Cover crops slow weeds

GLOBAL COVER RI Farm
n Cover crops can slow the development of herbicide resistant weeds such as horseweed. PICTURE: Joe Boggs, OSU Extension

COVER crops can play an important role in slowing the development of herbicide resistant weeds, field experiments in the United States show.

Pennsylvania State University used seven cover-cropping treatments over two growing seasons to explore how cover cropping tactics influenced the management of horseweed in no-till grain crops.

Horseweed, Erigeron canadensis, is an annual plant native throughout most of North America and Central America. It is widely naturalised in Eurasia and Australia. Common names include horseweed, Canadian horseweed, Canadian fleabane, coltstail, marestail, and butterweed.

The Pennsylvania researchers report that in comparison to fallow control plots, cover crop treatments reduced horseweed density at the time of a pre-plant, burndown herbicide application by 52 per cent in the first year and 86 per cent in the next.

This reduced the herbicide workload and lowered the selection pressure for resistant weeds. Cereal rye alone or in combination with forage radish was found to provide the most consistent horseweed suppression.

Winter hardy cover crops also reduced horseweed size inequality – meaning fewer large horseweed plants were found at the time of herbicide application. Researchers say this reduces the chance of a size-dependent fitness advantage for horseweed biotypes that develop herbicide resistance.

The researchers say proactive integrated weed management is critically needed in no-till production to reduce the intensity of selection pressure for herbicide-resistant weeds. Reducing the density of emerged weed populations and the number of larger individuals within the population at the time of herbicide application are two practical management objectives.

“Our hope is that understanding the complementary relationship between cover crops and herbicides can lead to new weed control strategies that slow the development of herbicide resistance,” researcher John Wallace says.

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