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Alfalfa persistence needs good soil, not just good genetics

David Lang Published on 06 February 2014

Alfalfa (Medicago sativa L.), the queen of the forage crops, is one of the most developed and researched forage and pasture crops in the U.S. and elsewhere in the world where it is well adapted.

It’s normally considered to be productive for three to five years throughout much of the Midwest and follows a regular stand decline pattern of 20 to 50 or more plants per square foot at establishment to less than 10 plants per square foot at the end of its productive life span.

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The ability of alfalfa crowns to send out many shoots allows a single plant to expand in size and dominance as other neighboring alfalfa plants die off over time. As few as four to six alfalfa plants per square foot can constitute a productive alfalfa stand that remains economically viable. The alfalfa plant expands its number of stems to fill the space as the crowns age and die out.

Following harvest, as the individual crowns produce new shoots, weed competition reduces the number of shoots produced by individual alfalfa crowns. With some of the herbicide systems and alfalfa tolerance to the herbicides, such as glyphosate, weeds can be managed to extend the productive life of alfalfa stands.

Alfalfa is well adapted to deep, well-drained soils that also have good surface drainage. Though it can be tolerant of slightly acidic soils (pH 5.5 to 6), it does best in a soil with pH of more than 6.

Soil calcium is a particularly important consideration, especially in slightly acidic soils. Alfalfa persistence is also greatly influenced by soil potassium (K), and potash is essential for proper storage of carbohydrates in alfalfa’s extensive taproot system.

Cycling of carbohydrate reserves stored in alfalfa’s taproot follows a pattern of buildup to as much as 40 percent of the taproot by weight at early flowering to as little as 12 to 15 percent as those carbohydrates are remobilized five to 10 days following a hay harvest or quick foliage removal during a grazing event. If alfalfa is harvested too soon, carbohydrate root reserves will not be given a chance to recover to sufficient levels for strong re-growth.

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These carbohydrate reserves are also critical for overwinter survival, and they are slowly consumed during the Midwest dormant period from October to April. Snow cover also aids survival by providing a protective blanket to moderate changes in soil temperature during cold winters.

Alfalfa survival in the Midwest generally needs a high level of carbohydrate root reserves built up in the fall, though this concept has been altered in recent years by advances in alfalfa genetics that has moderated this concept.

In the past, advances in alfalfa genetics have focused on resistance to various root and leaf diseases as well as resistance to various insects. More recently, scientists have modified alfalfa genetics to include resistance to herbicides such as glyphosate.

Roundup Ready alfalfa is now available in many varieties, including some well adapted to Southern conditions. Plant breeders have incorporated many of the needed resistances to insects and diseases necessary to make it adapted to Southern conditions.

It still needs to be grown on soils well adapted to alfalfa (well-drained soil, including internal and surface, pH more than 6 and moderate to high levels of soil phosphorus and potash as per soil test recommendations). Genetic advances are needed to provide adaptation of alfalfa to heavy clay soils.

Alfalfa was grown on many cropland acres throughout the South as late as the early 1900s. It was used primarily as feed for horses, oxen and other draft animals used before tractors became available. Much of the best well-drained cotton and soybean land in the Mississippi River alluvial floodplain, commonly called the Delta, was once planted to alfalfa.

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The Mississippi Agricultural and Forestry Experiment station released a variety of alfalfa called ‘Delta’ in the 1920s. Alfalfa can still be grown throughout the South if the soil conditions are suitable, particularly with recent advances in genetics, including the advent of Roundup Ready alfalfa.

Soil limitations include wet soils with poor drainage (both internal and surface) in Mississippi’s hot subtropical highly humid climate that receives 60-plus inches of rainfall per year. [Check with Web Soil Survey – to see if your soil has a capability rating of 1 or 2 without a small letter of s (shallow) or w (wet).]

Internal soil drainage is often limited by the presence of highly restrictive soil layers called fragipans or other root-restrictive layers that occur naturally in our soils due to ancient geological processes that occurred millions of years ago.

These are not the same as “plow pans” caused by repeated shallow tillage operations. Fragipans are generally deeper and cannot be alleviated by deep-ripping the soil with chisels the same as plow pans can be dealt with. One way to effectively deal with a fragipan is to allow mining companies to recover the lignite and other minerals deep in the earth under much of the Mississippi Embayment .

As part of this mining process, the land is drastically disturbed to depths of 100 to 300 feet and reclaimed to a productive landscape in an ongoing process as the mining operation moves across the area.

Such activities are currently occurring in Choctaw and Kemper counties in Mississippi on lands that have naturally occurring fragipans. New potential alfalfa land, as well as land suitable for most cropping and forestry situations, are being recreated as lignite coal is removed in these counties in Mississippi.

Historically, alfalfa has been grown in the South on well-drained deep soils in the alluvial Delta and also along the ridge of loess (wind-blown) soil and other Gulf Coastal Plain soils. Some of these loess soils are shallow with a fragipan at a depth of 18 inches and are not suited for traditional alfalfa.

Productive loess soils have no fragipan or one that occurs at depths of more than 3 feet and can be suitable for alfalfa. Most of these soils are acidic, and growers must go to great expense to incorporate great quantities of lime into the soil 12 inches or deeper.

During this type of land preparation, it is also wise to incorporate deeply potash and phosphorus according to soil test recommendations. Alfalfa’s taproot and secondary roots have the ability to penetrate deep into the soil and will do so in soil that is not restricted.

Some deep Midwest prairie soils have had alfalfa taproots and rooting systems that reach a 20-foot depth. This characteristic allows alfalfa to tap into deep moisture reserves making it very drought-tolerant, which explains why one of its centers of genetic origin is the very dry region near Iraq.

There are other deep well-drained soils in the mid-South that have great potential to grow alfalfa – so consult the local county extension office and obtain your land’s soil survey from the nearby NRCS office.

The new genetics and advent of Roundup Ready alfalfa are not “silver bullets” capable of overcoming the natural soil limitations, but alfalfa can be grown in the South if the conditions are suitable and you are willing to spend the time needed to make good decisions about its usage on your land.

Visit the National Alfalfa & Forage Alliance website and download the PDF for Growing Alfalfa in the South for more about utilizing alfalfa. PD

David Lang

David Lang
Professor of Agronomy
Forage and Pasture Crops
Mississippi State University

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