Silvopastoral Agroforestry Using Honeylocust
Honeylocust (Gleditsia triacanthos L.) is a pod bearing, leguminous tree native to the central United States and now grown in much of the temperate zone worldwide. Honeylocust is used extensively as an urban shade tree and occasionally in windbreaks and shelterbelts and has potential as a silvopastural agroforestry fodder tree.
Honeylocust produce pods and seeds high in carbohydrates and protein that provide animal feed during autumn and winter when pasture grass production declines. Livestock may harvest pods directly from under the trees, minimizing harvesting and processing costs.
Research on various aspects of honeylocust agroforestry are currently underway in five temperate zone countries. Following a brief historical review, this paper examines the highlights of this research. More extensive accounts of silvopastural honeylocust are available for the interested reader (Gold and Hanover, 1993; Scanlon, 1980; Wilson, 1994).
Early Honeylocust Planting and Research
George Washington and Thomas Jefferson shared an interest in honeylocust. While attending the Continental Congress in 1774 in Philadelphia, Washington wrote to his Mt. Vernon foreman William Pearce, "I shall send you by the first Vessel at least a bushel and half of clean honey locust seed which I would have raised in a nursery for the purpose of hedging." (Ditwiler, 1947) Jefferson planted 65 honeylocust as an ornamental allee along the east carriage lane at his home, Monticello. It's probable that Jefferson was the first to plant honeylocust as an ornamental shade tree. New trees have been planted in the original locations and can be viewed on a tour of the plantation.
In 1929 J. Russell Smith published his prophetic book, Tree Crops: A Permanent Agriculture, promoting trees, including honeylocust, as an answer to conventional agriculture on erodible soils (Smith, 1950). In the 1930s the Tennessee Valley Authority(TVA) initiated extensive tree crops projects. The TVA established clonal orchards of 90 tree at three southern universities, and conducted pod nutrition analysis and feeding trials with cattle. After World War II these projects were abandoned.
In the 1980s the Tennessee Valley Authority distributed grafted `Millwood' cultivars to several private growers, and two of these honeylocust orchards are still intact in Tennessee and Arkansas. At Michigan State University, Michael Gold established two provenance, half-sib progeny honeylocust plantings containing 250 sources (Gold and Hanover, 1993).
Silvopastural Honeylocust with Sheep
Honeylocust has several characteristics recommending it as a silvopastural species. Its open canopy produces a light shade, minimizing possible negative effect on summer grass production. Casual observations of field workers suggest that pasture grasses and legumes do well under honeylocust, growing right up to the trunk of the tree. Late spring leaf‑out and early leaf drop in fall reduce shading during these seasons. In addition, the tree's small leaflets are easily absorbed into pasture grasses during autumn leafdrop and provide an additional source of fertilizer. Pod yields averaging 43kg per tree have been recorded. Honeylocust is dioecious and planting staminate trees (one per 15-20 pistillate trees) is recommended.
Because sheep, in contrast to cattle and swine, can digest between 70% and 90% of honeylocust seed, they fit best with silvopastoral honeylocust. Trees require less protection when established in sheep pastures. The sheep reproduction cycle also coincides with pod production: pods can be fed appropriately in autumn to flushing ewes or weaned lambs. For cattle and swine to receive full nutrient value, pods must be machine-harvested and processed.
Using plastic tree protectors and/or electric fences, honeylocust orchards can be established in operating pastures and hayfields, allowing cash flows from livestock sales to continue while the trees mature.
Pods have a nutritional value between that of oats and barley, depending on the cultivar, growing culture, and location. Selected cultivars contain 10% crude protein and 32% total sugars (Scanlon, 1980). Economic evaluations of silvopastoral honeylocust indicate internal rates of return of 9% to 25%, depending on a variety of cost and production assumptions (Wilson, 1994). Although difficult to quantify, additional benefits include reduction of water runoff and topsoil erosion, shade for livestock, a productive pollen and nectar source for bees, a more diversified and aesthetically pleasing pasture environment, and timber upon project completion.
Research on honeylocust agroforestry is being preformed by Christian Dupraz and his coworkers at the plant breeding station of the National Institute for Agronomic Research (INRA) near Montpellier, France. Projects include: clonal selection for high pod yield, pod production from clonal honeylocust orchards, and carefully controlled pod feeding trials with sheep (Dupraz, 1987; Papanastasis et. al., 1999).
Based on high pod yields of parent trees found in southern France, sixteen honeylocust cultivars were selected and are being evaluated in five clonal orchards in France and Greece. Scionwood from the best of these cultivars will soon be released commercially.
In pod feeding trials, sheep fed a daily ration of 1.4kg of whole pods gained 135g/day (1993 trials) and 178g/day (1994). (Papanastasis et. al., 1999). Dupraz concludes that "Gleditsia pods may therefore cover not only maintenance requirements but also production needs." (Dupraz, C. and S.M. Newman. 1997).
He prefers feeding whole pods to ground pods because of greater digestibility due to longer retention time, as well as avoiding of the cost of processing pods. Dupraz calculated a seed budget for each cultivar and animal based on seeds fed per day per animal and deduced in vivo digestibility of 55%. (Papanastasis et. al., 1999).
The Department of Forestry at Virginia Polytechnical University, in collaboration with the Agricultural Research Service-USDA, has established two honeylocust plantings: one hectare of seedling honeylocust will be used to test understory light and grass interactions; a second hectare will test grafted `Millwood' honeylocust. Both plantings include several spacing and tree protection replications, and will be grazed by sheep.
A breeding and selection program cross pollinating four US honeylocust cultivars is being conducted by Jerry Cottingham in Texas. Mike Stilwell at Rhody Ranch Nursery in Virginia is experimenting with the propagation using hardwood and softwood cuttings (personal correspondence). Grafted honeylocust of several cultivars are available from Hidden Springs Nursery in Tennessee.
Nitrogen Fixation in Honeylocust. Nitrogen fixation by symbiotic bacteria is assumed to take place in root nodules of the Leguminosae family. (Allen and Allen, 1981) Honeylocust does not display nodules and was not believed to have bacterial nitrogen fixation. This conventional wisdom has recently been challenged by James Bryan and his colleagues at the Yale University Department of Forestry (Bryan, 1996). Working mainly with honeylocust, Bryan has found bacterial nitrogen fixation directly in the roots of non-nodulating Leguminosae, evidence that non-nodulating leguminous species do form a symbiotic relation with soil bacteria.
Bryan tested for bacterial activity in non-nodulating species with an electronic scanning microscope and ethylene evolution using acetylene reduction as an indication of nitrogenase activity. Bryan hypothesizes an evolutionary continuum between precursor non-nodulating, 'primitive' Leguminosae (including the genus Gleditsia) and the more sophisticated nodulating species. (Bryan, 1996)
Honeylocust leads a double life, as a thorny, evasive and usually unwanted species, and as an agroforestry species with considerable potential. Honeylocust naturally produce thorns (up to 45 cm) dangerous to livestock and tractor tires. Thornless honeylocust can be produced by grafting scionwood taken from the thornless upper branches of the desired cultivars. Nevertheless, livestock and wildlife defecating the undigested seeds in pastures will produce unwanted thorny seedlings. Seedlings from thornless grafted trees will usually present thorns and have to be controlled mechanically or with herbicide.
Further research is needed on pod production, nutrition, and cultivar performance in different growing conditions. Cultivar selection should be keyed to reduction of biennial pod bearing and seeds digestibility. Research on fertilizer combinations, alternate tree spacing patterns, and pruning regimes is also needed.
Bryan, J. A., G. P. Berllyn and J. C. Gordon. 1996. Toward a new concept of the evolution of symbiotic nitrogen fixation in the Leguminosae. Plant and Soil. 186:151-159.]
Detwiler, S.B. 1947. Notes on honeylocust. USDA, Soil Conservation Service (mimeo).
Dupraz, C. 1987. Un aliment concentre pour l'hiver, les gousses do Gleditsia triacanthos L. Communication presentec aux Jemes rencontres du Sous-Reseau Mediterraneen FEO pour le paturage et la production fourragere. FAO sous reseau Mediterraneenne pour le paturage et la production fourragere.
Dupraz, C. and S.M. Newman. 1997. Temperate agroforestry: the european way. In: A.M.Gordon and S.M.Newman. Temperate agroforestry systems. CAB International. Wellingford, U.K.
Gold, M.A. and Hanover, J. W. 1993. Honeylocust(Gleditsia triacanthos L.): Multipurpose tree for the temperate zone. International Tree Crops Journal 7(4):189-207.
Papanastasis, V.P., C.N.Tsiouvaras, O.Dini-papanastasi, T.Vaitsis,L.Stringi, C.F.Cereti, C.Dupraz, D.Armand, Meuret, and L.Olea. 1999. Selection and utilization of cultivated fodder trees and shrubs in the Mediterranean Region. Instituto Agronomic Mediterraneo de Zaragoza, Zaragoza, Spain.
Scanlon, D.H. 1980. A Case Study of Honeylocust in the Tennessee Valley Region. In Tree Crops for Energy Co‑Production on Farms. U.S. Solar Energy Research Institute. Golden, Colorado.
Smith, J.R. 1929. Tree Crops: A Permanent Agriculture. Harcourt, Brace and Co. New York, NY.
Wilson, A.A. 1994. Silvopastoral Agroforestry Using Honeylocust. Schultz, R.C. and J.P.Colletti (eds.) Opportunities for Agroforestry in the Temperate Zone Worldwide. Proceedings of the Third North American Temperate Agroforestry Conference. August 15-18. Ames, Iowa. pp.265-269.
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