In areas of Kansas, wheat stands are poor from the effects of dry soils, winter heaving, late planting, and other factors, some farmers might be considering recropping the fields to a row crop.

Before making any decisions, consider potential herbicide carryover that might cause problems for the spring-planted crop, says Kansas State University agronomist Dallas Peterson.

If producers still aren't sure whether to keep their wheat, but want to keep their recrop options open, their best options for weed control at this time of year would be MCPA, 2,4-D, dicamba, Affinity, or Harmony products, says Peterson, who is a weed science specialist with K-State Research and Extension.

"MCPA generally has better crop safety on small wheat than 2,4-D," he says. "MCPA can be applied to wheat from 3-leaf until the early boot stage, while 2,4-D can be applied between full-tiller and early boot stages. Dicamba can only be safely applied to wheat prior to the jointing stage. Application after jointing increases the risk of crop injury, he says. Harmony GT, Harmony Extra, and Affinity BroadSpec or TankMix can be applied to wheat until the flag leaf is visible."

All of these products have very short crop rotation restrictions, Peterson says. But, they do not provide much residual weed control.

"In contrast, many of the commonly used sulfonylurea herbicides, including Ally, Ally Extra, Agility, Amber, Finesse, Finesse Grass and Broadleaf, Glean, Maverick, Olympus, Olympus Flex, Peak, and Rave, are very persistent and have fairly long crop rotation guidelines," the scientist says.

In general, the most tolerant summer crop to residues of these herbicides, is STS soybeans, followed by grain sorghum. Product labels tend to specify grain sorghum, but forage sorghum and sudangrasses would likely have similar levels of tolerance.

One major exception to this guideline is sorghum and Maverick herbicide, Peterson says. Sorghum is extremely susceptible to Maverick and should not be planted for at least 22 months after application.

More information on cropping restrictions associated with herbicides is available in the K-State Research and Extension publication 2008 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 994). The publication is available at county and district Extension offices and online at www.oznet.ksu.edu/library/crpsl2/srp994.pdf.  

Always refer to the specific herbicide label regarding crop rotation guidelines and restrictions, Peterson says.

"Label guidelines for crop rotation are often complicated by soil pH and geography. Some product labels have very rigid crop rotation restrictions, while other labels allow shorter intervals in the case of catastrophic crop failure, as long as the producer is willing to accept the risk of crop injury.

"Another confusing issue may be the existence of supplemental herbicide labels with shorter crop rotation guidelines than the regular label. Herbicides with supplemental crop rotation labels include Finesse, Ally, and Ally Extra," he says.