With a narrow window to establish cover crops after corn harvest, some farmers have experimented with inter-seeding cover crops when corn is at the 6-8 leaf stage. However, the sensitivity of different cover crops to residual corn herbicides is unknown and could negatively affect their establishment. A multi-year study looked at the sensitivity of six different cover crop species to common corn herbicides used in Ontario.
The simple answer
Cereal cover crops (oats, rye, triticale) and oilseed radish were, in general, more tolerant to residual corn herbicides, and easier to establish than white and red clover. Integrity herbicide caused the least amount of injury across all cover crops. Acceptable levels of injury to cover crops existed with other herbicides but were species-specific. Converge Flexx, Primextra II Magnum, Acuron and AAtrex 480 caused the highest level of injury to all cover crops.
A little more information
Four experiments were conducted over two growing seasons. Eleven commonly used pre-emergence herbicides were applied to field corn. Six cover crop species were seeded with a drill that was modified to sow seed between the 30” corn rows at the 6-leaf stage of growth. Visible crop injury was evaluated at 28 and 56 days after cover crop planting. Biomass was collected the spring after application (data not shown). Oats and tillage radish did not overwinter. Biomass of surviving cover crops was closely associated with the level of crop injury observed. Table 1 shows the average visible injury observed by each cover crop species across all herbicides.
The full story
The amount of visible crop injury observed was specific to the cover crop species and herbicide. There was significant variability in the amount of crop injury observed by each species across years and locations. This is not uncommon with soil applied herbicides since soil texture, chemical properties (e.g. pH) and rainfall after application will affect the amount of active ingredient taken up by germinating plant species. In general, visible injury ratings of 10% or less are considered acceptable, although individual growers may tolerate a higher level of injury if the cover crop is able to establish.
Table 2 provides specific details about the herbicides applied, active ingredient(s), rate applied per acre and the amount of atrazine applied per acre (if applicable). Table 3 provides an overview of the average visible injury observed to each cover crop species along with the range in visible injury from the least to greatest amount of injury observed across all four trials.
