not cause berries to drop immediately from the plant," says Illinois' Hager.

Applying a herbicide to a weedy field prior to harvest may make combining easier. But the majority of soybean pods should be brown and most leaves should have dropped before the application is made, Hager notes.

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