Kevin Shelley and Dan Smith, UWEX Nutrient and Pest Management Program
If a cover crop will be planted later this cropping season, consider the rotational restrictions for any herbicides used in the field this spring, or the past few seasons. This is especially important if the cover crop might be used as a forage crop. A crop is classified as a cover crop when no biomass is harvested. A cover crop becomes a forage crop when its biomass is harvested for feed.
As part of its EPA registration, each herbicide product’s label provides directions for use on labeled (allowed) crops. It will also provide information on crop rotation intervals. A crop rotation interval is the amount of time required after application of the herbicide product before another (non-labeled) crop can be legally planted. The rotation interval is established for two reasons: To ensure that herbicide residues in the soil will not affect establishment of the following crop, and; To ensure the safety of a subsequent feed or food crop from accumulating herbicide compounds at un-safe levels in plant tissues.
Most corn, soybean and cereal grain herbicides do not have rotational intervals for non-harvested cover crops. If the crop to be used as a cover crop has a rotational interval listed on the herbicide’s label, this interval provides a guide as to the likelihood of successful establishment. But, if not intended for harvest, the time interval is not a legal requirement. If a farmer’s or agronomist’s experience suggests the cover crop will grow, or if the farmer is willing to assume the risk, a decision to plant prior to the expiration of the interval may be made. If, however, the cover crop is used as a forage, the rotation interval becomes a legal requirement for feed and food safety. If the specific crop is not listed on the herbicide label as an allowed crop, nor is among those with established rotation intervals, the maximum rotation interval must be used when planting for forage.
Many dairy farms in Wisconsin are planting a winter cereal grain, such as rye or triticale, following harvest of corn as silage. If planted early enough in the fall, this overwintering “cover crop” helps prevent soil erosion and nutrient losses from runoff and leaching on corn silage fields. Winter rye or triticale also provides the option of harvesting as a forage crop the following spring, usually about the third to fourth week in May in most of WI.
A farmer has multiple herbicide options for silage corn weed control, however their rotational interval restrictions are drastically different. For example, the label for one popular pre-emergence corn herbicide, in the rotational crops section, states that barley, oats, rye or wheat may be planted 4 1/2 months after application. Therefore, if planning to plant rye after corn silage by September 25, an application of this herbicide would need to made by May 13. It also states, “for all crops not listed, wait at least 12 months following the last application.” Triticale is not specifically listed. So, if this herbicide was used for this year’s corn silage crop, triticale planted this fall could not, technically, be used as forage crop next spring. Other corn herbicides have rotation intervals ranging from 0 to 18 months for cereal grain crops (grain or forage) and up to 26 months for other species.
Similarly, most herbicides labeled for use in wheat or other small grains will have rotation interval restrictions ranging from 0 to 18 months for crops that might be planted for forage (harvested or grazed) following small grain harvest. For current year information on rotation interval requirements for specific herbicides, consult the product’s EPA registered label. A good source for specimen pesticide product labels is the Crop Data Management System’s website at http://www.cdms.net/Label-Database.