There is research being conducted on perennial polycultures at The Land Institute and The Rodale Institute.30 The Land Institute, for example, is experimenting with polycultures involving warm-season grasses (such as eastern gamagrass), cool-season grasses (such as mammoth wild rye), legumes (such as Illinois bundleflower and wild senna), and composites (such as Maximilian sunflower) all growing together in mixed intercropping.
To answer this question more specifically, it is useful to replace it with four questions that The Land Institute asks itself:
• Can perennialism and increased seed yield go together?
• Can a polyculture of species overyield a monoculture?
• Can perennial polyculture sponsor all its nitrogen needs?
• Can perennial species adequately manage pests?
Laura Jackson at the University of Northern Iowa identified a mutant strain of eastern gamagrass whose seed production is four times greater than normal—without any corresponding loss of root mass or vigor.31 That alone should be sufficient evidence of a yes answer to the first question. There is ample evidence beyond that, however, that seed yields of perennials can be improved without significant loss of root mass. And most of this research has not involved direct genetic engineering of perennial plants—a potential route to further increases in yield.
Yields of a single perennial species can approach those of annual monocultures (70 percent or more). However, more important, can a perennial polyculture "overyield" an annual monoculture in the sense of producing more food per plot of land than does the annual mono
28 See The Land Institute general Web site and, specifically, "About Us: Introduction and Mission." As of April 13, 2007: http://www.landinstitute.org.
29 D. Pimentel, C. Wilson, C. McCullum, R. Huang, P. Dwen, J. Flack, Q. Tran, T. Saltman, and B, Cliff, "Economic and Environmental Benefits of Biodiversity," BioScience, Vol. 47, No. 11, December 1997, p. 755.
31 See Scott Russell Sanders, "Learning from the Prairie," The Witness, Vol. 84, No. 5, May 2001.
culture? Overyield in polycultures is common. One study concluded that the yield advantage of intercropping (interspersing rows of different crops) was small (5 to 15 percent) but consistent.32 The traditional corn/beans/squash polyculture of Mexico produces overyields as high as 50 percent,33 and other studies have found overyields as high as 150 percent.34
The remaining question, then, is whether an overyield is possible with a perennial polyculture. There is increasing evidence that it is. For example, The Land Institute has obtained a 19 percent overyield with a mixture of eastern gamagrass and Illinois bundleflower, and in a three-species mixture of eastern gamagrass, Illinois bundleflower, and the cool season (C3) mammoth wild rye, the institute observed a 26 percent overyield.35
There is prima facie evidence from ancient prairies that perennial polycultures can sponsor all their nitrogen needs. The more important question is whether a specific set of perennials can produce enough nitrogen to both yield a crop and maintain its nitrogen level. Here the evidence is more indirect, but there is experimental evidence, for example, that a plant like Illinois bundleflower can improve available soil nitrogen status over several years, despite having its seeds removed annually (see footnote 35).
There is ample evidence that plant mixtures aid pest control. As one source puts it:
There are two schools of thought on why this occurs. One suggests that higher natural enemy populations persist in diverse mixtures due to more continuous food sources (nectar, pollen, and prey) and favorable habitat. The other thought is that pest insects that feed on only one type of plant have greater opportunity to feed, move around in, and breed in pure crop stands because their resources are more concentrated than they would be in a crop mixture. Regardless of which reason you accept, the crops growing together in the mixture complement one another, resulting in lower pest levels. Intercropping also aids pest control efforts by reducing the ability of the pest insects to recognize their host plants. For example, thrips and white flies are attracted to green plants with a brown (soil) background, ignoring areas where vegetation cover is complete.36
32 R. W. Snaydon and P. M. Harris, "Interactions Below Ground—The Use of Nutrients and Water," in Proceedings of the International Workshop on Intercropping, R. Willey, ed., International Crops Research Institute for the Semi-Arid Tropics, Patancheru, Andhra Pradesh, India, 1981, pp. 188-201 (referenced in Geno and Geno, 2001).
33 S. R. Gliessman, "Sustainable Agriculture: An Agroecological Perspective," in John Andrews and Inez Tom-merup, eds., Advances in Plant Pathology, Vol. 11, Academic Press, London, 1995, pp. 45-57 (referenced in Geno and Geno, 2001).
34 M. Liebman, "Polyculture Cropping Systems," in Agroecology: The Science of Sustainable Agriculture, M. A. Altieri, ed., Intermediate Technology Publications, London, 1995, pp. 205-218 (referenced in Geno and Geno, 2001).
35 Jon Piper, "Natural Systems Agricultural Research," in M. Hackett and S. H. Sohmer, eds., Proceedings of the Ecology of Our Landscape: The Botany of Where We Live, Exploring the Interfaces Between Plants, People, and the Environment, The Botanical Research Institute of Texas and Texas Christian University, Fort Worth, Tex., 1996. As of April 13, 2007: http://www.landinstitute.org/vnews/display.v/ART/2001/03/10/37825ab33.
36 Preston Sullivan, Intercropping Principles and Production Practices, ATTRA Publication #IP135, National Sustainable Agriculture Information Service, Fayetteville, Ark., 2003. As of April 13, 2007: http://www.attra.org/attra-pub/intercrop.html.
This is not exactly the same as saying that perennial polycultures can adequately manage their pests, but the principles are the same, and there is every reason to expect that perennial polycultures will require less pesticide than do annual monocultures.37
These are not the only questions that can be asked about the feasibility of perennial polycultures, but they are the main questions. Even partial "yes" answers to all four are strong evidence that the potential of perennial polycultures is worth pursuing.
Given that perennial polycultures might be a feasible alternative to monocultures from an agricultural standpoint, the two most important follow-on questions would be about the food value of perennial polyculture species and their overall economic viability. With respect to food value, again there is preliminary evidence that species being tested in perennial polycultures could compete with monoculture foods. A study of eastern gamagrass, for example, concluded:
As a food source, the nutritional value of gamagrass grain is impressive. The protein content of the grain is 27% while that of wheat and corn is about 17 and 10%, respectively. Gamagrass grain also has twice as much of the amino acid methionine as corn and is about 51% carbohydrate. Gamagrass is readily digestible and, in addition, tastes good. It has a distinctive corn-nutty flavor when popped or ground into flour.38
In addition, one of the most promising nitrogen-fixing legumes, Illinois bundleflower, while a favorite food of grazing animals, is also a possibility for human consumption:
The seed of Illinois bundleflower is high in protein: 38% on a dry weight basis. For comparison, soybeans are 40% protein. To test for protein digestibility, we sent cooked and uncooked seed to the University of Nebraska's Food Protein Research Group. Protein from uncooked seed was 69% digestible; protein from seed boiled for 60 minutes was 83% digestible. An estimate of the degree to which protein is utilized in the human body is called the computed Protein Efficiency Ratio (c-PER); Illinois bundleflower's c-PER is close to that of cooked oats, at 1.8.39
The economic viability of perennial polycultures is more difficult to gauge at this point. Since the concept of perennial polycultures is still in its exploratory phase, overall economic comparability with annual monocultures is hard to judge. If the savings in energy and labor with perennial polycultures can be realized and issues related to harvesting can be solved expe-
37 The general ability of mixed plants to manage pests better than monocultures is widely acknowledged in the agricultural world. See, for example, Nadia El-Hage Scialabba and Douglas Williamson, The Scope of Organic Agriculture, Sustainable Forest Management and Ecoforestry in Protected Area Management, Environment and Natural Resources Working Paper No. 18, Food and Agriculture Organization of the United Nations, Rome, Italy, 2004. As of April 13, 2007: http://www.fao.org/docrep/007/y5558e/y5558e00.htm.
38 Patricia R. Boehner, "Eastern Gamagrass: In Transition from Forage to Food Crop," The Land Report, Vol. 31, 1987, pp. 8-12.
39 J. Vail, P. Kulakow, and L. Benson, "Illinois Bundleflower: Prospects for a Perennial Seed Crop, in D. D. Smith and C. A. Jacobs, eds., Proceedings of the Twelfth North American Prairie Conference: Recapturing a Vanishing Heritage, University of Northern Iowa, Cedar Falls, 1992, pp. 31-32.
ditiously, there is reason to believe that perennial polycultures could be more cost-effective than annual monocultures. Conversely, if management of perennial polycultures is more complicated than with annual monocultures, for example, the hoped-for savings could disappear. The one current attempt related to the costs of perennial polycultures is the Sunshine Farm Research Program of The Land Institute. According to the institute:
The Land Institute is using the prairie as a model for the Sunshine Farm. The main goal of the project is to conduct year-round accounting of energy, materials, and labor on the farm. The aims are to examine whether the Sunshine Farm can provide its fuel and fertility, and to determine how much industrial energy society must provide from sunlight to manufacture the farm facilities, equipment, and inputs. Prairies are characterized by species diversity, perennial plants, energy flows based on sunlight, and internal control of fertility and pest damage. Hence, the Sunshine Farm contains renewable energy technologies and innovative farming practices applied to conventional crops and animals. Import of nutrients is minimal, and some candidates are included from The Land Institute's natural systems agriculture research.40
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