Three steps to weed-free soybeans

After spending the last two years evaluating the efficacy and economics of winter weed management options, Dan Poston, a weed scientist at Delta Research and Extension Center in Stoneville, is convinced he’s found a system that works.

Poston evaluated 12 glyphosate-based herbicide programs at three different timings on a winter weed spectrum that included some difficult to control weeds like cutleaf eveningprimrose, horseweed and henbit.

His goal was to develop an efficacious and cost effective three-pass winter weed control system that would include a pre-plant burndown, planting in April without an additional burndown, and making only one well-timed in-season glyphosate application. He also wanted to know if tank mixing residual herbicides with glyphosate in a pre-plant burndown scenario were economically feasible.

The resulting three-pass system for early-planted Roundup Ready soybeans consists of one pre-plant burndown herbicide tank mix, an April soybean planting, and one in-season glyphosate application.

“Early planted soybean production is a system that works for us in the Mississippi Delta, and choosing an April planting date is one of the things that makes it work,” says Poston. “A stale seedbed system also seems to work better in our region than a completely no-till system. Unfortunately, we’ve only got about eight good planting days in April.

“One well-time glyphosate application tank mixed with a residual herbicide may be all that’s needed to burndown problem weeds such as horseweed, cutleaf eveningprimrose and henbit.”

To control cutleaf evening primrose, Poston says the best treatments were glyphosate mixed with 2,4-D. For horseweed or marestail control, his tests showed an 80 to 85 percent control rating with most tank mixes. Tank mixes with glyphosate and 2,4-D performed best with 95 percent plus control.

“Horseweed control generally decreased with most treatments the later applications were made,” he says. “Glyphosate-resistant horseweed populations do exist in Mississippi and it is extremely important that we do a good job controlling both susceptible and resistant populations of this weed.”

Poston said there are some benefits to tank mixing glyphosate with residual herbicides, because glyphosate and 2,4-D both lack residual activity. Some residual herbicides improve efficacy and can hold weed ground cover down until an in-season glyphosate treatment can be made in April or May.”

Soybean yields were higher where residual herbicides were used. Poston says that the net economic test results mirror his yield results. “The residual herbicides look like they will pay for themselves. We saw a significant yield response to an at-planting burndown in plots that had as little as 5 to 10 percent of ground cover,” he noted.

“As little as 10 percent ground cover at planting time may warrant a burndown treatment, both from an economic and from a yield standpoint. The only treatments that had less than 5 percent groundcover at-planting in mid-April were burndown programs that included residual rates of Valor and Canopy XL.”

With his three-pass system, Poston says residual activity may be necessary, especially in high yield and high weed pressure systems. Tankmixes with Canopy XL and Valor provided the most benefit in his studies.

“The residual herbicides in an early planted soybean production system have a real fit. It really adds some flexibility, and the fact that it pays for itself makes it an even better option,” he says. “Valor is the most versatile. However, from a efficacy and residual standpoint, Canopy XL is a better choice.”

His preferred tank mixes consisted of 1.5 pints of a 4-pound glyphosate product plus either 3.9 ounces of Canopy XL, or 2.5 ounces of Valor. “There’s a lot of flexibility to adjust the glyphosate rate upward based on the weed species present in your particular field,” he says.

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