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Hedging against erosion.

Stiff grasses could be a low-tech solution to a widespread problem. Just as grasses planted along beaches protect sand dunes from wind and water erosion, stiff grasses could help U.S. farmers solve a major problem-- protecting valuable topsoil.

In India in 1988, two World Bank agriculturalists extolled the virtues of using live hedges of vetiver grass (Vetiveria zizanioides) for controlling water erosion and argued their merits over U.S. bulldozer-built terraces. Doral Kemper, national program leader for soil management, was there with other ARS experts as an adviser on soil management technology for the U.S. Agency for International Development.

"It was a somewhat humbling experience," Kemper recalls. "As representatives of the country that has mounted the most extensive erosion control effort on planet Earth, we were less than wholly receptive to the message of the World Bank agriculturalists. They were saying that bulldozer-built terraces like those we use in the United States disturb topsoil and take water away too quickly--rather than letting it soak in. And that vetiver grass hedges do a better .job of controlling erosion and conserving water--at much less cost--than terraces formed by earthmoving equipment."

With the aid of literature and slides, these vetiver enthusiasts provided strong evidence that the grass was fulfilling these claims in India, the West Indies, and Fiji.

"Slim, green rows of vetiver grass planted by farmers across hillsides 10 to 30 years ago grew into thick, sturdy grass hedges 9 teet tall and 6 to 7 feet wide. They slowed runoff significantly and deposited sediment," Kemper says. "Over the years, these deposits have formed wide, productive bench terraces uphill from the hedges."

Stateside, Kemper, working with cooperators at USDA's Soil Conservation Service (SCS), found out more about the grass. Until the mid-1940's, vetiver was grown in the southern states and was prized for its valuable roots containing an aromatic oil used for making long-lasting perfumes. After chemists found out how to synthe-size perfume, commercial production of the grass practically ceased.

Seeds of vetiver collected in that era were found by ARS agronomist Gilbert Lovell at the Regional Plant Introduction Station, Griffin, Georgia. Some of those seeds were still viable after almost 40 years in storage.

Next, Kemper and SCS national plant materials specialist Curtis Sharp called together interested ARS, SCS, and university personnel to coordinate research and development activities on grass hedges for controlling erosion.

The group soon learned that using grass hedges is not a new idea here either. About 40 years ago, SCS proposed using hedges for developing natural terraces on steep lands.

"We're not sure why the idea didn't catch on," Kemper says. "We suspect that farmers were in a hurry to get their terraces built and earthmoving equipment became readily available to do the job. Since the government subsidized the construction of terraces, farmers adopted the quick alternative."

The group began testing different types of vetiver grass at a dozen U.S. locations to find out whether it would live up to World Bank expectations.

"All the vetiver types grew well over the summer. But winter freezes killed most stands," Kemper says, "except for several hedges planted by SCS plant materials specialist Mike Materne across shallow gullies at Fort Polk, Louisiana, where U.S. Amy tank maneuvers had denuded extensive areas. There, the 100-foot-long vetiver hedges not only flourished, but caught sediment 18 inches deep on the upslope side in just a year."

Evaluating Other Grasses

Materne' s findings were so encouraging that the group began a search for other grasses with stiff stems that wouldn't bend over when water ponds up against them and that are adapted to the climates of various U.S. regions.

Kemper found one candidate grass growing outside his office at the Beltsville Agricultural Research Center, Maryland. He had been watching an ornamental silvergrass, (Miscanthus sinensis Andersson), grow thick, closely packed stems and survive several cold winters.

Also thriving in nearby plantings were several closely related silver-grasses planted in 1980 by ARS' Jack Murray and Kevin Morris, of the National Tuff Grass Association. They provided vegetative silvergrass to Materne, who multiplied it in Louisiana plots during the winter months and made it available to all who needed it the following spring.

In 1991, silvergrass hedges were tested on erosion plots near Oxford, Mississippi. About a month before cotton planting, clumps of 1-foot-tall grass from Materne were planted about 7 inches apart and uphill from the lower ends of 72-foot-long plots with 5-percent slopes.

"Just one row of the grass held back nearly half of the soil lost or eroded from plots without hedges," says Keith McGregor, ARS agricultural engineer at the USDA National Sedimentation Laboratory in Oxford.

From May through September, about 14 tons an acre of soil were lost on grass-hedged plots that were conventionally tilled compared to 24 lost on plots without hedges. On untilled plots, these losses were 0.7 tons with hedges versus 1.3 without.

McGregor says, "A 5-percent slope is enough for runoff to cause serious erosion during rainstorms. Longer slopes in fields would cause even more soil to erode."

"Soil accumulated as far as 10 feet uphill from the grass hedges in just a year," he says. "It was fertile soil that would have been lost if it had been on unprotected cottonfields."

"How hedges work to protect fields is simple," explains McGregor. "As gaps between the hedges fill in, more water ponds up uphill from the hedges, more sediment is deposited in these ponds, and less soil and water are lost from the field."

He says that plans are to continue the study for at least 2 more years.

Another candidate the group found was switchgrass (Panicum virgatum L.), which showed promise for controlling both wind and water erosion.

ARS agronomist Seth Dabney, the Oxford project's grass expert, is testing several types of silvergrass and switchgrass as hedges. Last summer, he planted ten 3-foot-wide strips of switchgrass seed across the slopes of a 20-acre soybean field.

"We wanted to see under field-scale conditions how well the stiff-grass strips held back concentrated runoff water that would flatten most grasses," Dabney says.

"We've established a stand, but switchgrass grows slowly for the first year or so. The young hedges are not yet stiff enough to stand up against the concentrated runoff near the bottom of the field." Dabney is working with local farmers to develop practical methods to establish and manage hedges to best control erosion.

Using grass hedges will also help to maintain water quality downstream. "Any conservation practice that leaves more of the soil in place on the land also improves the water quality of our streams and lakes," McGregor says.

Dabney adds, "Hedges planted on the contour give farmers a guide to farm along. And by temporarily ponding runoff, hedges may be able to hold sediment on fields where crop residues must be turned under to prevent build-up of insects and diseases or to control weeds where herbicide applications are a problem."

Past research under the direction of Oxford ARS agricultural engineer Don Meyer with finer stemmed grasses like tall rescue, Festuca arundinacea, and bermudagrass, Cynodon dactylon, showed their ability to trap sediment from shallow flows. However, they were flattened by deeper flows.

"A major challenge now," says Meyer, "is to find grasses that will stand up to deeper concentrated flows coming off large, sloping fields."

Former ARS research associate, Gale Dunn, who worked with Dabney, evaluated the ability of grass hedges to slow runoff water in a test channel at the Oxford lab.

"We found that all the 2-year-old vetiver, silvergrass, and switchgrass types we tested were strong enough to pond flowing water up to 1 foot deep," Dabney says. "While all these grasses grow tall and erect, more important for hedges than plant height is an increase in the size and number of tillers-- sprouts from the base of the stem. Tillers help fill in the gaps between plants and slow the flow. Large tillers give hedges stiffness so they don't bend over in deep flows."

Meyer and Dabney are building a new test channel where they will study different types of hedges to see how crop residues and sediment in runoff affect the amount of water ponded and the length of time it remains.

Kemper says that grass hedges are also being studied at several other ARS labs.

* Based on 25 years of research by ARS soil scientists A1 Black and J. Kristian Aase at the ARS Northern Plains Soil and Water Research Center, Sidney, Montana, Kemper is convinced that grass hedges will also help control wind erosion.

Aase says that "Hedges of tall wheatgrass, Elytrigia elongata, seeded at 50-foot intervals, reduce wind velocity at the soil surface and provide protected areas where loose soil particles stay settled. Otherwise, unsettled particles become projectiles that blast additional particles loose to become airhome."

He says that in the northern plains, hedges also trap snow. Some water-borne sediment from their rare runoff events is deposited in the draws, causing terraces to form across the hedges.

* Near Big Springs, Texas, where only about 17 inches of rain fall a year, ARS agronomist James D. Bilbro is finding that hedges of Alamo switch-grass grow over 6 feet tall and have dense tillers and deep rooting.

"Growing cotton on these sandy soils makes them extremely vulnerable to wind erosion," says Bilbro. "In this semiarid area where crop residues are generally in short supply and the potential for wind erosion is quite high, switch-grass hedges can certainly reduce soil loss from wind erosion."

* Also adding to our knowledge of how to use grass hedges effectively to control erosion are studies by ARS agricultural engineer Larry Kramer at Deep Loess Research Station near Traynor, Iowa, and soil scientist E. Eugene Alberts on claypan soil erosion study plots near Columbia, Missouri.

Kramer has established a switchgrass hedge on a 15-acre corn field of highly erodible soil with slopes of up to 16 percent. He's using silvergrass plantings to reinforce the hedge where it must stand up to concentrated runoff.

"Perennial grass hedges don't grow quickly, so time is needed to give them the opportunity to show what they can do," Kemper said. "This time we're not going to let our impatience keep us from completely evaluating this technology!"--By Hank Becket, ARS.

To contact scientists mentioned in this article, write or call Hank Becker, USDA-ARS Information Staff, Bldg. 419, BARC-East, Beltsville, MD 20705-2350. Phone (301) 504-8547, fax number (301) 504-8030.
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Title Annotation:planted stiff grasses prevent soil erosion
Author:Becker, Hank
Publication:Agricultural Research
Date:Dec 1, 1992
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