Originally written by Gared Shaffer, former SDSU Extension Weeds Field Specialist.
Long residual pre-emergent or early post-emergent herbicides may cause stand reduction or complete failure of cover crops. Depending on efficacy of the herbicide, each situation can both affect in-season and/or post-harvest cover crop establishment. First consider the rotational restrictions for herbicides that are used in the spring or summer months. Rotational restrictions can be found on most herbicide labels under the title or similar wording to “rotational crop restrictions” or “rotational crop guidelines”. This can become a larger issue if the cover crop will be grazed. Specific guidelines usually found under “forage restrictions” must be followed for cover crops that are grown for feeding livestock (i.e. grazing or hay).
When the EPA registration for any pesticide is completed, the product label usually contains directions for use on labeled crops and crop rotation intervals. If a crop is not included on the EPA approved label and the product is applied to that crop, then it would be an off-label application. A crop rotation interval is the required time between application time and the time of next planting. The crop rotation interval is required for two main reasons, first, a rotation interval that ensures potential herbicide residues in the soil will not affect plant establishment, and second, it ensures there are no unsafe levels of herbicide in plant tissues.
A few chemical companies add common cover crops and rotation interval to their labels. If a cover crop is not listed on the label, it then falls into the “other” category. Most corn, soybean and small grains herbicide labels do not have rotational intervals for non-harvested or harvested cover crops. If a producer does not intend to harvest the cover crop, the rotation interval requirement is not a legal requirement, but if the producer plans on harvesting the cover crop the label restrictions must be followed. One example, is when a producer grows wheat and applies an herbicide with plant back restriction of six months for cover crops, and then plants the cover crop five months after application for non-forage use. The producer will then be fully responsible for any damage that occurs to the planted cover crop as a result of residual herbicide.