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University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Legume Cover Crops as a
Fallow Alternative

Background

Disking sweet clover green manure.

Legumes are recognized for their ability to contribute N in a cropping system. Crop rotations that include green manuring of alfalfa hay crops and other legumes have, however, declined in popularity since the advent of synthetic N fertilizers. More recently, renewed interest in the use of green manure alfalfa and other legumes have been the result of increasing N fertilizer prices, greater interest in organic crop production and rotation benefits including decreasing soil erosion, and increasing soil organic levels.

Cover Crops in Fallow Years

Wind erosion on a fallow field.

Soils are vulnerable to wind erosion during periods of summerfallow. Currently, on the Canadian prairies cover crops are used mostly during the summerfallow period. There are a wide range of crop types that can be used ranging from forage legumes (alfalfa, red clover, berseem clover) to larger seeded annuals (lentil, chickling vetch, peas and fababeans). See The Fertilizer Replacement Value of Legume Cover Crops or The N Benefits of Grain Legumes in Crop Rotation for information on the N benefits of these legumes.

Excess water standing in a spring seeded crop.

In wet areas of the prairies, cover crops help to improve soil tilth, thereby helping with water infiltration. Summerfallowing without cover crops in wet areas often results in more serious water excess problems. In dry areas of the prairies, cover crop types must be drought tolerant plus they cannot use too much water. Indian Head lentil and chickling vetch are good candidates for cover crops in dry areas.

Grain Yield Benefits of Legume Green Manure

Single-year alfalfa, berseem and red clover, chickling vetch, and lentil were evaluated for rotational yield and N benefits to the following wheat and barley crops. Green manue crops were compared to canola harvested for grain and to a tilled fallow control. All crops were seeded at full recommended rates and legumes were inoculated with the proper strain of bacteria. Only canola received fertilizer at 80 lb/ac N. The alfalfa, berseem and red clovers were cut twice for hay prior to incorporation. Vetch and lentil were incorprated at 50% bloom around the beginning of August. P was placed with the wheat and barley seed, but no N was applied to determine legume effects.

Previous Legume First Year After Fallow Second Year After Fallow
Wheat Yield bu/ac Barley Yield bu/ac
Annual* Alfalfa 36 28
Regular Alfalfa 34 31
Red Clover 35 28
Berseem Clover 29 24
Chickling Vetch 45 31
Lentil 42 29
Canola 25 27
Fallow 39 24
*No fall dormancy so plants keep growing in fall
    Berseem clover, an annual legume.
  • Rotational yield benefits were generally greatest for vetch and lentil > alfalfas > clovers.
  • The annual alfalfas resulted in higher grain yields and N uptake when compared to the regular alfalfas. These alfalfas continue to grow all fall and would fix more N than the other alfalfas.
  • Red clover performed better than berseem clover.
  • The legumes had a longer lasting benefit than the fallow treatment as shown by the barley yield in year 2.
  • Alfalfa and the clovers would have an economic benefit over the grain legumes because they were harvested for hay, however, establishment is more risky with these small seeded crops.

Recommendations

  • Single-year hay legumes and legume green manure crops boost yield and N uptake of subsequent cereal crops.
  • Success depends on good legume establishment and proper inoculation.
  • If soil erosion or excess water are a problem, consider a green manure crop as a fallow alternative.

Further Reading: Bullied, W.J., M.H. Entz, S.R. Smith, Jr., and K.C. Bamford. 2002. Grain yield and N benefits to sequential wheat and barley crops from single-year alfalfa, berseem and red clover, chickling vetch and lentil. Canadian Journal of Plant Science 82: 53-65.

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This page created August 2004.