Late Summer Planted Legumes For N Credits: Pencil Carefully

By Jane Fyksen Crops Editor
Thursday, August 7, 2008 10:04 AM CDT

Painfully high priced nitrogen has more Wisconsin producers interested this year in planting a legume crop after wheat or a canning-crop harvest. They’re after green manure and nitrogen credits for next year’s crops. Side benefits are organic matter and ground cover over winter to reduce erosion.

However, UW-Madison Forage Specialist Dan Undersander and Soil Scientist Carrie Laboski warn that the economics are iffy. Late-summer planted legumes may not be cost-effective.

How many N credits can you expect? Farmers can figure on 40 pounds per acre from alfalfa, red clover, sweet clover and vetch from less than six inches of growth at the time of killing frost. Over six inches, the crops supply the following: Alfalfa, 60 to 100 pounds; red clover, 50 to 80; sweet clover, 80 to 120; vetch, 40 to 90. If top growth of vetch is over 12 inches before killing the cover crop, the credit is up around 100 to 150 pounds of N per acre. (The upper end of the range is for spring seeded green manures plowed under the following spring, the lower end is for fall seeding.)

N credits are 50 pounds lower on sandy, coarse-textured soils; reduce the above credits by 50 pounds if that’s your type of land.

Undersander and Laboski remind growers of the risk; crops perform differently year to year depending on rainfall and temperature late in the season. Consider the following crop dry weight data (from Nebraska) for green manure plantings two years in a row: Austrian winter pea, 0.45 tons per acre and 0.35; hairy vetch, 1.4 and 0.41; alfalfa 0.47 and 0.15; rye, 0.57 and 0.62; black medic, 0.6 and 0.08; arrowleaf clover, 0.4 and 0.04.

“The amount of fall forage dry matter production from August-seeded legumes is highly variable from year to year,” they warn, “depending on the specific pattern of rainfall and growing degree days, as well as the length of the fall period.”

Now consider the cost of seed and the cost to establish a legume. Other than alsike (which is considerably cheaper), it’s apt be $32 to $69 an acre, according to these two UW experts. At a nitrogen cost of nearly $1 a pound, the legume would have to produce 32 to 69 pounds of N to break even with the cost of fertilizer.

Here’s what Undersander and Laboski offer in terms of seed cost of potential legumes for late summer seeding:

- Red clover - $2.70 a pound for seed; seeding rate of 10 pounds per acre; $27 per acre seed cost

- Berseem clover - $2.10 for seed; seeding rate of 10 pounds; total seed cost, $21

- Hairy vetch - $2.24 for seed; seeding rate of 20 pounds; total seed cost, $44.80

- Alsike clover - $1.40 for seed; seeding rate of 3 pounds; total seed cost, $4.20

- Alfalfa - $5 for seed; seeding rate of 12 pounds; total seed cost, $60

- Austrian winter pea n seed not available

- Medic n seed not available.

“The economics of planting a later summer seeded legume for nitrogen credits is questionable for most situations, because there is no guarantee that at least a 40 pounds of N per acre credit can be obtained,” they stress. What’s more, the cost of firing up the tractor (i.e. fuel) and your time need to be considered, too.

There are two situations, though, where late summer seeded legumes may be useful. One is where a cover crop is needed anyway to reduce erosion over winter. “Any cover crop planted will have some cost, so, for example, if rye is generally planted as a cover crop, the seed cost is $15 to $20 per bushel this year. A legume could be planted for the ground cover at approximately the same cost and then the nitrogen fixed is a bonus,” they explain.

Undersander and Laboski think a better possibility may be to seed oats at three-fourths to one bushel per acre along with the clover to provide rapid ground cover and still reap some N fixing ability from the clover.

The other place they see August seeding of legumes of N credits paying off is in organic systems where commercial fertilizer isn’t an option.

As noted, fall growth and N fixation is extremely variable. N credits come from both top growth and root growth. For perennial legume, producers can assume that half the growth is above ground and half below. For annuals like berseem, hairy vetch and pea, assume 60 to 70 percent of total growth is above ground.

Determine the yield of forage at frost this fall for actual N available in your field for next year’s corn. For perennials, determine above ground growth and multiply by two to determine total biomass. Then assume 3 percent N to determine N credits. If a ton of forage is produced, assume two tons or 4,000 pounds total biomass times 3 percent or 120 pounds of N per acre.

For annual legumes, determine the above ground growth and multiply by 1.4 to determine total biomass. Then assume 3 percent N to determine credits. If a ton of forage is produced, assume 1.4 tons or 2,800 pounds total biomass from an annual legume times 3 percent or 84 pounds of N per acre.

Actual N credits can be verified if the remainder of the N requirement for corn will be sidedressed next spring. The pre-sidedress nitrate soil test (PSNT) should be used to verify N credits, instead of using the “book values” above.

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