Suppressing weeds with a carpet of cover crops – timing is key!

Cereal rye in late October, planted following corn harvest in Beltsville, MD.

Cereal rye in late October, planted following corn harvest in Beltsville, MD (Photo: A. Klodd, Penn State University)

As more no-till farms across the country become interested in cover cropping for its numerous benefits, weed scientists and producers have also been working hard to learn how they can be used to manage problem weeds.

The success of cover crop management depends on a lot of factors, but the right set of factors can lead to high weed suppression of summer annuals and winter annuals. In Pennsylvania, Maryland, and Delaware, weed scientists and farmers have been working together for a number of years to test out the fine details of when to plant, what to plant, and when to terminate in order to coax the cover crop into a highly weed suppressive mulch in no-till fields. In this technique, a fall-planted cover crop like cereal rye overwinters, re-emerges in the spring, and is then terminated via herbicide burndown or a roller-crimper prior to cash crop planting to create a thick, weed-suppressive dead mulch. Even before the cover is terminated, it is already helping suppress weeds by competing for resources and shading the soil.

Dr. Claire Keene, a PhD graduate of Penn State and now a weed specialist at North Dakota State University, studied cover crop-based weed management in reduced-till organic fields. This was part of an on-going multi-state study involving organic fields in Pennsylvania, Maryland, and Delaware. Of the many lessons her study highlighted, this is among the most important:

The timing of cover crop termination is crucial for determining how much biomass the cover crop produces, which then affects how successful it will be at suppressing weeds. The optimal timing in the spring also depends on the cover crop species and growth stage.

If the cover crop is terminated too early in the spring, it may be too small to produce enough biomass to block out weeds as a mulch. If it’s terminated too late, it might go to seed or become too unwieldy to be rolled down or burned down.

Roller crimper

For a hairy vetch-triticale cover crop mixture, weed control was better if the cover crop was terminated later. The best weed control was achieved if the cover crop was terminated when it was in the late-flowering to early pod set stage. Terminating it late also meant fewer volunteer vetch seedlings popping up in subsequent soybeans and winter wheat.

Cereal rye biomass also increased as termination was delayed, with peak biomass achieved at late dough stage. However, the best late spring/summer weed control was achieved if the rye was terminated between 50% anthesis and early milk stages.

In conventional no-till fields, cover crop termination can be achieved with a herbicide burndown, leaving less risk of volunteer cover crop seedlings in the subsequent cash crop. However in organic fields, the lack of herbicide burndown means that if some cover crop plants have produced seed by the time of roller-crimper termination, those seeds may sprout into volunteers in the subsequent cash crop. In this case, proper termination timing is again critical in ensuring that the cover crop does not persist long enough to produce viable seed. In Dr. Keene’s study, most of the cereal rye was effectively terminated by roller-crimper before seed production. However, she saw 3-11% wheat grain contamination by cereal rye that had produced seed before being terminated. Careful monitoring of grain fill can help ensure that rye is terminated between 50% anthesis and early milk stages.

Read the full results of this study in Agronomy Journal. Funding for the reduced-tillage organic systems experiment was provided by USDA Organic Research and Extension Initiative.

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