Before destroying a stand of freeze-damaged wheat and re-cropping to a summer row crop, producers will have to consider possible herbicide carryover from the wheat crop, says Kansas State University agronomist Dallas Peterson.
Many of the commonly used sulfonylurea herbicides used on wheat, such as Ally, Ally Extra, Finesse, Glean, Amber, Peak, Rave, Maverick, Olympus, and Olympus Flex, are persistent and have fairly long crop rotation guidelines, says Peterson, who is a weed management specialist with K-State Research and Extension.
"In general, the most tolerant summer crop, to residues of these herbicides, is STS soybeans, followed by grain sorghum. One major exception to this guideline is sorghum and Maverick herbicide. Sorghum is extremely susceptible to Maverick and should not be planted for at least 22 months after application," he says.
Product labels on most of these herbicides recommend not planting cotton or non-STS soybeans until the following year, Peterson says. The Maverick and Olympus labels allow shorter re-crop intervals in case of catastrophic events if a field bioassay shows it is safe to plant the crop.
"Corn, sunflowers, canola, and alfalfa tend to be the most susceptible crops to the sulfonylurea herbicides and have rotation guidelines of 12 months or longer," he adds. "Wheat fields that have been treated with Beyond herbicide can be re-cropped in the spring with any type of soybean or Clearfield sunflowers, but not with sorghum or corn. Most other commonly used wheat herbicides in Kansas have very short crop rotation restrictions."
Another consideration is how to kill the wheat crop, if producers plan to re-crop, says Dave Regehr, K-State Research and Extension weed management specialist.
"For glyphosate to be effective, it has to be absorbed by healthy, growing plant parts. Wheat that has been injured and is not dead yet, but not growing well, will be hard to kill with glyphosate. Paraquat is not a good alternative. Paraquat will burn back the top leaves, but is not translocated in the plants and will not kill the crowns and buds if they are still alive. The best approach is probably to wait until healthy re-growth from the crown is 6 inches tall before applying glyphosate," he says.
Both scientists cautioned that producers always refer to specific herbicide labels regarding crop rotation guidelines and restrictions. Label guidelines for crop rotation are often complicated by soil pH and geography. Some product labels have rigid crop rotation restrictions, while others 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, Peterson says, 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.