K-State Agronomist Gives Tips for Doublecropping into Wheat

Grain sorghum, soybean farmers have weed control options. Compiled by staff 

Published on: Jun 28, 2006

Much of Kansas remains dry, but some areas have had enough moisture that producers might consider doublecropping soybeans, sorghum or another summer crop after wheat this year. 

"Most wheat fields have weeds in them that need to be controlled," says Dave Regehr, weed management specialist with K-State Research and Extension. "Doublecropping offers a chance to manage what grows in those stubble fields, instead of letting them grow up in weeds."

What's the best timing for weed control, planting, and fertilizing when doublecropping?

"As you harvest the wheat, note the weed situation," Regehr says. "If there are tall weeds - winter annuals such as prickly lettuce and marestail or summer annuals such as sunflower and kochia - that have gotten an early start in thin wheat, they'll be topped by the combine sickle. These weeds will have to grow new tops and top leaves before they can be controlled with herbicides. This could delay application by two weeks or more."

In thicker wheat that had a fall or spring herbicide treatment, winter annual weeds should be absent, and summer annuals should still be below cutter bar height, he says. After a few days of post-harvest winds to settle the straw, glyphosate can readily control these weeds.

Growers should not add 2,4-D to the glyphosate unless they are prepared to delay planting by seven days per pint of 2,4-D (4 pounds acid equivalent/gallon).

Regehr has these tips for growers to keep in mind:

Soybeans

• If planting Roundup Ready soybeans for grain, plant as soon after wheat harvest as possible - even right behind the combine - to give the beans enough growing time.  

• Soybeans to be used as forage or cover crop can be planted later. To keep costs down, consider using reduced seeding rates (60,000 to 90,000 seeds per acre). Some seed companies offer discounted seed for double cropping.

• Don't neglect the seed inoculant. Most stubble fields will have enough weed pressure to warrant a glyphosate application within a week after planting. Where few weeds are present and where topped weeds need more time to make new leaves, a 2- to 3-week delay may be warranted. As they grow, soybeans in wheat stubble use up much of the soil moisture from summer rains. This and the shade they produce will help reduce development of weeds and volunteer wheat.  

Sorghum

• When doublecropping with a grain or forage sorghum, be sure to "burn down" any weeds and grasses that may be present after wheat harvest. If soil moisture is good, you may want to plant as soon as possible after harvest and then apply the burndown herbicide before the sorghum emerges. The timing will depend on whether there is enough time to get back into the field, including whether rain is in the forecast (which could delay the spray application and speed up the date of emergence)

If the weeds have been topped, producers will have to wait until regrowth begins before spraying, even if that means a delay in planting. No herbicides are available for postemergence control of grasses in sorghum and few for broadleaf weed control.

• After the sorghum emerges, atrazine and crop oil can control very small grasses, volunteer wheat and broadleaf weeds. Avoid using any postemergence herbicide that could delay heading, such as 2,4-D.

• When doublecropping grain sorghum after July 1, use shorter-season hybrids. If wheat harvest is running late, do not cut corners and rush the harvest just to get the sorghum planted a little earlier. The better course is to take the time to do the best possible job with harvesting. If it gets too late to plant grain sorghum, your planting possibilities still will include a forage sorghum, sudangrass, or millet for forage production.

"In most cases, the first thing producers should do is get the soybeans or sorghum planted. Then worry later about weed control," Regehr says. "The grain yield potential of doublecropped beans or sorghum usually starts declining rapidly the later it's planted."