Capturing the potential of alfalfa – Organic Matters

A study by a University of Manitoba graduate student indicates that farming with alternating strips of alfalfa and grain can provide many of the benefits of a green manure without the drawbacks of tillage or the loss of grain income.

Including alfalfa in a crop rotation offers substantial benefits. Alfalfa can suppress weeds such as wild oats, Canada thistle and green foxtail. Alfalfa forms partnerships with bacteria that fix nitrogen. Tapping this source of nitrogen does not require the use of large amounts of economically, environmentally and socially expensive fossil fuels. The nitrogen is then available for the crops that follow.

So why do so few producers include alfalfa in their rotations? Alfalfa does have some liabilities. It can deplete moisture and long-term alfalfa stands can reduce phosphorus and sulfur availability in the soil.

The main limitation for many organic producers who do not have livestock is that an alfalfa field produces no cash crop unless large amounts of valuable nutrients are exported from the field when the alfalfa is cut, baled and sold.

Strip farming is a concept that uses alfalfa as an organic nitrogen fertilizer in grain crops. Alfalfa is grown in strips across the field. Annual crops are planted between the alfalfa strips. The alfalfa is harvested and applied directly as a mulch to the annual crop strips. Eventually, the alfalfa strips are moved. A three year alfalfa strip is suggested for greatest benefit.

The study was done by graduate student Matthew Wiens under the supervision of U of M plant scientist Martin Entz and in collaboration with Ralph Martin and Andy Hammermeister of the Organic Agriculture Centre of Canada.

Wiens paid particular attention to the effect of alfalfa mulch on AC Barrie wheat. He harvested the alfalfa strips with a flail mower and applied the mulch to the wheat crop at different times and at different rates. He repeated his study at four sites, over two years.

The mulch was applied to the wheat plots either before the wheat emerged or at the three-leaf stage. Three amounts of alfalfa mulch were tested:

  • The amount that would be generated if alfalfa and wheat were in equal sized strips.
  • The amount if the alfalfa strips were twice as wide as the annual crop.
  • The amount if the alfalfa strips were half as wide as the annual crop.

Alfalfa did not smother the wheat crop even when the amount applied was more than 2.5 tons per acre of dry matter, or 2,300 kilograms per acre. Wheat received a nitrogen boost from the alfalfa. Treated plots were a darker green.

Nitrogen uptake and wheat yield increased as the amount of alfalfa was increased. Wheat that received the most alfalfa mulch yielded nearly twice as much as wheat that got no mulch.

Alfalfa mulch suppressed weeds when applied at higher rates. At low rates, weed numbers were higher than in plots without alfalfa mulch. It appears that small amounts of alfalfa improve the conditions for weed establishment, perhaps by reducing moisture loss at the surface.

Higher rates discourage weeds, perhaps by shading or by creating a physical barrier between the soil and the surface.

Wiens’ work indicates that alfalfa can be used as mulch on spring wheat to get value from alfalfa hay. This opens the door for broader thinking about rotations. Organic producers may find additional benefits from such diversification, including reduced erosion potential and greater habitat for beneficial organisms.

This study also has important implications for reducing tillage in organic systems. Tillage to incorporate the alfalfa green matter was not necessary to gain substantial nitrogen and weed control benefits.

With green manure crops, there is a balance between the benefits of nitrogen, organic matter buildup and weed suppression with the risks of tillage. This study may help shift this balance and be another step forward on the path to healthy soils.

Matthew Wiens collaborated in the writing of this column. He may be reached at umwiensm@cc.umanitoba.ca. Frick is the prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Sahewan. Frick can be reached at 306-966-4975, at brenda.frick@usask.ca, or www.organicagcentre.ca.The opinions expressed in this column are not necessarily those of The Western Producer.

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