Corn is a lot less competitive than soybean early in the season. This heavy population of waterhemp and grass will soon be robbing the crop of water and nutrients.
A giant ragweed poking out of a corn or soybean field late in the season may hurt your pride, but early season weed competition can actually do more to hurt your pocketbook.
"Early season weed interference has a huge effect [on yield]," says Wesley Everman, Michigan State University weed scientist. "In one study that we have been running for five or six years, we have seen a 20-bu. yield increase in corn when weeds are removed when they are 3" tall instead of waiting until they reach 9"."
A study at Canada’s University of Guelph found that when pigweed emerged with the crop, one-half plant per square meter led to a 5% yield loss. When pigweed emerged after corn was established, it took as many as 20 plants to cause the same yield loss.
Aaron Hager, University of Illinois Extension weed specialist, reports weeds are not all created equal when it comes to competition. Neither are crops.
"Corn is less competitive with early season weeds than soybeans," Hager says. "We have known for decades that there is a period of time when corn and weeds can coexist without reducing yield and that there is a critical time when crop yields will be reduced. There are important differences in weed management in corn and soybeans. Weed interference begins to reduce yields sooner in corn than in soybeans."
Research at The Ohio State University finds that weeds do the most damage to yield potential four to six weeks after planting. Controlling early season weeds is critical to maximizing yields. But how early is early?
Timing matters. The biggest challenge in any scenario is knowing the exact day weed interference begins to reduce yield. "You can’t predict it to the day," Hager notes. "Perhaps it was when weeds were 3" tall in corn last year, but 2" tall this year. We can’t be as accurate as we would like. We can’t look at a field and say, ‘you need to remove weeds now.’"
It also is difficult to correlate weed size at removal with bushels lost. There are differences among weed species.
"For example, Pennsylvania smartweed doesn’t reduce yields as much as giant ragweed," Hager notes. "Of course, you don’t just see a single species in the field but a mix."
Glyphosate-resistant crops became popular for many reasons—a big one being application flexibility. It’s handy to be able to wait to spray until after planting. However, that’s not always the best strategy when it comes to weed competition.
"One of the riskiest strategies is to put all of your weed management tools on at one time, whether soil-residual or post-applied," Hager says. "It’s difficult to get season-long weed control with a single post application. That’s not the fault of the herbicide but the nature of today’s weed spectrum."
Wet springs present a timing challenge for post applications, too.
"It’s better to err on the side of reducing weed interference too soon rather than too late. If you spray too soon, you may have to come back with a follow-up spray. But if you wait too late, crop yield is reduced, and you can’t get it back," Hager says.
- March 2011