Forage legumes like alfalfa and red clover together with manure were once the only way to provide additional nitrogen (N) to non-legume crops. The ability to synthetically produce nitrogen fertilizers greatly decreased agricultural reliance on traditional nitrogen sources beginning in the early 20th century.

Legumes still can provide valuable N to today’s cropping systems. Legumes also contribute a non-nitrogen rotation effect due to addition of soil organic matter and improvement in soil health. Corn grown following alfalfa stands that are 2+ years old (and contained at least 50% alfalfa) require no nitrogen fertilizer on many soils. Red clover N credits are less than for alfalfa. Details of nitrogen fertilizer rates following legume crops can be found in N credit from other previous crops.

How does of one year of legume growth benefit following crops?

Alfalfa and red clover are typically grown in rotations for 2-4 years before termination, however there are many benefits from growing legumes for only one year in the cropping rotation as well. In a rotation experiment in southern Minnesota, alfalfa and red clover were planted alone (solo seeding with no weed control) or with a spring wheat companion crop, followed by corn in year 2. The spring wheat was harvested for grain (average yield 59 bu/acre) and the legumes were allowed to regrow until they were plowed under in November. When solo seeded, the legumes were harvested once for forage (average yield 1.5 tons/acre) and the regrowth was incorporated in early November.

At fall plow-down, N contained in herbage regrowth and roots of the alfalfa established with wheat was greater than for red clover (Table 1). Corn yield following alfalfa was similar whether it was solo seeded or seeded with a wheat crop. Corn yields were greater when planted following the red clover monoculture compared to when the red clover was seeded with wheat.

Table 1. Fall forage dry matter yield and N for incorporation in herbage and roots of alfalfa and red clover and corn grain yield the following year. All crops produced under organic production conditions without pesticide inputs. 

How does underseeding affect yields of small grains and legumes?

Because of their small size, seedling red clover and alfalfa do not compete with the small grain companion crops. Research conducted in northwestern and west central Minnesota showed that wheat grain yield (44.8 bu/acre), test weight (56.2 lb/bu) and protein level (14.4%) were not affected by intercropping with red clover or alfalfa.

Companion cropping often affects legume yields and N available for incorporation. While effects on alfalfa forage and N yields can be minimal as shown in Table 1, red clover yields were somewhat better when seeded in monoculture. Those results were influenced by weed competition when legumes are grown in monoculture. When a herbicide is used for weed control, seedling year alfalfa forage and root and crown biomass yields are maximized with the greatest opportunity for contributing N to subsequent crops (Table 2). Harvesting the small grain for forage at the boot stage shortens the time of competition for light and moisture and results in greater forage, root/crown and fall nitrogen yields.

Table 2. Establishment of alfalfa with post-emergent herbicides or oat companion crops affects seasonal forage and fall root/crown yield, as well as fall N available for incorporation. All values from seeding year.

Tips for successful establishment

Practices for successful establishment of forages in the spring are described in several resources on Extension's Forage establishment webpage. To maximize establishment and productivity in the seeding year special consideration should be given to:

• Seeding depth. Optimum seeding depth for all methods is ¼ to ½ inch. A ¾ inch seeding rate is allowed in sandy soils. For drills used in small grain companion crop seeding it is critical to seed legumes through a separate legume seed box. Also, use separate delivery tubes that allow deeper placement of the small grain and shallow placement of the legume. Dragging or harrowing legume seed into the soil surface is not recommended because much seed is often buried too deeply.
• Soil seed contact.Drills should use press wheels or be followed with cultipacker seeders to promote contact of the small legume seed with soil and soil moisture. Smooth or tire rollers do not improve seed to soil contact like a cultipacker/brillion roller does.
• Competition with legume seedlings. Using postemergence herbicides to reduce weed pressure will provide for the greatest seedling year yields and nitrogen production. Small grain companion crop grain and straw should be harvested promptly at maturity to reduce shattering, lodging. and smothering by straw windrows. Minimize grain shattering because volunteer small grains will complete with legume seedlings.

How much nitrogen credit can one year of alfalfa or red clover provide?

N credits for 1 year old stands of alfalfa or red clover are less than for stands 2+ years old. This is because of the smaller root system, reduced biological nitrogen fixation and less forage production compared to older stands.

By extrapolating from nitrogen recommendations comparing 1 and 2+ year old alfalfa stands provided in N suggestions for 1st and 2nd year corn following alfalfa, it appears that a fall incorporated 1 year old stand of alfalfa can contribute from about 30-60 lb/N per acre to a subsequent corn crop on medium or fine textured soils.

N contribution is increased by incorporating greater amounts of forage in the fall or spring since roots contain less nitrogen than forage. For example, assuming that alfalfa forage contains about 50% of its N from biological nitrogen fixation, each 1000 lb of forage incorporated contributes about 15-20 lb/acre of biologically fixed N.

Alfalfa and red clover also contribute beneficial non-nitrogen rotation effects to following crops due to improvement of soil health.

The economic advantages of using 1-year of alfalfa or red clover in crop rotations can be determined when considering seed costs and value of the N supplied.

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