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Corn+Soybean Digest

Kill 'Em To Grow 'Em

Have you ever considered planting conventional soybeans mixed with Roundup Ready lines to boost yields and reduce your herbicide costs?

It may sound obscure, but a South Carolina soybean agronomist is saving one glyphosate application and seeing a 2-4 bu. increase in Roundup Ready soybean yields by using conventional soybeans as a cover crop.

This practice doesn't sound economically feasible because a 50-lb. bag of conventional seed runs $16-17. But by using conventional seed you grow yourself (bin run) as the cover crop, you can skip that second Roundup application and save money, says Jason Norsworthy, a Clemson University agronomist.

“With soybean prices the way they are now, 50 lbs. of conventional seed per acre you grow yourself (about 150,000 seeds/acre) will cost $5.50-6/acre,” says Norsworthy. “So you're looking at saving about $4/acre or more if you are able to apply only a single application of Roundup. And that doesn't include the potential yield benefits.”

Norsworthy has been studying the conventional/Roundup Ready mix for more than five years. In this system, a 50-50 mixture of conventional and glyphosate-resistant seed is drilled into 71/2 in. rows at a rate of 150,000 seeds/acre for each type. The drilled beans help provide a dense canopy, which creates a weed-reducing shade.

The plants grow together for about three or four weeks after emergence. Then a standard over-the-top glyphosate application is made. The conventional plants are terminated, leaving mulch.

“The mulch reduces evaporation of soil moisture,” says Norsworthy, adding that the Roundup Ready beans likely receive a boost from nutrients released by the mulched plants. “This can be a definite benefit when the plants start setting, producing and filling pods. It could be the reason behind our increase in yields of 2-4 bu./acre.”

Those extra bushels can easily add close to $25/acre to the bottom line, based on a 4 bu. increase at about $6/bu. Added savings are seen when the initial Roundup application is the only one needed to maintain weed control.

“The early canopy system prevents weed development early on,” says Norsworthy. “And our research has shown that the remaining canopy enables us to get by with one Roundup application.”

Cutting the second glyphosate application can save $8-10/acre, considering the chemical cost of about $6 and application costs of $2-4/acre.

Add that to the $15-18 earned from added production, and the return can approach an extra $25-28/acre, excluding the cost of the conventional seed.

“To make the system economical, the conventional seed should be grown by the grower or bought for the going soybean price,” says Norsworthy.

Norsworthy says Group VII and VIII varieties work best in the program in South Carolina. “We have looked at Groups V, VI, VII and VIII, which are common in South Carolina,” he says. “The VIIs and VIIIs seem to work better.”

He notes that with Group Vs, for example, if there is a late planting, the Roundup Ready plants begin flowering soon after the conventional plants are terminated. Yields are likely to decline because the plants aren't able to form a dense cover.

The system looks more promising for the South. “Southern soybean varieties are usually determinate beans and will flower a couple of weeks later in the growing season,” he says. “Indeterminate beans in the Midwest have a shorter window from the time of emergence to flowering.”

The system will help growers stay on schedule with their herbicide plans.

“Essentially, this system makes the farmer properly time the Roundup applications,” says Norsworthy. “We can often kill weeds at six to seven weeks after emergence. But those weeds have already had a negative effect on yields. By growing the conventional beans, farmers know they have to come back in three to four weeks to clean up.”

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