Long-Term Danger

This article appears in the January 2022 issue of Potato Grower.

Herbicides are an important weed management tool for agriculture. The use of herbicides in multiple crops makes it necessary to track what has been applied to crops in rotation with potatoes to avoid damage due to carryover from the previous season(s). The time after an herbicide is applied and when a particular crop can be planted in ensuing growing seasons is called plant-back interval, plant-back restriction, or another similar term. This information is listed on herbicide labels. The table below outlines examples of commonly grown rotation crops and applied herbicides, and the required plant-back interval for potatoes. Tracking herbicide use in previous crops before planting potatoes can be complex, especially if the land is being rented or leased. But this is extremely important to avoid any unnecessary damage to the potato crop.

The risk of herbicide carryover from one crop to the next is determined by a myriad of factors including but not limited to:

1. Type of herbicide and chemical structure influencing the innate persistence of herbicides in the soil: Some of these characteristics can result in slow or no metabolism and/or chemical processes, so a particular herbicide might persist longer in the soil than others.
2. Soil type and characteristics: Heavy soils tend to prolong herbicide persistence in the soil. Soil pH and organic matter also affect herbicide persistence considerably. Read the label carefully to understand how your soil characteristics may affect rotational crop restrictions.
3. Late-season application timing or high rates: This can lead to not enough time with optimum conditions needed for herbicide breakdown before potato planting.
4. Amount of moisture: Most herbicides are broken down by soil microbes and various chemical processes. These processes all require soil moisture. Dry weather means all these processes would either be slowed down or brought to a screeching halt and increase the risk of herbicide carryover.
5. Cultural practices: Soil compaction, poor drainage and other conditions impacted by tillage can cause a reduction of herbicide breakdown. A highly soluble herbicide might move far enough down in the soil profile, but a complete inversion of the soil by plowing can bring the herbicide-concentrated layer back up to the surface. This can create the risk that the herbicide may be at the upper profile when potatoes are planted the next year.

What can you do to avoid herbicide carryover damage to your potato crop? Read and follow the herbicide label carefully and keep good records. If renting the ground, ask for previous records of crop rotation and herbicide applications.

During herbicide planning, even before purchase and application, read and follow label instructions carefully to identify plant-back restrictions for potatoes. The process to avoid herbicide carryover damage to your potatoes is incorporated into each herbicide decision for all rotation crops. Also, accurate records involving herbicide applications are an important step in maintaining a smooth, trouble-free rotation that will help to maximize yields for every crop in the rotation.

Cultural Practices

Appropriate and properly timed tillage (especially in the fall) can play a major role in reducing the persistence of herbicides in the soil. Tillage can help “dilute” herbicides concentrated in the topsoil through mixing with herbicide-free soils from below. However, highly soluble herbicides likely to leach into the subsoil could be brought back up to the surface with a complete inversion by plowing.

Bioassay

If herbicide application records in the previous year(s) are not known, or if conditions were not conducive enough for herbicide breakdown and safe plant-back, a bioassay is recommended to determine whether the field is safe to plant to potatoes. A bioassay in this case means planting potatoes, or another crop sensitive to the herbicide, in the soil that was treated with that herbicide. An indoor or greenhouse bioassay conducted the fall/winter after use of the herbicide would speed up assessment time and provide enough information to allow for planting potatoes in the spring without the likelihood of herbicide injury. The one issue with using seed potatoes in the bioassay is that they may still be dormant or slow to sprout and emerge. This could delay results or mask some herbicide damage. If you would like to conduct a bioassay, contact your area extension specialist or agronomist to find out the steps to follow.  

Laboratory Analysis

Instead of or in addition to a bioassay, soil from the field can be collected and sent to a lab for herbicide residue analyses. Follow the laboratory’s guidelines for sample collection and submission, as it can differ depending on the potential herbicide applied. It is helpful to know the herbicide application history, or at least have a good guess as to what was applied, so the laboratory can conduct analyses for specific herbicides rather than a broad screen, which can be very expensive. 

Overall, the best prevention for herbicide carryover from one season to the next is to know the previously applied herbicides, identify the plant-back restriction months on the label, and factor in weather and cultural practices that may influence herbicide residues. Integrating these actions will help limit unintentional potential damage to your potato crop.

Albert Adjesiwor is a weed scientist and extension specialist based at the University of Idaho’s Kimberly Research & Extension Center. He can be contacted at aadjesiwor@uidaho.edu.

Nora Olsen (norao@uidaho.edu) is a professor and potato specialist also based at the Kimberly R&E Center.

Pamela Hutchinson (phutch@uidaho.edu) is a weed scientist and extension specialist of potato cropping systems based at UI’s Aberdeen R&E Center.

Examples of commonly used herbicides in Idaho and rotational restrictions to potatoes

*To simplify information, trade names have been used. No endorsement of named products is intended, nor is criticism implied of similar products not mentioned.