Manure improves organic forage-grain crop production

‘Experimental lakes of agriculture’ find organic crops can produce on par with conventional crops

Organic crop producers can match the productivity of their conventional farming neighbours with a little help from some four-legged friends.

Composted beef and dairy manure restores important nutrients that can be mined from the soil under organic management systems, Martin Entz, an agronomy professor with the University of Manitoba’s Glenlea research station told participants in a recent field day.

But he said when treated with composted manure, the forage-grain organic crop rotation can produce just as much as conventional grain-only crops.

The Glenlea plots, which were started 23 years ago, are home to Canada’s oldest organic rotation study. The replicated trials compare the performance of four-year rotations of annual crops against a rotation that includes two years of perennial legumes under conventional and organic management.

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truck being filled with grain

“We’re like the experimental lakes of agriculture,” Entz said. Crops rotate between classic Prairie crops — Waskada wheat, Bethune flax, Leggett oats and soybeans combined with forages such as alfalfa and hairy vetch. The ongoing study, now into its fifth rotation, allows researchers to more accurately measure long-term effects of different approaches.

A lot can change in a couple of decades.

For the first eight years of the study, organic system crops yielded 10 to 15 per cent less than conventional crops because weeds slowed production. By the 12th year, however, the forage-grain system had become increasingly weed resistant and produced comparably to grain-only conventional crops.

“Forage in the system does a terrific job of dealing with the wild oats,” said Entz.

But repeated alfalfa hay harvesting can also remove up to four times as much phosphorus as the grain crops. After 15 years, the soil began to run out of phosphorus, which in turn reduced the amount of nitrogen available to crops. Crop productivity suffered. Subsoil carbon levels had also dropped.

At that point the researchers applied manure and began seeing better results.

Hairy vetch reintroduces nitrogen to the soil.

Hairy vetch reintroduces nitrogen to the soil.
photo: Meghan Mast

They have also started growing hairy vetch. A plot of the legume adds about 160 kg of nitrogen to the soil. At the end of every year the vetch is harvested. Wheat is planted in its place and the cycle continues.

Entz hopes to have sheep grazing on the alfalfa soon to recycle nutrients that would otherwise be removed back onto the land. Even in an intensive grazing system, animals remove only 15 to 25 per cent of the nutrients.

Composted manure can also change the weed dynamics.

When composted manure was spread on flax crops, wild mustard thrived. Mustard is a brassica, which does a poor job of working with mycorrhizal populations to access soil phosphorus. So depleting the phosphorus actually had a weed-suppressing effect.

Overall, Entz said weed pressure was comparable to the conventional plots. But energy efficiency was higher.

Near the end of the tour, Entz led the group to a plot of wheat.

“Look at the difference between the front half of the plot and the back half,” he said. The difference is staggering. The crop started small and nearly doubled halfway through, where the soil was treated with manure.

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