Saving nitrogen fertilizer with grass-legume mixtures

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included. 

As nitrogen fertilizer costs increase, the potential benefits of using legume cover crops to supply nitrogen is becoming more economically attractive. However, legumes tend to be less effective at suppressing weeds and have relatively high seed costs compared to other cover crops. Mixtures of legumes with grasses can help overcome these problems, and provide additional benefits compared to legumes grown alone.

Why bother with mixtures? Mixtures have several potential advantages relative to legumes grown alone:

  • Mixtures of legumes can reduce seed costs by allowing substitution of high-cost legume seeds with lower cost grasses. Legumeseeding rates can often be reduced by about 50 percent in mixtures with grasses.
  • Mixtures are often better at suppressing weeds than monocultures of legumes alone. Legumes tend to be slower growing and less capable of smothering weeds than non-legume counterparts. Complementary grass cover crops can serve as nurse crops for legume establishment and take up space and resources that might otherwise be occupied by weeds.
  • Mixtures reduce risks associated with pure stands of either cover crop; if pests or environmental stresses restrict growth of one species, the other can often fill the gap. Similarly, variation in soil moisture or soil nutrients across fields will result in less variation in cover crop growth when two or more complementary species are included in the mix.
  • Legumes in mixture can be more efficient at fixing nitrogen, since the non-legume draws down soil nitrogen, resulting in stimulation of legume nodulation and nitrogen fixation. For example, in our studies comparing cowpea grown alone with cowpea grown in combination with Japanese millet, more nitrogen was fixed per plant in the mixture, resulting in lower costs per pound of nitrogen fixed.
  • Grass and legume mixtures provide a residue with a balanced C:N ratio for which the timing of nitrogen release is better matched to crop demand. In contrast, nitrogen from residues of pure stands of legumes like hairy vetch is often quickly mineralized and lost before crops can use it, while high C non-legume residues like rye can tie-up nitrogen and suppress early crop growth.

Potential problems with legume-grass mixtures

The most serious problem with legume-grass mixtures is the potential suppression of the more valuable and costly legume by the non-legume. For example, in experiments evaluating sorghum-sudangrass in combination with either soybean or cowpea, sorghum-sudangrass suppressed both legumes, resulting in poor nitrogen-fixation. In this case, pure stands of either crop would be preferable. Similarly, aggressive cover crops like buckwheat do not usually do well in mixtures with legumes. Rapid canopy closure by buckwheat cover crops suppresses early legume growth. Buckwheat also needs to be mowed about 40 days after seeding to avoid seed production. This combination of shading and mowing leaves little opportunity for most legumes to fix nitrogen. Fields with high soil nitrogen fertility are also poor candidates for legume-grass mixtures. High soil nitrogen tends to favor non-legumes in mixtures and suppress nitrogen-fixation resulting in poor legume stands. Temperature can also have a strong impact on legumes grown in mixture. For example, cool temperatures tend to favor rye in rye-vetch mixtures, so planting mixtures of these species late in the fall can result in dominance of rye at the expense of vetch.

Some legume mixtures worth trying

For mid-summer cover crop plantings, cowpeas (Red Ripper or Iron Clay) have performed well in combination with summer grasses including pearl millet, German millet and Japanese millet (Photo 1). Oats and peas are a proven combination for early spring or late summer plantings. In late summer, they can produce substantial biomass and nitrogen before winter-kill, leaving a manageable residue in the spring (Photo 2). Winter rye and hairy vetch are a reliable combination for fall plantings (Photo 3). In addition to the benefits described above, this combination is thought to improve overwinter survival of vetch, and reduce harmful allelopathic effects of rye on subsequent crops. Several new early flowering, winter-hardy hairy vetch varieties developed atUSDA Beltsville, including ‘Purple Bounty’ and ‘Purple Prosperity’ are being tested for compatibility with winter rye, and may expand the already strong potential of this combination for vegetable growers. However, hairy vetch can become a formidable weed problem in perennial crops like asparagus or winter grains, so it should be avoided on farms with either of these crops in their rotation.

Mid-summer planting of cowpea
Mid-summer planting of cowpea and Japanese millet.

Late summer planting of oats
Late summer planting of oats and field peas.

Fall planting of winter rye
Fall planting of winter rye and hairy-vetch.

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