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Kill weeds while they're small to prevent yield loss

Controlling weeds in corn fields before they reach four inches tall can help growers eliminate "invisible yield loss," according to University of Wisconsin Extension weed scientist Chris Boerboom. He says when control measures are put off until weeds are larger, the yield loss can be 10 to 20 bushels per acre.

It doesn't take long, maybe a week, for weeds to grow from 4 to 8 inches tall," Boerboom says. A soil-applied herbicide can help growers manage risk on Roundup Ready fields, he says, by increasing overall weed control and reducing the risk of glyphosate-resistant weeds.

"Preemergence applications increase the overall level of weed control, which is important for fast-growing weeds like giant ragweed and later-emerging weeds like crabgrass and waterhemp," he says. "By adding a residual herbicide to a system at the start of the season, we're reducing the number of weeds we're spraying postemergence and stunting many weeds that remain, which buys more time for postemergence applications."

According to research trials last year in Minnesota conducted by University of Minnesota Extension weed scientist Jeff Gunsolus and his team, an application of glyphosate on V3 to V4 corn following the use of a half-rate of a soil-applied herbicide provided the most consistent economic return over a wide range of annual weed species and environmental conditions.

One trial evaluated yields when post applications of glyphosate were used at five different weed height stages. Those yields were then compared to a two-pass program of an acetochlor-based herbicide applied preemergence at a half-rate, followed by a post application of glyphosate.

Using the soil-applied herbicide resulted in a 10 bushels-per-acre advantage over controlling weeds at that were 5 to 6 inches high. That jumped to 18 bushels per acre for weeds controlled at 7 to 8 inches, and 44 bushels per acre for weeds controlled at 9 to 12 inches.

In this case, the study found the grower would have recovered the cost of the soil-applied herbicide at the 3- to 4-inch weed height, and earned profits with soil-applied herbicides versus a glyphosate-only spray at 5-inch weed heights and above. At the 9- to 12-inch weed height -- using a $10 approximate herbicide cost and $2 per bushel corn price -- that profit would have been $78 per acre.

The result of competition between emerging corn and weeds is made clear when during a five-day period between the 7- to 8-inch and 9- to 12-inch weed treatments, a yield loss of 26 bushels per acre occurred.

At first glance, cost may be a factor that causes growers to turn toward an all-post glyphosate program, but Boerboom says growers should take a realistic look at their operation. He recommends growers consider the number of acres they need to treat, typical spring weather conditions and equipment availability. When it's unlikely growers can treat their acres within the ideal application window, Boerboom says they should consider the benefits and extra yield protection they're going to get with a preemergence program.

While a post-only glyphosate program can work successfully, Boerboom says growers are setting themselves up for herbicide-resistant weed populations if glyphosate is over-used. Using soil-applied herbicides helps control weeds and cuts the risk of resistance, he says.

"With a two-pass program, you're not relying on one single treatment to work perfectly," Boerboom says. "You're building some redundancy and risk management into the system. Growers should evaluate where they get the greatest value from glyphosate and when they can use herbicides with other modes of action in their crop rotation, and then manage accordingly."

Controlling weeds in corn fields before they reach four inches tall can help growers eliminate "invisible yield loss," according to University of Wisconsin Extension weed scientist Chris Boerboom. He says when control measures are put off until weeds are larger, the yield loss can be 10 to 20 bushels per acre.

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