Herbicide considerations when interseeding cover crops

By Jenny Rees - Extension Educator | Steve Melvin - Extension Educator Irrigated Cropping Systems | Amit Jhala - Extension Weed Management Specialist | Chris Proctor - Weed Management Extension Educator

With increasing use of cover crops for different goals, we receive more questions each year regarding potential impacts of different herbicide programs on cover crops.

When it comes to herbicides, keep in mind that herbicides have been tested for safety on the crops they are labeled for and often cover crops are not listed as a labeled crop. Thus, working with cover crops requires thinking outside the box on how to use herbicides effectively. One also needs to respect what the herbicide is labeled to do: kill weeds for foliar active herbicides and/or prevent weed seeds from emerging when applying residual herbicides. Thus, it’s important not to attempt seeding cover crops in situations where they aren’t going to succeed due to an herbicide with longer residual activity or an herbicide that may injure a specific cover crop species being considered.

The following are the herbicide programs we have used when interseeding cover crops into corn and soybean early in the growing season. This effort was a partnership amongst The Nature Conservancy, Upper Big Blue NRD, Nebraska Extension, Kelloggs, and cooperating farmers. We successfully achieved germination and establishment of a variety of interseeded cover crops (grasses, legumes, brassicas) in the

2019-2022 growing seasons in Nebraska.

Corn Herbicide Program for Drill Interseeding into V3-V4 Corn

We have utilized PRE- herbicides at the recommended label rate. PRE’s included a variety of products (Table 1).

By the time the corn was V3-V4, the PRE herbicide residual was no longer impacting the seeding zone (1/4-inch to one inch) for the interseeded cover crops.

Often the day before, day of, or day after interseeding cover crops, producers applied: glyphosate, dicamba (often Status®), Liberty® or a combination of products with no issues.

One grower conducted an herbicide comparison between a full rate of generic Lexar® PRE- vs. split applied. The split application applied half the rate PRE and the other half rate was applied seven days prior to interseeding. In that case, every species still emerged but growth was thin compared to the full rate of product applied PRE-plant. This video shows what he learned.

None of the growers chose this option, but for those desiring a residual herbicide, a Group 15 product (ex. Zidua®, Outlook®, Warrant®, Dual®) can be used once the cover crop is at least one inch tall as this herbicide mode of action has little to no postemergence activity.

Soybean

Herbicide Program for Drill Interseeding into VC Soybean

Of note on this study, we used winter wheat as a cover crop species. Had it survived the growing season into the fall, that can be problematic creating a “green bridge” for the wheat curl mite in areas where wheat is often planted. In our stud- ies, the winter wheat died during July, but if one wishes to err on the side of caution, a different grass species can be interseeded (or just interseed red/white clover) in areas where higher acres of wheat are planted.

Soybean PRE- emergence herbicide was only used if it was applied at least three

Outdoor plant containers

Choosing a container for plants is important. Plastic pots are cheap, light, and durable and they keep the soil moist; however, they can turn brittle and crack.

Terra-cotta pots are made from baked clay and are very attractive; however, they are heavy, break easily, and are subject to frost damage.

Glass fiber containers have a wide variety of molds and colors. They can look like stone but are much lighter. They are strong, durable, and repairable, but are expensive.

Reconstituted stone pots are crushed stone molded into shape. Imitation classical urns and vases are very attractive but extremely heavy and should be use with caution in roof gardens or balconies.

If using new concrete plant containers, leave them outdoors for a few weeks to weather fully, as some concrete can contain setting agents that may be harmful to plants.

Wooden half barrels are useful plant containers. Line the barrels with strong plastic to stop the compost from coming in contact with the wood. Make drainage holes both in the plastic and the barrel.

When using heavy plant containers, make sure to position them in their intended final sites before filling, as they are difficult to move. If they do have to be moved, put length of old scaffolding pipes under the pot to use as rollers.

Take the wheels off old suitcases, shopping carts, or old strollers and fix them to the bottom of heavy patio pots. This will allow them to be easily moved around the garden or pathways.

To reduce the weight of compost in a large pot, fill the container with empty plastic bottles to a depth of 8” (12 cm). Cover with a layer of polyethylene, making holes in it for drainage. Fill to the top with compost.

When stacking terra-cotta pots, place a few sheets of newspaper between them to prevent the pots from jamming together and making them difficult to separate.

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