By Tim Longwell, Nick Miller, and Melissa Schreiweis
Many farmers, livestock producers and other landowners have areas of their property that are not appropriate for traditional management practices. These marginal lands have poor soil quality, prohibitive adverse slopes or hydrologic conditions which exclude them, either economically or environmentally, from intensive production. Thus these areas are either mismanaged or abandoned giving little long-term benefits to the land ownership. The best land use for these areas is usually forest resource production.
When converting marginal lands to either woodlots or forest plantations, tree species selection based on site conditions and market value are critical in assuring economic viability. Although forestry is less management intensive than many other crops, certain activities are needed to maximize both survival and growth of the trees. Grasses, weeds and shrubs can drastically decrease the establishment and future success of the tree resource. These existing shrubs, weeds and grasses are typically pioneer or early succession species that compete fiercely for existing nutrient and water resources. Tree survival can be decreased drastically in the initial establishment of seedlings and growth can be significantly reduced in young and mature forests due to competition with other vegetation.
Additionally, these "weed" species are typically small in diameter and thus dry quickly during drought creating hazardous "flashy" fuels in wildfire situations. Often young plantations and mature tree quality is destroyed or reduced due to forest fires. The effective control of this undesired vegetation is needed if growth potential and quality risks are to be optimized.
Traditional weed management includes mechanical control either with manual cutting or mowing and herbicide application. Both involve labor and cost expenditure with no direct economic returns or negative environmental consequences.
An attractive alternative is the managed grazing of this vegetation with livestock, such as sheep, to provide not only the desired weed reduction but also income through out the rotation of the forest resources. This agroforestry system produces multiple products including timber, fruits, and animal products from trees, forage and livestock. The markets for these products are usually well established, resulting in less economic risk. The cost of production is reduced since all three products are produced on the same land and the income from the livestock and the selected trees is relatively constant. The property value of the land itself may be increased due to the provided windbreak and long-term resource improvements.
The benefits to the livestock, trees, and pasture increase the profitability of the agroforestry practice. The grazing of the livestock throughout the woodlot or forest plantation can effectively control the competition between the trees and the undergrowth of moisture, nutrients, and sunlight. An example of this is the control of the exotic, invasive kudzu (Pueria lobata) in the southeastern United States. Kudzu is a vine that dominates the undergrowth and wraps around the trees, causing deformed tree boles. Additionally, it competes fiercely for many of the nutrients and water in the soil. To control the kudzu, goats were successfully introduced to allowing the increased growth and quality of the tree resources.
Control of weedy species with livestock can be implemented without the use of herbicides. This quality of agroforestry makes the practice attractive form an environmental standpoint, as well as more cost efficient. The trees can utilize the fertilizer, which is applied to the forage crops and manure from the livestock can also supply nutrients to the trees and forage such as nitrogen, phosphorus, potassium, and sulfur.
Another incentive for using the silvopastural system is the benefits that the trees give to the livestock and to the forage pasture. The performance of the livestock herd can be greatly improved due to the shelter the trees provide. The trees shelter the livestock from extreme temperatures and can reduce direct cold by 50% and wind velocity by 70%. This, in turn, lowers the energy required for maintenance and increases animal performance.
The trees also shelter the forage that grows underneath the forest canopy cover. These forages mature more slowly, so there will be less lignin and cellulose accumulation in the forage, making it more digestible than forage grown in an open field. Forage plants that are most appropriate for this management system are those that are tolerant to drought, and require less light. Trees can also increase the amount of snow retained during the winter and thus add moisture to the soil as the snow melts.
This agroforestry system also protects the land from erosion, compaction, and fire. Less soil erosion will occur in the forage patures grown on hillsides because the trees decrease the velocity of the raindrops that, in an open field, break the soil into many tiny particles, increasing erosion. Livestock are less likely to congregate underneath only one tree, which would cause compaction. Instead, the livestock can disperse themselves among the extensive forest cover. The control of weeds and undergrowth controls the potential fire hazards due to the elimination of fine fuels.
Environmental and aesthetic benefits include the diversity of plant and animal species that the system promotes. There are fewer concerns about the quality of water, odors, noise, and disease problems since most of the feces are incorporated rapidly into the environment as opposed to the feedlot operations. The silvopastural system is also more aesthetically appealing to the public and to the landowner.
Even though there are many advantages of using a silvopastural system, there are some disadvantages if the system is not properly implemented. These disadvantages, including soil disturbance, soil erosion, and damage to trees may be overcome, but, nevertheless, they are important. The potential for tree damage is high if the sheep are kept in the same pasture throughout the entire grazing season. Tree damage is defined as browsing the trees, which may result in decreased growth or malformation. There is always going to be some damage done to trees, but no more than would normally be caused by deer in the wild. Providing more attractive forage can reduce tree browsing and older and more experienced the sheep may also be used, causing less damage to trees. For example, an ewe with a new lamb is more prone to do damage to the trees than a dry ewe.
The maintenance of the sheep should be a consideration in the use silvopastoral systems. These management considerations can help to maximize the profit and reduce the environmental effect to both the sheep and the land.
As in all sheep production systems you, minerals should be available to the flock. Ewes short of essential minerals will strip bark from trees to satisfy their cravings, causing a detrimental effect on the health of the tree. Sheep have a varied diet and many times relish the forbs that are causing problems for the timber stand. Along with beneficial scavenging of weeds sheep can also browse the leaves and twigs of trees. This could be a concern if the sheep are ingesting a toxic plant or are causing tree form damage and thus decreasing the future value of the tree. If sheep are allowed to browse young trees they may consume the terminal leaders or upper branches causing undesirable growth patterns. This could lead to severe malformation or death. Larger trees where the crown is out of reach of the animals are not at risk of such damage. It has been found that younger lambs are more apt to damage trees than older stock. If a producer decides to graze in younger timber stands the use of tree shelters or animal repellents is recommended. Many products exist to provide protection to the tree while allowing the sheep to graze around it.
A producer interested in using sheep in this management system may want to consider some form of rotational grazing. By rotating the livestock you do not allow them to inflict severe grazing damage or soil compaction. Soil compaction is a major concern because it ruins the soil structure, causing reduced water and air infiltration and increasing water surface runoff. Proper animal management and rotational grazing can reduce these concerns.
If these management practices are correctly carried out, the rewards can be innumerable. The land is not only more economical, but the environmental and aesthetical benefits attained from using this practice are immeasurable.
Silvopastoral Web Page Links
Intercropping Black Walnut in Oregon's Willamette Valley
Oklahoma Projects Combine Timber Products with Cattle Grazing
Silvopasture: An Agroforestry Practice
Silvopasture Design with Animals in Mind
Silvopasture Systems Demonstrated by Agroforest Wisconsin
The Biology of Silvopastoralism
Trees and Pastures: 40 Years of Agrosilvopastoral Experience in Western Oregon