| Nitrogen fixing trees for fodder production - A Field Manual |
Ross C. Gutteridge, Janet Stewart, H.P.N. Gunasena, F.B. Patil, P.K. Mutty and N.N. Pathak
Seed to be used for multiplication should be of superior genetic quality, and also as genetically diverse as possible. Leucaena leucocephala is the only NFT species for which seed has been produced in commercial quantities and for which a seed certification scheme exists in some countries. Seed of other NFT species is produced in a number of countries, but the quality and genetic origins of the seed is often unknown.
Rather than relying on external sources of seed supply, it may be more appropriate for extension personnel to collect and multiply seed of selected NFTs for distribution in their particular regions. This chapter provides broad guidelines on how to approach this process.
Local Seed Collection
In most cases, seed or cuttings for multiplication must be selected from locally available material. Although a detailed evaluation of fodder yields and quality may not be feasible, a selection of superior specimens is possible. Occurring in farm fields, many fodder trees are privately owned. The farmers who use the trees may be able to identify superior trees in terms of yield, response to repeated lopping, timing of fodder production and even perceived effects on animal production. Seed should be collected from these superior trees. It will often be necessary to pay farmers for the right to collect seed and for the fodder foregone during seed maturation. Generally, trees used for seed production can not be lopped for at least one year.
Seed should never be collected directly from provenance trials. Most nitrogen fixing fodder trees are thought to be outcrossing (an exception is L. Ieucocephala which is largely selfing). Cross pollination between provenances will result in seed of mixed parentage. The trees grown from this seed will not have the same characteristics for which the superior provenance was selected. Once superior material has been identified, the organization which proved the original seed may be able to provide larger quantities of the same provenance for establishment of a seed production area (seed orchard).
It is very important to ensure adequate genetic diversity in seed collections. Genetic diversity will minimize the risk of inbreeding and vulnerability to pests and disease, as well as provide a broad genetic base for any future selection. Seed to be used for seed orchard establishment should be collected from at least 50 different trees located at least 100 m apart. Since most nitrogen fixing fodder trees are outcrossing, seed from trees dose together is more likely to be paternally related - received pollen from the same tree. In natural ecosystems, mother trees close together are more likely to be maternally related originated from seed of the same tree. The aim of the seed collection strategy is to minimize relatedness of the seed and so capture as much genetic diversity as possible.
Some species, such as Gliricidia septum, are easily propagated from cuttings which can be used instead of seed as the basis for multiplication. However, it is still important to ensure adequate genetic diversity by including cuttings from at least 50 plants in the multiplication area. Again, the cuttings should come from widely separated plants. In areas without a pronounced dry season, G. septum does not set seed well. Cuttings may be the only means of propagation.
Caution must be exercised when collecting seed or cuttings from any planted stand of trees, including fence-lines, windbreaks, woodlots or large plantations. Often, such stands have been propagated with seed collected from a small number of trees. They already have a narrow genetic diversity. If these stands do not show superior quality over natural stands, seed and cuttings should not be collected from them. When possible, investigate the genetic origins of the seed used to establish these stands.
Establishment of Seed Orchards
Multiplication of good quality seed (or cuttings) in an orchard provides an easy and reliable way of repeatedly obtaining planting material for distribution to farmers, without having to repeat the lengthy seed collection procedure described previously.
It is essential to locate seed orchards of open-pollinated species as far away as possible from unselected trees of the same species. This distance will minimize cross-pollination which would reduce the gain achieved through the selection of superior trees. The distance required to achieve this isolation depends on the mode of pollination employed by the tree species. Ideally, the distance should be at least 1 km. This may not always be practical. Alternatively, nearby trees of the same species may be lopped before flowering to eliminate the source of undesirable pollen. Seed set within the stand can also be improved by encouraging the appropriate pollinators. For instances, by placing healthy beehives in orchards of bee-pollinated species.
Seed trees should be planted at wide spacing, for example 4 x 4 m. Some nitrogen fixing fodder trees, such as L. leucocephala and G. septum, are precocious seeders - setting seed within 1 to 2 years. Other species, such as Prosopis cineraria, take 7 to 8 years to set seed. This interval can be reduced to 3 to 4 years by using vegetative material to establish the seed orchard. Vegetatively propagated seed orchards have the additional advantage that the genetic identity of selected superior trees is maintained, whereas with open-pollinated seed the identity of the father (pollen source) is unknown.
Seed Handling Storage
Only mature, well-developed pods should be harvested for seed. Where possible, harvesting should be undertaken during dry periods to avoid contamination by fungal pathogens. Some species, for example Acacia nilotica, display variation in flowering time and seed set which leads to variability in seed quality and viability. To avoid this problem seed should be harvested only during the season which gives the highest quality seed.
The harvested pods should be shelled and the seed dried under shade to a moisture content of 10 to 15%. This process should take 3 to 5 days. The seeds should be winnowed, screened and sorted to remove foreign objects and poor quality seeds. Only fully developed, healthy, undamaged seed should be stored. During storage, seedlots should be labeled with the species name, date of collection, collection site, number of mother trees from which the seed was collected and seed weigh. This information will maintain the identity of the seed source and make subsequent evaluation possible.
Seed should be stored in airtight, insect-proof containers at temperatures below 4Ã¸C. If possible, it should be dusted with an appropriate insecticide to minimize damage by bruchids. A sample of seed should be tested for viability before storage and again just prior to distribution. This information will enable managers to determine appropriate planting densities.
Availability of Selected Seed
There have been studies of provenance variation in many nitrogen fixing fodder trees, including; A. nilotica, Calliandra calothyrsus, Faidherbia albidia, G. septum, L. Ieucocephala and P. cineraria. Unfortunately, even when superior provenances have been identified it is often not possible to acquire seeds of these provenances in large quantities. An exception is L. Ieucocephala, for which varieties such as cv. Cunningham (K500) and Taramba (K636) are now commercially available. A hybrid of L. Ieucocephala x Leucaena pallida (KX2) is now commercially available from the Hawaiian Agricultural Research Center. The KX2 hybrid was selected for psyllid resistance and fodder production. In Australia there are plans for the release of commercial cultivars of Sesbania sesban derived from ILCA (International Livestock Centre for Africa) accession no. 15036.
For G. septum, superior material has been identified through a worldwide provenance trial organized by the Oxford Forestry Institute (OFI). The Guatemalan provenance 'Retalhuleu' (or 'Maya type' - OFI nos. 14/84, 60/87 125/91) has shown consistently superior growth on a wide range of sites. Seed of this provenance has been distributed to several countries for establishment of seed multiplication areas. Small quantities are available for multiplication purposes from OFI. A list of organizations able to supply Retalhuleu seed is being prepared.