Herbicide Carryover Considerations When Re-cropping Damaged Wheat

By Sarah Lancaster

Growers who decide to terminate their wheat crop because of poor stands or weather-related damage need to consider the persistence of herbicides applied to the wheat. Many wheat herbicides have fairly long crop rotation restrictions. Selected herbicides and rotation intervals to certain crops are listed in Table 1.

Herbicide dissipation is affected by many factors, so be sure to check herbicide labels for specific information for your fields, particularly for the intervals marked with an asterisk in Table 1. Additionally, some 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. Supplemental herbicide labels may further complicate your decision.

Table 1. Minimum rotation intervals for key summer crops following application of selected wheat herbicides.

Abbreviations: d=day, m=month
*Consult herbicide label for geographic, soil, precipitation, application rate, or other conditions associated with this crop rotation interval.

Another important factor to consider is the availability of herbicide-resistant varieties or hybrids. Many of the herbicides listed in Table 1 are ALS-inhibiting herbicides that are associated with resistance in certain varieties or hybrids. Some examples are Clearfield sunflowers, which can be planted following Beyond, or Bolt soybeans which have a shorter rotation interval following sulfonylurea herbicides such as Ally.

In general, it would be best to wait as late as possible before planting sensitive crops to allow for maximum herbicide dissipation. Tilling the soil to try to “dilute” the herbicide residue likely will not have a great benefit. 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. If you are in doubt about your decision, consider conducting a field bioassay to determine if it is safe to plant the crop you are considering. A field bioassay is simply a short strip of crop planted across the direction of the herbicide application. Plants in the strip are monitored for herbicide injury as they emerge.

Terminating the wheat crop

Another consideration at this time is how to kill the wheat crop. Wheat that has been injured but is not yet dead may be hard to kill with glyphosate because of reduced absorption and movement through the plant. Producers should wait until 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. You can also consider a ‘planting green’ strategy, which is employed in some cover cropped fields.