Selecting species of nitrogen fixing fodder trees

James M. Roshetho, J.C. Dagar, Sunil Puri, D.Y. Khandale, P.S. Takawale, G. Bheemaiah, and Narayan C. Basak

There are numerous nitrogen fixing tree species which can be used as fodder resources. Each of these species has a unique combination of environmental requirements and secondary uses. When establishing tree fodder production systems, be sure to select species that are appropriate for local environmental characteristics and provide the necessary products and services. The selection process should include evaluation of the planting site; identification of the primary and secondary uses of the trees in the systems; and review of information on suitable species.
The minimum site characteristics to be evaluated include elevation, average annual rainfall, and average annual temperature. Additionally, number of months with less than 50 mm of rainfall, annual maximum and minimum temperatures, and soil parameters are also useful. Important soil parameters are texture, depth, drainage, pH, and nutrient content (nitrogen, phosphorus and potassium). Table 1 provides a list of common nitrogen fixing fodder trees summarized by average annual rainfall and temperature ranges. Table 2 lists common nitrogen fixing fodder trees and their other uses. More detailed information on select species is available in extension bulletins and research journals. The Forest, Farm, and Community Tree Network (FACT Net) publishes fact-sheets and a research journal on multiple-purpose trees species. Fact-sheets are available in English, Spanish, Indonesia, Chinese, Vietnamese and Khmer for many of the species listed in Table 1. Some English-language Fact-sheets are reproduced in Appendix D. The International Centre for Research in Agroforestry (ICRAF) also publishes information on multiple-purpose trees and shrubs which can be used as fodder resources. ICRAF resources include a computerized database and research reports.

If a large number of species are suitable for the environmental conditions of the planting site, it may be appropriate to conduct a species screening trial. This is particularly important when exotic trees are introduced to an area for the first time. Always include native or naturalized trees in a screening trial. These commonly known species will provide a standard by which exotic species can be judged. There is a chance that exotic species may become weeds outside of their native range. If exotic species show signs of becoming weeds, remove them immediately. Be sure to remove the complete root system, otherwise trees may re-establish from by stump sprouts.

Screening trials should always be established where soil and environmental conditions are similar to the proposed planting areas. If planting areas differ greatly, then more than one screening trial may be necessary. Establish one trial in each distinct planting area. The screening trial site should be uniform, so that differences in tree growth and survival are not confused with differences in soil fertility, water availability or other factors. Each species included in the screening trial should be established in a single block of 25 to 36 trees. Tree spacing will vary by site and proposed management system, 1 x 1 or 2 x 2 meters is common. If the trees are being screened for use in a hedgerow, a linear layout should be used. Depending on hedgerow design and management, in-row spacing may vary from 10 cm to 1 or more meters.

Trees on the outside of blocks, or ends of hedgerows, may be influenced by conditions adjacent to the trial. When evaluating the screening trial, these 'border trees' should not be measured. A minimum of 9 to 16 trees should be measured to evaluate each species. Depending on the objectives of the screening trial, evaluation parameters may include survival, height growth, diameter growth, total biomass production, or leaf biomass production. Height and diameter is recorded for each non-border tree and the mean reported for each species. Basal diameter is recorded for the first year or until diameter at breast height can be measured. Biomass production can be recorded and reported per tree or per area basis. When trial results are reported, always include: location of trial, site and soil characteristics, number of trees measured, age of trees at measurement, planting method, tree layout and spacing, management practices (weeding, fertilization, etc.), and other details of interest. This information will make results meaningful to other people and make comparison with other trials possible.

Readers interested in establishing screening trials, research trials or provenance trials are encourage to consult Burley and Wood (1976), Webb et al. (1980), Briscoe (1989), Macklin et al. (1989), or MacDicken et al. (1991).

Table 1 a. Common nitrogen fixing fodder tree species summarized by mean annual rainfall and mean annual temperature: species suitable for high-rainfall areas.

Table 1b. Common nitrogen fixing fodder tree species summarized by mean annual rainfall and mean annual temperature: species suitable for medium- and low-rainfall areas.

Table 2. Common nitrogen fixing fodder trees species summarized by use.

Uses include living fence (LF), windbreak (WB), intercropping (IC), pasture improvement (PI), home garden (HG), shade for perennial crops (SP), small or large timber production (TP), fuelwood production (FP), soil improvement (SI), human food (HF) and ornamental (OR). "X" denotes that the species has actually been used for the purpose designated; "P" denotes that the species has potential for such a use.