The high price of nitrogen fertilizer has increased interest in planting a legume crop after wheat or canning crop harvest as a green manure to provide some nitrogen credits for next year’s crop. This practice can provide some nitrogen and organic matter as well as increase ground cover to reduce erosion from fields. However, it may not be cost effective.

The following provides some data to help growers decide about options for producing nitrogen credits from late summer planted legumes.

How much nitrogen credits can we expect? The table 1 below shows the green manure nitrogen credits for several legumes. The nitrogen credit depends on the amount of crop growth and when the legume was planted. Nitrogen credits are 50 lb N/a lower on sandy, coarse-textured soils. Legumes other than those listed below would likely produce a range of N credits similar to those in table 1; however, not enough data exists on N credits to corn from these crops.

Table 2. Crop dry weight for plantings in Lincoln, NE
	Aug 7, 1985	Aug 26, 1986
	tons/acre
Austrian winter pea	0.45	0.35
Hairy vetch	1.40	0.41
Alfalfa	0.47	0.15
Rye	0.57	0.62
Black medic	0.60	0.08
Arrowleaf clover	0.40	0.04
Data from Power and Koerner, 1994, Agron J

 

 

 

 

 

 It is important to note that crops will perform differently from year to year depending on rainfall patterns, temperature and other considerations as shown by data from Power and Koerner (table 2). Thus the amount of fall forage dry matter production from August seeded legumes is highly variable from year to year depending on the specific pattern of rainfall and growing degree days, as well as the length of the fall period.

If we then consider the seed cost (table 3) and seeding cost ($15 to $25/acre) the establishment cost of a legume (other than alsike) will be $32 to $69/acre. At nitrogen cost of nearly $1.00/lb, the legume would have to produce 32 to 69 lbs of nitrogen to break even with fertilizer cost. The economics of planting a late summer seeded legume for nitrogen credits is questionable for most situations because there is no guarantee that at least a 40 lb N/a credit can be obtained.

Table 3. Seed cost of potential legumes for late summer seeding
Legume	Seed Cost per lb	Seeding Rate (lb/acre)	Seed cost/acre
Red clover	$2.70	10	$27.00
Berseem clover	$2.10	10	$21.00
Hairy vetch	$2.24	20	$44.80
Alsike Clover	$1.40	3	$4.20
Alfalfa	$5.00	12	$60
Austrian winter pea	Seed not available
Medic	Seed not available

 

 

 

 

 

One situation where late summer seeded legumes may be beneficial is where a cover crop is needed 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/bu rye this year. A legume could be planted for the ground cover at approximately the same cost and then the nitrogen fixed is a bonus. A better possibility may be to seed oats at ¾ to 1 bu/a oats along with the clover to provide rapid ground cover and still have some nitrogen fixing ability from the clover.