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Panicum virgatum

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Switchgrass
Scientific classification
Kingdom:
Plantae
(unranked):
Angiosperms
(unranked):
Monocots
(unranked):
Commelinids
Order:
Poales
Family:
Poaceae
Genus:
Panicum
Species:
P. virgatum
Binomial name
Panicum virgatum

Panicum virgatum, commonly known as switchgrass, is a perennial warm season grass native to North America, where it occurs naturally from 55° N latitude in Canada southwards into the United States and Mexico. Switchgrass is one of the dominant species of the central North American tallgrass prairie and can be found in remnant prairies, in native grass pastures, and naturalized along roadsides. It is used primarily for soil conservation, forage production, game cover, as an ornamental grass, and more recently as a biomass crop for ethanol, fibre, electricity, and heat production. Other common names for switchgrass include tall panic grass, Wobsqua grass, blackbent, tall prairiegrass, wild redtop and thatchgrass.

Properties

Root system of switchgrass grown at The Land Institute

Switchgrass is a hardy, deep rooted, perennial rhizomatous grass that begins growth in late spring. It can grow up to 1.8-2.2 m high but is typically shorter than Big Bluestem grass or Indiangrass. The leaves are 30-90 cm long, with a prominent midrib. Switchgrass uses C4 carbon fixation, giving it an advantage in conditions of drought and high temperature.[1] Its flowers have a well-developed panicle, often up to 60 cm long, and it bears a good crop of seeds. The seeds are 3-6 mm long and up to 1.5 mm wide, and are developed from a single-flowered spikelet. Both glumes are present and well developed. When ripe, the seeds sometimes take on a pink or dull-purple tinge, and turn golden brown with the foliage of the plant in the fall. Switchgrass is both a perennial and self-seeding crop, which means farmers do not have to plant and re-seed after annual harvesting. Once established, a switchgrass stand can survive for ten years or longer.[2] Also, unlike corn, switchgrass can grow on marginal lands and requires relatively modest levels of chemical fertilizers.[2] Overall, it is considered a resource-efficient, low-input crop for producing bioenergy from farmland.

Background

Much of North America, especially the prairies of the Midwestern United States, was once prime habitat to vast swaths of native grasses, including Switchgrass (Panicum virgatum), Indiangrass (Sorghastrum nutans), Eastern Gamagrass (Tripsacum dactyloides), Big Bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium), and others. As European settlers began spreading west across the continent, the native grasses were plowed up and the land converted to growing crops such as corn, wheat, and oats. Introduced grasses such as fescue, bluegrass, and orchardgrass [3] also replaced the native grasses for use as hay and pasture for cattle.

Distribution

Switchgrass is a very versatile and adaptable plant. It can grow and even thrive in many weather conditions, lengths of growing seasons, soil types, and land conditions. Its distribution spans south of latitude 55°N from Saskatchewan to Nova Scotia, south over most of the United States east of the Rocky Mountains, and further south into Mexico.[4] As a warm season perennial grass, most of its growth occurs from late spring through early fall, becoming dormant and unproductive during colder months. Thus, the productive season in its northern habitat can be as short as three months, but in the southern reaches of its habitat, the growing season may be as long as eight months, around the Gulf Coast area.[5]

Switchgrass is a diverse species, with striking differences between plants. This diversity, which presumably reflects evolution and adaptation to new environments as the species spread across the continent, provides a range of valuable traits for breeding programs. Switchgrass has two distinct forms, or "cytotypes": the lowland cultivars, which tend to produce more biomass, and the upland cultivars, which are generally of more northern origin, more cold tolerant, and therefore usually preferred in northern areas. Upland switchgrass types are generally shorter (≤ 8 ft, or 2.4 m, tall) and less coarse than lowland types. Lowland cultivars may grow to ≥ 9 ft, or 2.7 m, in favorable environments. Both upland and lowland cultivars are deeply rooted (> 6 ft, or 1.8 m, in favorable soils) and have short rhizomes. The upland types tend to have more vigorous rhizomes. Subsequently, the lowland cultivars may appear to have a bunchgrass habit, while the upland types tend to be more sod forming. Lowland cultivars appear more plastic in their morphology, and produce larger plants if stands become thin or when planted in wide rows. On the other hand, lowland types seem to be more sensitive to moisture stress than upland cultivars.[6]

In native prairies, switchgrass is historically found in association with several other important native tallgrass prairie plants, such as big bluestem, indiangrass, little bluestem, sideoats grama, eastern gamagrass, and various forbs (sunflowers, gayfeather, prairie clover, and prairie coneflower). These widely adapted tallgrass species once occupied millions of hectares.[7]

Establishment and management

Switchgrass can be grown on land that typically isn’t well suited to row crop production, including land that is too erodible for corn production as well as sandy and gravelly soils in humid regions that typically produce low yields of other farm crops. No single method of establishing switchgrass can be suggested for all situations. The crop can be established both by no-till and conventional tillage. When seeded as part of diverse mixture, planting guidelines for warm season grass mixtures for conservation plantings should be followed. Regional guidelines for growing and managing switchgrass for bioenergy or conservation plantings are available. Several key factors can increase the likelihood of success for establishing switchgrass. These include:[8]

  • Planting switchgrass after the soil is well warmed during the spring.
  • Using seeds that are highly germinable and planting 1/4 inch to 1/2 inch deep, or up to 3/4 inch deep in sandy soils.
  • Packing or firming the soil both before and after seeding.
  • Providing no fertilization at planting to minimize competition.
  • Controlling weeds with chemical and/or cultural control methods.

Mowing and properly labeled herbicides are recommended for weed control. Chemical weed control can be used in the fall prior to establishment, pre-plant and post-plant. Weeds should be mowed just above the height of the growing switchgrass. Hormone herbicides such as 2,4-D should be avoided as they are known to reduce development of switchgrass when applied early in the establishing year [9]. Plantings that appear to have failed due to weed infestations are often wrongly assessed, as the failure is often more apparent than real. Switchgrass stands that are initially weedy commonly become well established with appropriate management in subsequent years. [8] Once established, switchgrass can take up to three years to reach its full production potential.[9] Depending on the region, it can typically produce 1/4 to 1/3 of its yield potential in its first year and 2/3 of its potential in the year after seeding. [10]

After establishment, switchgrass management will depend on the goal of the seeding. Historically, most switchgrass seedings have been managed for the Conservation Reserve Program in the US. Disturbance such as periodic mowing, burning, or discing is required to optimize the stand’s utility for encouraging biodiversity. Presently, increased attention is being placed on switchgrass management as an energy crop. Generally, the crop requires modest application of nitrogen fertilizer as it is not a heavy feeder. Typical nitrogen (N) content of senescent material in the fall is 0.5% N. Fertilizer nitrogen applications of about 5 kg N/hectare (ha) applied for each tonne of biomass removed is a general guideline. More specific recommendations for fertilization are available regionally in North America. Herbicides are not often used on switchgrass after the seeding year, as the crop is generally quite competitive with weeds. Most bioenergy conversion processes for switchgrass, including those for cellulosic ethanol and pellet fuel production, can generally accept some alternative species in the harvested biomass. Stands of switchgrass should be harvested no more than twice per year, and one cutting often provides as much biomass as two. Switchgrass can be harvested with the same field equipment used for hay production and it is well-suited to baling or bulk field harvesting. If its biology is properly taken into consideration, switchgrass can offer great potential as an energy crop. [8][11]

Uses

Switchgrass can be used as a biomass feedstock for energy production, as ground cover for soil conservation and to control erosion, for forages and grazing, and as game cover. It can be used by cattle farmers for hay and pasture and as a substitute for wheat straw in a diversity of applications, including livestock bedding, straw bale housing, and as a substrate for growing mushrooms.

Panicum virgatum 'Heavy Metal', an ornamental switchgrass, in early summer

Additionally, switchgrass is grown as a drought resistant ornamental grass in average to wet soils and in full sun to part shade.

Bioenergy

Switchgrass has been researched as a bioenergy crop since the mid-1980s, because it is a native perennial warm season grass that has the ability to produce moderate to high yields on marginal farmlands. It is now being considered for use in a diversity of bioenergy conversion processes, including cellulosic ethanol production, biogas, and direct combustion for thermal energy applications. The main agronomic advantages of switchgrass as a bioenergy crop are its stand longevity, drought and flooding tolerance, relatively low herbicide and fertilizer input requirements, ease of management, hardiness in poor soil and climate conditions, and widespread adaptability in temperate climates. In some warm humid southern zones such as Alabama it has the ability to produce up to 25 ODT/ha. A summary of switchgrass yields across 13 research trial sites in the United States found the top two cultivars in each trial to yield 9.4 to 22.9 t/ha, with an average yield of 14.6 ODT/ha.[12] However, these yields were recorded on small plot trials, and commercial field sites could be expected to be at least 20% lower than these results. In the United States, switchgrass yields appear to be highest in warm humid regions with long growing seasons such as the US Southeast and lowest in the dry short season areas of the Northern Great Plains.[12]

The energy inputs required to grow switchgrass are favorable when compared with annual seed bearing crops such as corn, soybean, or canola, which can require relatively high energy inputs for field operations, crop drying, and fertilization. Whole plant herbaceous perennial C4 grass feedstocks are desirable biomass energy feedstocks, as they require fewer fossil energy inputs to grow and effectively capture solar energy because of their C4 photosynthetic system and perennial nature. One study cites that it takes from 0.97 to 1.34 GJ to produce 1 tonne of switchgrass, compared with 1.99 to 2.66 GJ to produce 1 tonne of corn.[13] Another study found that switchgrass uses 0.8 GJ per oven dry tonne (ODT) of fossil energy compared to grain corn's 2.9 GJ/ODT.[14] Given that switchgrass contains approximately 18.8 GJ/ODT of biomass, the energy output-to-input ratio for the crop can be up to 20:1. [15] This highly favorable ratio is attributable to its relatively high energy output per hectare and low energy inputs for production.

Presently, a great deal of effort is being put into developing switchgrass as a cellulosic ethanol crop in the USA. In his 2006 State of the Union address, President George W. Bush proposed the usage of switchgrass for ethanol [16] [17][18]; since then, over $100 million has been invested into researching switchgrass as a potential biofuel source[19].Switchgrass has the potential to produce up to 100 gallons (380 liters) of ethanol per metric ton harvested.[20] However, current technology for herbaceous biomass conversion to ethanol is about 90 gallons (340 liters) per tonne.[21] In contrast, corn ethanol yields about 106 gallons (400 liters) per tonne.[22] The main advantage of using switchgrass over corn as an ethanol feedstock is that its cost of production is generally about 1/2 that of grain corn and more biomass energy per hectare can be captured in the field.[15] Thus, switchgrass cellulosic ethanol should give a higher yield of ethanol per hectare at lower cost. However, this will depend on cellulosic ethanol plant construction and ethanol processing costs being reduced considerably. The switchgrass ethanol industry energy balance is also considered to be substantially better than that of corn ethanol. During the bioconversion process, the lignin fraction of switchgrass can be burned to provide sufficient steam and electricity to operate the biorefinery. Studies have found, for example, that for every unit of energy input needed to create a biofuel from switchgrass, four units of energy are yielded.[23] In contrast, corn ethanol yields about 1.28 units of energy for each unit of energy input.[24] A recent study from the Great Plains [25] indicated that for ethanol production from switchgrass, this figure is 5.4, or alternatively, that 540% more energy was contained in the ethanol produced than was used in growing the switchgrass and converting it to liquid fuel. However, there remain commercialization barriers to the development of cellulosic ethanol technology. Projections in the early 1990s for commercialization of cellulosic ethanol by the year 2000 [26] have not been met. The commercialization of cellulosic ethanol is thus proving to be a significant challenge, despite noteworthy research efforts.

Thermal energy applications for switchgrass appear to be closer to near-term scale-up than cellulosic ethanol for industrial or small-scale applications. For example, switchgrass can be pressed into fuel pellets that are subsequently burned in pellet stoves used to heat homes (which typically burn corn or wood pellets). [9] Switchgrass has been widely tested as a suitable fuel for substituting for coal in power generation. The most widely studied project to date has been the Chariton Valley Project in Iowa [10]. As well, the Show-Me-Energy Cooperative (SMEC) in Missouri [11] is using switchgrass and other warm season grasses along with wood residues as feedstocks for pellets used for the firing of a coal-fired power plant. In Eastern Canada, switchgrass is being used on a pilot scale as a feedstock for commercial heating applications. Combustion studies have been undertaken and it appears to be well-suited as a commercial boiler fuel. Research is also being undertaken to develop switchgrass as a pellet fuel because of lack of surplus wood residues in Eastern Canada,[27] as a slowdown in the forest products industry in 2009 is now resulting in wood pellet shortages throughout Eastern North America. Generally speaking, the direct firing of switchgrass for thermal applications can provide the highest net energy gain and energy output-to-input ratio of all switchgrass bioconversion processes.[28] Research has found that switchgrass, when pelletized and used as a solid biofuel, is a highly efficient strategy for displacing fossil fuels. Switchgrass pellets were identified to have a 14.6:1 energy output to input ratio, which is substantially better than that for liquid biofuel options from farmland.[14] As a greenhouse gas mitigation strategy, switchgrass pellets were found to be a highly effective means to use farmland to mitigate greenhouse gases. Using farmland to produce switchgrass pellets could mitigate 7.6-13 tonnes per hectare of CO2. In contrast, switchgrass cellulosic ethanol and corn ethanol were found to mitigate 5.2 and 1.5 tonnes of CO2 per hectare, respectively.[29]

Historically, the major constraint to the development of grasses for thermal energy applications has been the difficulty associated with burning grasses in conventional boilers, as biomass quality problems can be of particular concern in combustion applications. These technical problems now appear to have been largely resolved through crop management practices such as fall mowing and spring harvesting that allow for leaching to occur, which leads to fewer aerosol forming compounds (such as K and Cl) and N in the grass. This reduces clinker formation and corrosion and enables switchgrass to be a clean combustion fuel source for use in smaller combustion appliances. Fall harvested grasses likely have more application for larger commercial and industrial boilers.[30][29]In regions where the potassium and chlorine contents of switchgrass cannot be successfully leached out for thermal applications, it may be that biogas applications for switchgrass will prove more promising. Switchgrass has demonstrated some promise in biogas research as an alternative feedstock to whole plant corn silage for biogas digesters.[31] Switchgrass is also currently being used to heat small industrial and farm buildings in Germany and China through a process used to make a low quality natural gas substitute.[2]

Soil conservation

Further information: [[:Mine reclamation, Restoration ecology, and Revegetation]]

Switchgrass is useful for soil conservation and amendment, particularly in the United States and Canada where switchgrass is endemic. Switchgrass has a deep fibrous root system – nearly as deep as the plant is tall. Since it, along with other native grasses and forbs, once covered the plains of the United States that are now the Corn Belt, the effects of the past switchgrass habitat have been beneficial, lending to the fertile farmland that exists today. The deep fibrous root systems of switchgrass left a very deep rich layer of organic matter in the soils of the Midwest, making those mollisol soils some of the most productive in the world. By returning switchgrass and other perennial prairie grasses as an agricultural crop, many marginal soils may benefit from increased levels of organic material, permeability, and fertility, due to the grass's deep root system.

Soil erosion, both from wind and water, is of great concern in regions where switchgrass grows. Due to its height, switchgrass can form an effective wind erosion barrier.[32] Its root system, also, is excellent for holding soil in place, which helps prevent erosion from flooding and runoff. Some highway departments (for example, KDOT) have used switchgrass in their seed mixes when re-establishing growth along roadways.[33] It can also be used on strip mine sites, dikes[32], and pond dams. Conservation districts in many parts of the United States use it to control erosion in grass waterways because of its excellent ability to anchor soils while also doubling as native habitat for wildlife.

Forages and grazing

Switchgrass is an excellent forage for cattle; however, it has shown toxicity in horses, sheep, and goats[34][35][36] through chemical compounds known as saponins, which cause photosensitivity and liver damage in these animals. Researchers are continuing to learn more about the specific conditions under which switchgrass causes harm to these species, but until more is discovered, it is recommended that switchgrass not be fed to them. For cattle, however, it can be fed as hay, or grazed.

Grazing switchgrass calls for watchful management practices to ensure survival of the stand. It is recommended that grazing begin when there is 18 – 22 inches of growth, to stop grazing when there are 8 – 12 inches of stubble left, and to rest the pasture 30 – 45 days between grazing periods[37]. Switchgrass becomes very stemmy and unpalatable as it matures, but during the target grazing period, it is a highly favorable forage with a relative feed value (RFV) of 90-104[38]. The grass' upright growth pattern places its growing point off the soil surface onto its stem, so leaving 8 – 12 inches of stubble is important for regrowth. When harvesting switchgrass for hay, the first cutting occurs at the late boot stage – around mid-June. This should allow for a second cutting in mid-August, leaving enough regrowth to survive the winter.[39]

Game cover

Switchgrass is well-known among wildlife conservationists as a favorite forage and habitat among upland game bird species such as pheasant, quail, grouse, wild turkey, and song birds, with its plentiful small seeds and tall cover. Depending on how thickly switchgrass is planted, and what it is partnered with, it also offers excellent forage and cover for a wide variety of other wildlife across the country. For those producers who have switchgrass stands on their farm, it is considered an environmental and aesthetic benefit due to the abundance of wildlife attracted by the switchgrass stands. Some members of Prairie Lands Bio-Products, Inc. in Iowa have even turned this benefit into a profitable business by leasing their switchgrass land for hunting during the proper seasons.[40] The benefits to wildlife can be extended even in large scale agriculture through the process of strip harvesting, as recommended by The Wildlife Society, which suggests that rather than harvesting an entire field at once, strip harvesting could be practiced so that the entire habitat is not removed, thereby protecting the wildlife that has inhabited the switchgrass.[41]

See also

References

  1. ^ Silzer, Tanya (2000). "Panicum virgatum L., Switchgrass, prairie switchgrass, tall panic grass". Rangeland Ecosystems & Plants Fact Sheets. University of Saskatchewan Department of Plant Sciences. Retrieved 2007-12-08. {{cite web}}: Unknown parameter |month= ignored (help)
  2. ^ a b c Secter, Bob. "Plentiful switch grass emerges as breakthrough biofuel". The San Diego Union-Tribune. Retrieved 2008-05-24. {{cite web}}: Text "The San Diego Union-Tribune" ignored (help)
  3. ^ Ernst Seed Catalog Web Page (2007). "Switchgrass and Warm Season Grass Planting Guide". Ernst Conservation Seeds. Retrieved 2007-12-08.
  4. ^ "PLANTS Profile for Panicum virgatum (switchgrass)". United States Department of Agriculture, Natural Resources Conservation Service. Retrieved 2008-05-21.
  5. ^ Ball, D.M. (2002). Southern Forages (3rd edition ed.). International Plant Nutrition Institute. p. 26. ISBN 0-9629598-3-9. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Establishing and Managing Switchgrass as an Energy Crop, Forage and Grazinglands, 2008.
  7. ^ Switchgrass as a Bioenergy Crop, ATTRA - National Sustainable Agricultural Information Service, 2006.
  8. ^ a b c [1] David J. Parrish, John H. Fike, David I. Bransby, Roger Samson. Establishing and Managing Switchgrass as an Energy Crop. Forage and Grazinglands. 2008.
  9. ^ a b Samson, R. (2007). "Switchgrass Production in Ontario: A Management Guide" (PDF). Resource Efficient Agricultural Production (REAP) - Canada. Retrieved 2008-05-24.
  10. ^ Bransby, David (2005). "Switchgrass Profile". Bioenergy Feedstock Information Network (BFIN), Oak Ridge National Laboratory. Retrieved 2008-05-24.
  11. ^ [ttp://www.reap-canada.com/online_library/feedstock_biomass/Optimization%20of%20switchgrass%20management%20for%20commercial%20fuel%20pellet%20production%20(Samson%20et%20al.,%202007).pdf] Samson, R., Bailey-Stamler, S., & Ho Lem, C. Optimization of Switchgrass Management for Commercial Fuel Pellet Production (Final report prepared by REAP-Canada for the Ontario Ministry of Food, Agriculture and Rural Affairs (OMAFRA) under the Alternative Renewable Fuels Fund). 2008.
  12. ^ a b McLaughlin, S.B., Kzos, L.A. Development of switchgrass (Panicum virgatum) as a bioenergy feedstock in the United States. Biomass and Bioenergy. Volume 28. 2005. 515-535.
  13. ^ [2], Dale, B., Kim, S. Cumulative Energy and Global Warming Impact from the Production of Biomass for Biobased Products. Journal of Industrial Ecology. Volume 7, Number 3-4. 2004. 147-162.
  14. ^ a b [3], Samson, R. et al. The Potential of C4 Perennial Grasses for Developing a Global BIOHEAT Industry. Critical Reviews in Plant Science. Volume 25. 2005. 461-495.
  15. ^ a b Samson, R. et al. Developing Energy Crops for Thermal Applications: Optimizing Fuel Quality, Energy Security and GHG Mitigation. In Biofuels, Solar and Wind as Renewable Energy Systems: Benefits and Risks. D. Pimental. (Ed.) Springer Science, Berlin, Germany. 2008. 395-423.
  16. ^ Adrienne Mand Lewin (2006-02-01). "Switchgrass: The Super Plant Savior?". ABC News.
  17. ^ "Switch Grass: Alternative Energy Source?". National Public Radio. 2006-02-01.
  18. ^ Dana Bash, Suzanne Malveaux; et al. (2006-02-01). "Bush has plan to end oil 'addiction'". CNN. {{cite web}}: Explicit use of et al. in: |author= (help)
  19. ^ "Since that mention in the 2006, investment in switch grass has exploded, thanks in large part, experts say, to the president's speech. Venture capitalists have poured over $100 million dollars into private companies that are exploring the technology necessary to convert switchgrass into fuel, and large publicly-owned companies are also directing their research dollars into biofuels."Jessica Yellin, Katie Hinman, Nitya Venkataraman (2007-01-23). "What Happened to Bush Call for Switchgrass?". ABC News.{{cite web}}: CS1 maint: multiple names: authors list (link)
  20. ^ "Switchgrass: Native American Powerhouse?". Renewable Energy Resources. Retrieved 2007-01-05.
  21. ^ [4] Iogen Corporation, 2009.
  22. ^ Farrell, A.E. Ethanol Can Contribute to Energy and Environmental Goals. Science. Volume 311. 2006. 506-508.
  23. ^ "Switch Grass: Alternative Energy Source?". NPR. Retrieved 2007-01-05.
  24. ^ Wang, M., Wu, M., & Huo, H. Life-cycle energy and greenhouse gas emission impacts of different corn ethanol plant types. Environmental Research Letters. Volume 2. 2007. 1-13.
  25. ^ Schmer, M. R., Vogel, K. P, Mitchell, R. B., and Perrin, R. K. Net energy of cellulosic ethanol from switchgrass. PNAS. 105. 2008. 464-469.
  26. ^ Lynd, L. R., Cushman, J. H., Nichols, R. J., and Wyman, C. E. Fuel Ethanol from Cellulosic Biomass. Science. Volume 251, Number 4999. 1991. 1318 – 1323.
  27. ^ Bailey Stamler, S., R. Samson and C. Ho Lem. Biomass resources options: Creating a BIOHEAT supply for the Canadian greenhouse industry. Final report to Natural Resources Canada, Ottawa. 2006. 38 pages.
  28. ^ [5] Samson, R., and S. Bailey Stamler. Going Green for Less: Cost-Effective Alternative Energy Sources. C.D. Howe Institute Commentary 282. Economic Growth and Innovation. 2009. 25 pages.
  29. ^ a b [6] Samson, R., Bailey-Stamler, S., & Ho Lem, C. Optimization of Switchgrass Management for Commercial Fuel Pellet Production (Final report prepared by REAP-Canada for the Ontario Ministry of Food, Agriculture and Rural Affairs (OMAFRA) under the Alternative Renewable Fuels Fund). 2008.
  30. ^ [7] R. Samson, S. Bailey and C. Ho Lem. The Emerging Agro-Pellet Industry in Canada. 2007.
  31. ^ [8] J.C. Frigon, P. Mehta, S.R. Guiot. The bioenergy potential from the anaerobic digestion of switchgrass and other energy crops. National Research Council Canada. Growing the Margins Conference: Energy, Bioproducts and Byproducts from farms and food sectors. April 2-5 2008, London, Ontario.
  32. ^ a b "Plant Fact Sheet, Panicum virgatum (switchgrass)" (PDF). United States Department of Agriculture, Natural Resources Conservation Service, Plant Materials Program. 2006-05-06. Retrieved 2008-05-21.
  33. ^ "KDOT Bid Tabs". Kansas Department of Transportation. Retrieved 2008-05-20.
  34. ^ Lee, S.T. (2001). "The isolation and identification of steroidal sapogenins in switchgrass". J Nat Toxins. 10 (4): 273–81. Retrieved 2008-05-24. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  35. ^ Johnson, A.L. (2006). "Fall Panicum (Panicum dichotomiflorum) Hepatotoxicosis in Horses and Sheep". Journal of Veterinary Internal Medicine. 20 (6): 1414–1421. doi:10.1892/0891-6640(2006)20[1414:FPPDHI]2.0.CO;2. Retrieved 2008-05-24. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  36. ^ Stegelmeier, B.L. (2007). "Switchgrass (panicum Virgatum) Toxicity In Rodents, Sheep, Goats And Horses". Poisonous Plant Global Research and Solutions. 19: 113–117. Retrieved 2008-05-24. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  37. ^ Ball, D.M. (2006). "Table 28. Guidelines for rotational stocking of selected forage crops". Forage Crop Pocket Guide. International Plant Nutrition Institute. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  38. ^ Ball, D.M. (2006). "Table 33b. Total Digestible Nutrients (TDN) and Relative Feed Value (RFV) Ranges for Various Forage Crops". Forage Crop Pocket Guide. International Plant Nutrition Institute. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. ^ Wolf, D.D. (1995). "Planting and managing switchgrass for forage, wildlife, and conservation" (PDF). Virginia Cooperative Extension Publication: 418–013. Retrieved 2008-05-24. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  40. ^ Hipple, Patricia C. (2002). "Farmers' Motivations for Adoption of Switchgrass" (PDF). In Jules Janick, Anna Whipkey (ed.). Trends in New Crops and New Uses. Fifth National Symposium, New Crops and New Uses, Strength in Diversity. Alexandria, Virginia: American Society for Horticultural Science. pp. 252–266. ISBN 0097075655. Retrieved 2008-05-23. {{cite conference}}: Cite has empty unknown parameter: |month= (help); External link in |conferenceurl= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |conferenceurl= ignored (|conference-url= suggested) (help)
  41. ^ Bies, Laura (2006-11-01). "The Biofuels Explosion: Is Green Energy Good for Wildlife?". Wildlife Society Bulletin. 34 (4): 1203–1205. doi:10.2193/0091-7648(2006)34[1203:TBEIGE]2.0.CO;2. {{cite journal}}: |access-date= requires |url= (help)
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