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A cover crop ‘failure,’ but the story doesn’t end there

Black medic is a soil improver, and can be a good fit in situations where nitrogen is limited

Under dry conditions, black medic growth is limited, meaning it won’t compete with field crops for nutrients.

A cover crop that self-seeds year after year, grows whenever environmental conditions are right and doesn’t compete with principal crops. Sounds like a dream, but it exists: it’s called black medic.

It’s been planted in Western Canada since the 1990s, but it’s not widely used, and in fact “it’s kind of a failure,” says Martin Entz, natural systems agriculture professor at the University of Manitoba.

Biotypes of medic, a type of leguminous plant native to the Mediterranean region, have been successfully used as self-regenerating cover crops in eastern Australia. In the 1990s, Entz travelled to Adelaide to study how medics might be adapted to Manitoba growing conditions. His team screened germplasm and zeroed in on one cultivar — the only black medic they could find that germinates in Western Canada.

This variety — “George” — has been studied in long-term plots across Western Canada since the early 2000s in collaboration with Agriculture Canada agronomist Bill May.

“What we’ve learned is that it will survive in a no-till system even when farmers are using herbicides. It’s survived for over 15 years in the long-term plots. The proof of concept has been really useful,” Entz says.

Are producers using it? Not many, he says. “I would have to say it’s kind of a failure. If nitrogen is restricted, it has some virtues, but it hasn’t been super-impressive given what other cover crops would give us in terms of growth and nitrogen.”

But Entz gets calls every year from producers interested in adding black medic to their systems.

That’s due to the appeal of a cover crop that doesn’t require reseeding, he says. Also, black medic reliably regenerates every year, even if the principal crop is terminated early. Another advantage is that under dry conditions, black medic growth is limited, meaning it won’t compete with field crops for nutrients. And in low-input systems, it helps establish and maintain soil aggregate stability.

Soil stability improvement

Medic’s influence on aggregate stability is the focus of a new research publication based on research conducted by May’s team at Indian Head.

The study, which found that black medic improves soil physical properties, looked at soil aggregate size in a long-term no-till grain rotation (wheat-flax-oat from 2002 to 2013, and wheat-flax-canary seed from 2014 until the present) with and without black medic. The rotations were treated at a range of fertilizer rates (20, 60 and 100 per cent of required N based on soil tests).

In 2017, soil samples for aggregate stability and bulk density were collected from the plots. In the low-N treatment, there was a 21 per cent increase in the mean weight diameter of soil aggregates in wheat when medic was present. At the full rate of N, there was no increase in mean weight diameter. In wheat, the proportion of soil aggregates increased overall in soils with medic, except at the full N rate. There was no statistical difference for aggregate size in flax when medic was present.

“Although significant improvements by the medic cover crop were only observed in wheat, the dataset consistently showed that medic reduced the mass of small aggregates and increased the mass of large aggregates. In this way, medic improved the status of an important soil health indicator,” note the authors of the paper. “The effect of medic in increasing aggregate stability was similar, in many cases, to the effect of fertilizer N additions.”

Agronomics

The effect of nitrogen on black medic performance is key to management of the cover crop.

Black medic is officially listed as a noxious weed in Canada, but the classification stems from early days when control wasn’t understood. Nevertheless, May says its tenacity could cause some concern for producers. “It makes people a little cautious. I think it’s a good idea to be sure you want this cover crop in your system. Once it’s there you’ve got to work to get it out.”

But May’s research makes it clear that the number one control measure for unwanted medic is nitrogen.

“Black medic works best in low-input systems,” says May. “Once we get to 60 per cent recommended N, the medic starts to disappear. So for guys trying to remove medic, N application is the best form of weed control.”

Work published by Leanne Wilson with Martin Entz in 2017 looked at black medic seed dormancy, and showed that regeneration of black medic is mediated by temperature. Extreme low-temperature exposure also affected regeneration. In soils subjected to temperatures in the -23 C range for long periods in January and February, germination rates were lower.

Nevertheless, the crop shows a lot of promise, particularly in situations where N inputs are limited. “You don’t have to spend money on seed, and when conditions are conducive to it, it’ll thrive,” May says. “It’ll develop into a cover crop whenever the conditions are right but it won’t get in the way of your main crop. If we’re forced to limit N, black medic starts to have some big benefits in supplementing inputs and stabilizing soil.”

About the author

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Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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