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.
Everman says a pre-emergence, soil-residual herbicide program usually works well in Michigan. Farmers can apply a full-season rate of a residual product or use a lower rate to set up a planned post-treatment.
"Recently, we did a study in conjunction with Purdue University and Ohio State looking at pre versus post. We discovered that combining the two approaches adds an extra layer of security and helps bump yields in many cases," Everman says. In fields with moderate to high weed populations, the pre-emergence plus postemergence approach provided more consistent control with less risk of corn injury. It has been especially effective in fields with giant ragweed, burcucumber, moderate to high annual populations of annual grass, velvetleaf, triazine-resistant lambsquarters and perennial weeds.
Weed scientists advise corn growers who depend on a total postemergence program to carefully evaluate weed populations and look for products with residual activity to control weeds that emerge after application.
In soybeans, pure postemergence programs typically require a second application when weeds continue to emerge and aren’t suppressed by the leaf canopy. Using the pre-plus-post approach to allow for a slightly delayed post application can result in more consistent control of late-emerging weeds, such as foxtails, giant ragweed, black nightshade and waterhemp.
Removing early season weed interference provides a hedge against the weather, can increase yields and give growers more peace of mind.