By Dallas Peterson, Weed Management Specialist
Growers who decide to terminate their wheat crop this spring because of poor stands or recent weather-related damage need to consider crop rotation restrictions relative to any herbicides that may have been applied to the wheat. Many of the commonly used sulfonylurea type herbicides, including metsulfuron, Ally, Ally Extra, Agility SG, Finesse, Glean, Amber, Peak, Rave, OutriderOlympus, and Power Flex HL are persistent and have fairly long crop rotation guidelines.
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 Outrider herbicide. Sorghum is extremely susceptible to Outrider and should not be planted for at least 22 months after application. In addition, fields should not be planted to sorghum for at least 14 months following Amber or Rave application according to label guidelines.
Producers who want to recrop to sorghum on their wheat acres that have received one of the other residual sulfonylurea herbicides should wait as long as possible to plant. Ideally, sorghum should not be planted on these fields until mid-June.
Cotton and non-STS soybeans are generally intermediate in tolerance to these herbicides. Many of these product labels recommend not planting cotton or non-STS soybeans until the following year, while others have a 3 or 4 month waiting interval or a clause that allows shorter recrop intervals in the case of catastrophic events if a field bioassay indicates it is safe to plant the crop. However, in those situations, the grower assumes all risk of crop injury.
Corn, sunflowers, canola, and alfalfa tend to be the most susceptible crops to the sulfonylurea herbicides and generally have rotation guidelines of 12 months or longer with most of these herbicides. With the high price of corn, many farmers may be interested in planting corn, but corn is very susceptible to residues of these herbicides.
Several herbicide labels make reference to shorter recrop intervals if planting IR corn. However, IR corn is obsolete and current Clearfield corn hybrids do not have the same level of cross resistance to sulfonylurea herbicides as did the IR corns.
Wheat fields that have been treated with Beyond herbicide can be recropped in the spring with any type of soybean or Clearfield sunflowers, but not to sorghum or corn.
Most other commonly used wheat herbicides in Kansas have short crop rotation restrictions. In fields where herbicide carryover is a concern, it would be best to wait until later in the spring before planting to allow as much time as possible for herbicide dissipation. Tilling the soil to try to “dilute” the herbicide residue likely will not have a great benefit and could increase the risk of soil erosion and moisture loss. Lowering residue managers on planters so that an inch or two of topsoil is thrown out of the rows could help get the seed into soil with lower herbicide levels.
Another consideration is how to kill the wheat crop if producers plan to recrop. 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, may be hard to kill with glyphosate.
The best approach is to wait until the wheat is actively regrowing before applying glyphosate. Paraquat is not a good alternative as it burns back the treated leaves, and is not translocated to the crowns and lower buds, thus the wheat plant often can regrow from these structures.
Always refer to the specific herbicide label regarding crop rotation guidelines and restrictions. 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.