Herbicide carryover considerations when recropping wheat
The recent cold weather and freeze damage to some of Kansas’ wheat crop has raised a number of questions regarding the recropping options following application of various wheat herbicides. Many of the commonly used sulfonylurea herbicides, including Ally, Ally Extra, Finesse, Glean, Amber, Peak, Rave, Maverick, Olympus, and Power Flex 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 Maverick herbicide. Sorghum is extremely susceptible to Maverick 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 However, IR corn is obsolete and current Clearfield corn hybrids do not have the same level of cross resistance to sulfonylurea herbicides as did the IR corns.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.