Alfalfa's Bioenergy Appeal

It's becoming increasingly clear that corn-based ethanol alone won't make a definitive dent in the nation's fuel needs. Instead, a diversity of materials from corn to crop residue will be needed to produce significant amounts of ethanol. A perennial plant that is gaining attention for this purpose is alfalfa.
By Jessica Ebert | August 27, 2007
Although some people refer to alfalfa as the "queen of the forages," many would be surprised to know that the deep-rooted legume is the third most valuable crop in the country. Although its production is overshadowed by its cousins—corn and soybeans—nearly 21.5 million acres of alfalfa, valued at over $7.5 billion was harvested last year, according to the National Agricultural Statistics Service. "It's a wonderful, sustainable crop that is very good for the environment," says Beth Nelson, executive director of the National Alfalfa and Forage Alliance (NAFA), a nonprofit organization that supports the alfalfa industry. However, "this is also a crop that people are not very aware of."

Thought to be one of the oldest crops cultivated by humans, alfalfa was first discovered and grown in the deserts of northern Iran. The leafy, palatable plant was introduced to the continental United States during the gold rush and the cultivation of it has since spread across the country. Although the rugged alfalfa crop grows best in fertile, well-drained soils that receive plenty of rainfall, breeding efforts have developed varieties that are winter hardy, pest resistant and more heat and drought tolerant.

Alfalfa is rich in protein, fiber, vitamins and minerals, and has historically been raised on farms and harvested for dairy and beef cattle feed. However, as cattle breeders have moved toward feeds with more grain and less forage, there has been a steady decline in the amount of alfalfa grown the past 25 years. Concurrently, more land is being turned over to continuous row cropping of annuals like corn and soybeans with little or no rotation with perennial plants. This is troublesome because in addition to its nutritional attributes, alfalfa—as well as other perennial forages—is known for its environmental benefits.

The plant is seeded once every three to five years and harvested several times each growing season. Side benefits of growing this crop include, improved soil and water quality, reduced erosion and increased soil organic matter. Alfalfa also forms a symbiotic relationship with a soil bacterium that lives in a pouch-like organ on its roots. The bacteria living in this nodule scrub nitrogen from the air and convert it into a form that the plant can use for growth. In other words, alfalfa essentially makes its own nitrogen fertilizer. Better yet, it leaves some of this naturally formed compound behind when the crop is plowed under, which increases soil fertility. In a rotation of alfalfa and corn for instance, a plowed field of alfalfa usually provides all the nitrogen needs for the first year of corn and about half of the nitrogen required by the second crop, says Hans Jung, a dairy scientist with the USDA Agricultural Research Service (ARS) in St. Paul. In addition, "when you have rotations you typically get an increase in yield of the following crop," he explains. "From an environmental standpoint and an agricultural management standpoint there are some real advantages to bringing in a short-lived perennial like alfalfa."

Developing New Varieties
Jung and his colleagues at the USDA-ARS and the University of Minnesota have been studying the potential for raising alfalfa as a bioenergy crop since the early 1990s. "At the time we thought this was a unique thing to do with alfalfa but we discovered that a couple of other people had proposed this during the OPEC oil embargo of the '70s," Jung explains. The group initially studied the feasibility of using alfalfa in a combustion system for generating electricity. The focus has changed, however, as interest in cellulosic ethanol arose and concern for the environmental consequences of continuous corn cropping has increased. That prompted Jung and several of his colleagues including JoAnn Lamb, a plant geneticist with the USDA-ARS, to start breeding new types of alfalfa and to develop a modified management system for the production of biofuels from alfalfa. "We've seen more and more farmers switching to continuous corn because of the grain ethanol industry," Jung says. "Environmentally that doesn't make sense. So the question becomes, what can we integrate into a rotation with corn such that you don't eliminate corn from the system but you produce something else that can feed into that same ethanol system so that the facility is still getting its supply of cellulose? We think that alfalfa fits and we're working with a USDA lab in Peoria (Ill.) to look at processing alfalfa to make ethanol."

If alfalfa stems (not the protein-rich leaves, which can be sold and used as feed) and corn stover are to be essentially interchangeable feedstocks for the production of ethanol then the alfalfa stems must contain the same amount of cellulose as the corn crop residue and produce high biomass yields. To achieve this, Lamb bred an experimental variety of alfalfa that could be grown for longer periods of time between harvests to accumulate more cellulose. Normally, when alfalfa is allowed to mature, it falls over—a trait called lodging, which makes the harvesting process a bit tricky. This new biomass-type of alfalfa, however, does a better job of resisting gravity. Based on the amount of cellulose and other fermentable carbohydrates produced, theoretical ethanol yield from alfalfa stems was doubled by switching from an animal feed-type alfalfa, cut early and often, to this biomass-type alfalfa, cut later and less frequently. These results will soon be reported in the journal Crop Science, Jung says. In addition to harvesting later and less frequently, the scientists adjusted the management scheme for alfalfa and found that planting fewer plants per square foot provided more space for each plant to grow and to produce more biomass. The only drawback is that there's more lignin in the cell walls of these new varieties so the alfalfa stems require a more stringent acid pretreatment to release the sugars used in the fermentation process.

To solve this problem, a company called Forage Genetics International is working with Jung's group to cross its nonlodging varieties with a low-lignin line of alfalfa created by Richard Dixon's team of researchers at the Samuel Roberts Noble Foundation Inc., an agricultural research center in Oklahoma. This work originally began more than 15 years ago with the aim of improving the forage quality—the digestibility—of alfalfa by modifying the lignin. To that end, Dixon and colleagues took certain genes from a similar species of legume and inserted those fragments of DNA into alfalfa. These pieces of genetic material interfered with the production of the proteins needed to make lignin thereby resulting in alfalfa plants with less lignin in the cell walls. "We generated alfalfa plants where we had modified the lignin pathway at many different points," Dixon explains. "All these different lines have different amounts of lignin and different compositions of lignin and some of these lines have been evaluated over several years from the forage quality standpoint."

About two years ago, the U.S. DOE announced funding opportunities for bioenergy research that focused on modifications of plant cell walls to improve the fermentation process. "We couldn't believe this because we had all these alfalfa lines sitting there that we had been putting into cattle to test digestibility," Dixon says. "All we really needed to do was put them through the processing steps for bioethanol production."

That's just what they did and in a recent paper in the journal Nature Biotechnology, Dixon and colleagues show that when the low-lignin alfalfa lines are subjected to acid pretreatment followed by treatment with enzymes, much more sugar is released from the cell wall. "The removal of the lignin makes the acid pretreatment a little more efficient but has a huge impact on the subsequent ability of the enzymes to degrade the cell walls," Dixon explains. In fact, "you don't even have to do the acid pretreatment to get as much sugar as you would get from a normal plant."

Dixon continues, "A lot of folks in the bioenergy area think this is really cool but they say this is a model system really, that it's basically a proof-of-concept. I see no reason why this proof-of-concept won't translate to other species like switchgrass and poplars based on what I know about cell walls. But at the same time I also think that alfalfa is viable as a bioenergy crop. Other people don't think it's going to be major, but it may be a player."

One way to change people's minds is to raise alfalfa's profile, Nelson says. To do that she organized a renewable fuel summit in early August that featured speakers from the DOE, the USDA, the Senate Energy and Natural Resources Committee and the House Agriculture Committee among others.

"Alfalfa is an important crop in our U.S. agricultural landscape as far as its environmental and rotational benefits [are concerned] Nelson says. "As the country is trying to move toward 25x'25 (a renewable energy initiative with the goal of getting 25 percent of our energy from renewable sources by 2025), I think everybody realizes it's not going to come just from corn. It's not going to come just from corn and alfalfa either but it's going to come from a host of crops. Because of some of the preliminary research that people have done, it really does look like alfalfa is a very good candidate crop for a feedstock."

At press time, Gene Sandager, a director on the board of the National Corn Growers Association, was scheduled to speak to the summit attendees about how a corn-alfalfa rotation works. "I was called because we have raised alfalfa in rotation with corn since I was a little kid and because we harvest a lot of grass in our operation," Sandager says. "I want to show this group that it's about adapting crops to your environment. If you can adapt those crops to produce food and fuel I think it's a win-win situation," he says.

Jessica Ebert is an Ethanol Producer Magazine staff writer. Reach her at [email protected] or (701) 746-8385.