| Nitrogen fixing trees for fodder production - A Field Manual |
James M. Roshetko and Ross C. Gutteridge
Livestock play an important role in small-scale farming systems throughout the world. They provide traction to plow fields, manure to fertilize crops, and food products for human consumption. Most often livestock graze fallow fields, pastures and woodlands deriving most of their sustenance from crop residue, grasses and other herbaceous plants. A smaller but important component of livestock diets comes from tree fodder. Farmers harvest tree fodder from natural forests, savanna and woodlots. Additionally, they often deliberately propagate trees on their farms to expand fodder resources.
Many of the most important fodder trees are nitrogen fixing species. Through a symbiotic relationship with Rhizobium soil bacteria these species are able to fix atmospheric nitrogen into a form they can use for growth. This ability enables nitrogen fixing trees (NFTs) to tolerate infertile sites and produce protein-rich fodder without high inputs of artificial N fertilizer. Nitrogen fixation is discussed in more detail in Chapter 2.
There are a number of characteristics that make NFT species particularly valuable for fodder. These include:
• the ability to fix atmospheric nitrogen
• adaptability to poor and difficult sites
• fast growth
• the ability to compete with other vegetation
• multiple uses and complementary tree architecture
• productivity under repeated harvesting
• high nutritive value and acceptability to animals
• easy propagation
In order for a NFT to be productive it must have the ability to survive and compete in the climatic and soil conditions of the site in question. Humid zone species are not productive in arid environments, lowland species are not productive in alpine regions. When propagating NFTs, the site's environmental conditions should be matched with the environmental requirements of potential species. Elevation, mean annual precipitation, length of annual dry season, mean annual temperature and soil characteristics are of particular importance. Tree selection for specific sites is discussed in Chapter 3. Many NFTs grow well on infertile or difficult sites. This ability is valuable as most fertile soils are used for food crop production, leaving marginal or poor soils for fodder tree production. NFTs help improve the sites when their nitrogen-rich leaves and branches senesce, fall to the ground as litter and are incorporated into the soil.
Seed germination and seedling growth of fodder trees should be relatively fast. This will provide a rapid supply of livestock feed and enable trees to compete with other vegetation. Competition between tree seedlings and pre-existing grasses can be intense. Normally, several months of weed control is necessary to assure successful fodder tree establishment. Fodder tree propagation and establishment methods are discussed in Chapter 2. In some situations tree species can be excessively competitive; do not allow these trees to dominate or eliminate useful companion plants. Before introducing a new species to an area establish a small evaluation trial to determine its potential. If the trees show signs of becoming weedy, early prolific seeding or root sprouts, remove them before these problems occur.
Most nitrogen fixing fodder trees provide multiple products and services. They are planted on contours to stabilize soil, on boundaries as living fences, with grasses and shrubs to form multiple layer fodder banks, or scattered around the farm where site characteristics are too poor to support food crops. In all of these systems, fodder trees may also provide fuelwood, poles, timber, fruit or other products for home consumption and sale in local markets. Simultaneously, fodder trees may decrease erosion, rejuvenate soil, or serve as ornamentals. Many NFTs have deep roots that utilize nutrients in the subsoil below the rhizophere of companion crops. Through leaf and branch litter, these trees recycle nutrients to the soil surface for use by other plants. Conversely, trees with shallow roots may compete heavily with other plants. Such trees are inappropriate components of most agroforestry or intercropping systems. Some NFTs have wide spreading crowns which cast light shade. These trees improve the micro climate for livestock, pasture grasses and shade loving crops. NFTs with vertical crowns cast little shade and are ideal companion plants for crops which need full sunlight. Always choose a fodder tree species that provides products and services that are complementary to the landowner's objectives and management system. Detailed information on various fodder production systems is provided in Chapter 4.
The main purpose of establishing nitrogen fixing fodder trees is to produce reliable quantities of livestock feed. To achieve this goal, trees must be able to with stand repeated defoliation by cutting or grazing. Neither tree growth nor survival should be adversely affected by an intensive management regime. Vigorous resprouting and high foliage productivity under repeated harvesting over many years is a trait required of fodder trees. Such long-lived hardy trees will enhance the sustainability of a livestock production system. A species intolerant of repeated harvesting is useless as a reliable fodder source.
Generally, nitrogen fixing fodder trees have a high nitrogen/protein content in their foliage. This characteristic is significant because protein is often the nutrient that limits livestock performance. During dry seasons and droughts, grasses and other herbaceous feeds desiccate and suffer a decrease in protein and nutrient value. Trees have deep root systems that allow tree fodder to remain green and nutritious during dry periods when it can be utilized as a supplement to improve the digestibility and nutritive value of dry feeds. Besides high nutrient content, tree fodders often contain levels of anti-nutritive compounds that assist digestion without harming the animal. More information on the nutritive value of tree fodders is found in Chapter 5. The selected fodder tree should also be acceptable to the livestock being raised. Large quantities of some fodders can be safely consumed only by ruminants (cattle, buffalo, goats, sheep, etc.). Other fodders have offensive smells or textures. Livestock must "learn" to eat these fodders before the species can be fully utilized. Before introducing a new species to an area, test local livestock's acceptance of the new fodder.
A final characteristic required of fodder trees is ease of propagation. A species can not furnish reliable quantities of fodder if establishment is difficult or prolonged. Fodder trees must produce adequate sup plies of viable seed, or be easily propagated by vegetative methods. Vegetative propagation is time consuming when establishing large areas and only practical with a limited number of fodder trees (see Chapter 2). In most circumstances, it is best to select a fodder species which produces viable seed in the environment in which it is planted, or a fodder species for which dependable inexpensive supplies of seed are available on a commercial basis. When producing seed of NFTs the factors to be considered include genetic diversity, quality of mother trees, location of collection area, collection techniques and seed handling and storage procedures. Basic guidelines for seed collection and multiplication of nitrogen fixing fodder trees are summarized in Chapter 7.