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Benefits of perennial forages for soils, crops and water quality

Bill Jokela, Michael Russelle Published on 02 March 2010

Alfalfa is the most widely grown perennial forage species in the U.S., with the greatest acreage in the Upper Midwest and Western states. But alfalfa acreage has been declining steadily for the past 50 years, while the acreage of soybeans and, more recently, corn, has been increasing.

There are costs associated with the displacement of perennial forage acreage. As we enter a period with heightened public scrutiny of agriculture’s environmental “footprint” (water and air quality, greenhouse gas production and energy use), farmers need to consider the total value of adding perennial forages to their crop rotations.

This article covers the direct benefits of perennial forages used in rotation or as a cover crop and the more far-reaching benefits of perennial forages in terms of improved soil and water quality.

One of the reasons for the increase in corn acreage, and decrease in alfalfa acreage, is that the amount of corn silage fed to U.S. dairy herds has increased significantly in the last 20 years.

This has occurred for several reasons, according to Randy Shaver, University of Wisconsin dairy scientist. Reasons include higher yield and energy per acre from corn silage, improvement of corn silage hybrids, problems with winterkill of alfalfa, high rumen degradability of alfalfa protein and better opportunities for manure management.

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There are, however, a number of economic and environmental benefits from production of alfalfa and other perennial forages that are sometimes overlooked when comparing them to silage corn.

One of the most important economic reasons to incorporate perennial forages into a rotation is the nitrogen (N) credit for the crop following the forage. Nitrogen released from decomposing alfalfa tissue and from newly accumulated soil organic matter can supply most or the entire N requirement of a following corn crop (see Figure 1).

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Estimates of fertilizer N credit vary depending on the forage species, the quality of the stand (see Table 1), and the amount of regrowth. For example, University of Wisconsin recommends an additional first-year N credit of 40 pounds per acre when 8 inches or more regrowth is incorporated.

In addition, first-year corn grown after alfalfa may have 10 to 15 percent higher yield potential due to factors other than N supply (“rotation effect” in Figure 1), including improved soil tilth and fewer pest problems. Because of lower insect pressure, additional savings can be had in seed or insecticide with corn rotated after a perennial forage.

A major concern with corn silage production is the increased risk of soil erosion when there is little residue to protect the soil surface. Erosion can be greatly reduced by incorporating alfalfa into the rotation.

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An example from southern Wisconsin shows that soil erosion can be reduced to 3 tons per acre soil loss in a corn silage-alfalfa rotation, compared to 10 tons per acre with continuous corn silage (see Figure 2 on page 6). Erosion also can be reduced with a winter cover crop after corn silage, but not as much as with a perennial crop.

Decreasing soil erosion helps protect surface water from sediment, one of the most frequently cited impairments of water quality. In addition, most phosphorus (P) lost in surface runoff from row crop systems is associated with eroded sediment. Both sediment and P impair habitat for high-value fish species, and excessive P can increase algae growth, leading to water quality degradation and fish kills.

Producers should be aware, however, that significant soil erosion and associated P loss can occur during establishment of perennial forages, and that dissolved P can be present in runoff from freeze-damaged forage. Reduce erosion during establishment by including a companion small grain or forage grass. Reduce runoff of dissolved P by removing regrowth in late fall.

Much of the benefit in controlling erosion is derived from the year-round vegetative cover perennial forages provide, but another important factor is increased accumulation of soil organic matter compared to annual cropping. Soil organic matter supports better water infiltration, water-holding capacity, aeration, and soil aggregate stability.

The composite effects on physical, chemical, and biological soil properties can be estimated by calculation of a soil quality index.

One such index, the Soil Management Assessment Framework (SMAF), incorporates several soil characteristics, including bulk density, water-stable aggregates, organic matter, extractable P, pH and microbial biomass.

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Legume and grass forages improve the SMAF soil quality index, both as rotation crops (see Figure 3) and as companion crops or cover crops (see Figure 4), compared to continuous corn.

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Another environmental concern is the leaching of nitrate into subsurface tile drains. Land seeded with alfalfa or Conservation Reserve Program vegetation in Minnesota had lower tile line discharge and negligible nitrate-N loss (less than 5 pounds N per acre) compared to continuous corn or a corn-soybean rotation (30 to 80 pounds N per acre). Nitrate losses also can be reduced by planting grass or alfalfa ‘waterways’ directly over tile lines.

What about manure on perennial forages? Land in perennial forages provides an opportunity for manure application during mid-season when land in annual crops is not available. Manure can supply needed nutrients for forages – potassium, P, sulfur, micronutrients, and, for nonlegumes, N. Large nutrient removal in the harvested forage helps with nutrient balance and minimizes nitrate leaching.

These advantages to manure application on forages need to be considered in the context of some potential concerns – plant damage from manure or wheel traffic, pathogen transmission in the feed, nutrient runoff, and excessive N at stand termination where there already is a large N credit.

Some of these risks can be minimized by careful management; for example, by spreading soon after harvest, avoiding wet soil conditions, and avoiding application at stand termination if the N credit from the forage is adequate for the next crop.

Several innovative liquid manure application methods offer additional options to improve N utilization, minimize contamination of forage, decrease runoff of nutrients, and provide more uniform application of manure. These include shallow injection, band spreading directly on the soil surface beneath the canopy, and combining band spreading with an aerator implement.

Other benefits from production of alfalfa and other perennial forages in rotation with corn include better distribution of labor needs and improved wildlife habitat. For grain producers who do not want to purchase a new line of equipment, contract planting and harvesting often is available.

And on top of all these benefits, nearly every economic analysis has concluded that alfalfa production is, on average, more profitable than corn. The fact that forages also can produce substantial direct and indirect economic returns should make the decision to include more forages in the rotation an easy one.  FG

Bill Jokela
  • Bill Jokela
  • Soil Scientist
  • USDA Dairy Forage Research Center