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3.3 Establishment of Forage Tree Legumes

H.M. Shelton


Introduction
Root Systems of Tree Legumes
Factors Affecting Seedling Growth Rates
Planting Methods
References


Introduction

Some of the great advantages of many tree legumes, once established, are their longevity, vigorous growth and apparent immunity against competition from lower-growing herbaceous species. The tall stature and deep-rooted habit of tree legumes effectively insulate them from the most serious effects of competition. Consequently, in pasture systems, management of tree legumes to maintain grass/legume balance is simpler than with herbaceous legumes.

However, most tree legumes exhibit very slow growth as seedlings and at this stage of growth are vulnerable to competition from weeds (Maasdorp and Gutteridge 1986) and predation from wildlife. Long-lived species are often particularly slow as seedlings as much of their early growth is directed to establishing a strong root system. The poor establishment record of Leucaena leucocephala in subhumid Queensland is currently the major impediment to its more rapid adoption. This often results in an extended period before first grazing can occur and in a high rate of establishment failure (Lesleighter and Shelton 1986).

Methods for establishment of tree legumes are required which are quick and more reliable. Slow and/or unreliable establishment increases the time and level of risk before economic returns are feasible, and farmers are much less likely to adopt new technology when establishments risks are significant.

This section summarises what is known about establishment of tree legume species for utilisation in agricultural systems.

Root Systems of Tree Legumes

The poor establishment characteristics of tree legumes can be partly related to their rooting characteristics. Trees have evolved for long-term survival rather than quick early productivity. Their root systems have a high component of permanent structural roots as well as a system of fine roots responsible for nutrient and water uptake. A greater proportion of assimilates is translocated to non-productive structural development in tree roots.

Trees also have a much lower root length density than grasses (Bowen 1985). Atkinson (1980) reported root length densities ranging from 0.8 to 69 cm/cm3 for horticultural trees compared with 1004,000 cm/cm3 for grasses. Swasdiphanich (1993) measured root length densities in the surface 50 cm of soil of 0.5 cm/cm3 for L. leucocephala and 2 cm/cm3 for Calliandra calothyrsus. Low root densities make it difficult for roots to access poorly mobile nutrients (H2PO4-, K+ and NH4+) or immobile nutrients (Cu2+ and Zn2+) in the soil (Bowen 1985). Since the major concentration of tree roots occurs in the surface 15-30 cm of soil (Bowen 1985) it is clear that grasses will have a competitive advantage over trees especially in the seedling stage. Weeds, and particularly grass weeds, must be carefully controlled during the seedling phase of tree legumes. Eucalyptus species, which have adapted to grow in low nutrient soils, have quite fine roots (Bowen 1980) and this may make Eucalyptus seedlings more competitive with grasses.

There are a number of factors which can be manipulated to improve seedling growth rates and these are now considered.

Factors Affecting Seedling Growth Rates

Seed treatment

Scarification

The hard or waxy coats on the seed of many tree legumes inhibits the absorption of water and prevents uniform germination. The seed coat must be broken or scarified before germination will occur. Without scarification, the germination percentage may be <10%. Withington (1986) provides a summary of scarification methods for a range of tree legumes (Table 3.3.1). The most common method is hot water treatment, but sulphuric acid or mechanical scarification methods are also used.

Rhizobium inoculation

Forage tree legume plants which are not effectively nodulated will be pale in colour and will not grow vigorously due to inadequate nitrogen fixation leading to nitrogen deficiency. In preliminary evaluation trials it is possible to confuse this problem with poor adaptation to the environment.

Quite a number of tree legume species have specific Rhizobium requirements for effective nodulation and nitrogen fixation (Moloney et al. 1986). However, in many cases, the specificity of the species is not known, and the most effective Rhizobium strain has yet to be identified (Section 3.4)

Table 3.3.1. Pre-germination treatment for some nitrogen fixing trees (Anon. 1989).

Species

Treatment.

Seeds/kg

Acacia acuminate

C, D

60,000-80,000

Acacia aneura

A, C

75,000-95,000

Acacia angustissima

A, C

90,000-100,000

Acacia auriculiformis

A for 30 s, B for 15 min. C

30,000-90,000

Acacia crassicarpa

A for 30 8, C

40,000-60,000

Acacia holosericea

A for 1 min. C

70,000-80,000

Acacia mangium

A for 30 s, C

80,000-100,000

Acacia mearnsii

A, C

48,000-85,000

Acacia melanoxylon

A, B for 15 min. C

60,000-100,000

Acacia nilotica

A, C, D

7,000-11,000

Acacia polyacantha

D

10,000-25,000

Acacia saligna

A, C

14,000-25,000

Acacia senegal

C, D

10,000-30,000

Acacia tortilis

A, C, D

12,000-18,000

Albizia lebbeck

A, C, D

6,000-16,000

Albizia procera

A

20,000-24,000

Albizia saman
(syn. Samanea saman)

A, C

6,000-8,000

Alnus species

no treatment needed

200,000-2,000,000

Cajanus cajan

no treatment needed

5,000-12,000

Calliandra calothyrsus

A, C, D

18,000-20,000

Casuarina species

no treatment needed

150,000-1,500,000

Chamaecytisus palmensis

A for 4 min

38,000-42,000

Dalbergia spp.

D

10,000-30,000

Enterolobium cyclocarpum

C, D

800-2,000

Erythrina poeppigiana

D

3,000-5,000

Faidherbia albida
(syn. Acacia albida)

A, B for 20 min. C, D

20,000-40,000

Flemingia macrophylla

no treatment needed

50,000-80,000

Gliricidia sepium

no treatment needed

7,000-12,000

Leucaena spp.

A, B for 5-10 min. C

22,000-35,000

Mimosa scabrella

C, D

60,000-90,000

Paraserianthes falcataria
(syn. Albizia falcataria)

A, B for 10 min. C

40,000-50,000

Pithocellobium dulce

no treatment needed

9,000-25,000

Prosopis spp.

A, C

20,000-50,000

Pterocarpus indicus

no treatment needed

1,500-2,000

Robinia pseudoacacia

A, B for 20-60 min. C

35,000-50,000

Sesbania grandiflora

C, D

20,000-30,000

Sesbania sesban

C, D

85,000-100,000

Tipuana tipu

no treatment needed

1,600-2,500

* Treatments:

A. Pour boiling water over seeds, about 1 litre water per 250 9 of seeds or about five times as much water as seed, stir gently, pour off after 2 min (or as specified), replace with tap water and soak overnight.

B. Cover seeds with concentrated sulphuric acid, stir gently for recommended soaking time, pour off acid and rinse well in water.

C. Scratch or nick the round end of each seed with a file, knife or nail clipper. Do not cut the cotyledon.

D. Soak in cold/tepid water for 24 h.

The Rhizobium requirements of leucaena are now well known and it is possible to obtain peat cultures of effective Rhizobium from seed suppliers, when ordering seed. This is not the case with most other species.

Gliricidia sepium (Mzoma 1989) and Sesbania sesban are known to require specific Rhizobium. In the latter species, there is a host-strain interaction and different accessions of S. sesban require different strains of bacteria (M. Masafu, unpublished data).

Until more is known about host plant - bacterial strain specificity, care should be exercised when evaluating new varieties of tree legumes. In some cases, it may be wise to apply nitrogen fertiliser, or to use soil from around the roots of well-grown nodulated trees.

Effective strains of Rhizobium for many tree legume species are available from NIFTAL at 1010 Holomua Road, Paia, Maui, Hawaii 96779, USA.

Site selection

Successful establishment of tree legume species will only be achieved if the characteristics of the proposed planting site are matched against the climatic and edaphic requirements of the species. If the establishment requirements are not fully met, growth of seedlings will be poor unless the soil is amended or an alternative site is found. Table 3.2.1 in Section 3.2 shows the environmental requirements of the main species of tree legumes.

Inherent vigour of seedlings

The vigour of young seedlings is largely determined by genetic growth potential, the environmental characteristics of the site chosen and the level of amendment of the soil. The vigour of young leucaena seedlings varies greatly depending on site characteristics, and there are genetic differences within the Leucaena genus. Leucaena pallida and its hybrids show superior vigour to L. leucocephala cv. Cunningham (Sorensson et al. 1993).

There are also differences among other species of tree legumes in seedling vigour. For instance, S. sesban consistently shows superior seedling vigour compared with other species (Figure 3.3.1). This can be related to the lower mycorrhizal dependency of S. sesban, higher root mass, higher root density, higher root surface area, higher root length, smaller root diameter and higher root hair incidence in S. sesban relative to other species such as L. leucocephala (Manjunath and Habte 1991).

Fertiliser application

The responsiveness of the tree legumes to applications of fertiliser depends on their external nutrient requirements for maximum growth, and the fertility of the soil being fertilised. There are many experiments in the literature providing information on the response of tree species to fertiliser application but these tend to be site specific (Bray et al. 1987) and therefore of limited value for predicting fertiliser requirements elsewhere. However, Moloney et al. (1986) and Ruaysoongnern et al. (1989) have reported data on the generalised external and internal nutrient requirements of some tree legumes; the latter work provided estimates of critical concentrations in index leaves as well as potential fertiliser requirements for a range of nutrients for L. leucocephala (Table 3.2.6). These indicate that nodulated leucaena plants have quite a high requirement for phosphorus and calcium. More work is required to elucidate the nutrient requirements of the other commonly used species.

Fig. 3.3.1. Seedling growth rates of five tree legume species at Redland Bay in southeast Queensland (Swasdiphanich 1993).

Vesicular arbuscular mycorrhizae

Most tree legume species form symbiotic associations with naturally occurring soil fungi called vesicular arbuscular mycorrhizae (VAM). This association assists the roots to exploit more fully the soil volume and to gain improved access to available nutrients especially phosphorus. Nutrient ions are transferred to the roots via the hyphae. VAM therefore compensate for the low root length densities of trees. VAM can produce 80 cm of hyphae per cm of fine root infected (Sanders and Tinker 1973) with a length/weight ratio of 500 times that of fine roots. Mycorrhizal infection is therefore an important strategy complementary to, and sometimes replacing, fine root production (Bowen 1985).

Both Calliandra calothyrsus and L. leucocephala are known to form mycorrhizal associations. Young leucaena seedlings are very dependent on rapid early mycorrhizal infection of the roots for adequate phosphorus supply. Ruaysoongnern (1989) showed that the growth of L. leucocephala without VAM infection was only 5% of that obtained in unsterilised soil; phosphorus concentration in young leaves was reduced from 0.31 to 0.07% and nodulation was reduced from 297 to 0 mg/plant.

Brandon and Shelton (1993) reported that leucaena plants growing in a soil inherently low in VAM activity (Mt Cotton soil) suffered a period of P deficiency until VAM infection increased to effective levels. This was not observed in a soil which had a higher inherent VAM activity (Theodore soil). Very high levels of phosphorus fertilisation (1,200 kg P/ha) negated the effect (Figure 3.4.2) while competition from grasses exacerbated the effect. Soils vary in natural VAM activity according to cropping and cultivation history. VAM levels are high under most tropical grass pastures but are reduced by long periods of cultivation or intermittent waterlogging and may be low in virgin sites under Australian native vegetation.

Weeds

The slow seedling growth of many tree legumes makes them susceptible to competition from fast growing weeds which may slow or completely dominate their growth. It is therefore vital that young seedlings be protected from weed competition until they are well established. This can be performed by hand, using hand-held or tractor drawn machinery, or by the use of herbicides.

Chemical weed control with leucaena is now well understood. Pre-emergence control of weeds can be obtained with Trifluralin (0.5 kg a.i./ha) and Alachlor (3 kg a.i./ha) when incorporated, or with 2,4-D amine (6 kg a.i./ha), Dacthal (8-10 kg a.i./ha) and Oryzalin (3 kg a.i./ha) when surface sprayed (Brewbaker et al. 1985). The post-emergence herbicides Fluazifop (2 kg a.i./ha) and Bentazone (2 kg a.i./ha) are effective against grass and broadleafed weeds respectively without being excessively phytotoxic to leucaena.

Little is known about herbicides for use with other tree legumes although Glover (1986) reported that Glyphosate (1 kg a.i./ha) and Simazine (1 kg a.i./ha) were effective and non-phytotoxic pre-emergent herbicides for control of grass and broadleafed weeds in Gliricidia sepium.

Wildlife

Damage by wildlife can be a serious hazard to establishment. In Australia, marsupials, hares, cockatoos and ducks all seek out young leucaena seedlings and can chew plants to ground level making them more susceptible to domination by weeds. Economical control methods are not available. The planting of larger areas tends to reduce the percentage of the total crop damaged. New varieties with improved seedling vigour will also reduce this problem.

Planting Methods

Direct seeding

Planting of tree legumes by seed to form hedgerows is the most common method for broad-acre sowings. Appropriate row spacings for leucaena in pasture sowings vary from 3 to 10 m with wider spacings used in drier environments. The current recommendation in central Queensland is single or double rows 1 m apart with approximately 4-5 m between centres. If plant spacing within rows is 30-50 cm, this gives a population of 13,000-33,000 plants/ha, vastly less than the 75,000-140,000 plants/ha reported to be necessary to achieve peak forage yield (Brewbaker et al. 1985). Smaller plant populations, in wider rows, may provide better rationing of limited water supply and an opportunity to intercrop the rows with grass.

In Australia, tree legumes (primarily leucaena are normally directly seeded using a seed drill, into fully prepared and clean cultivated seedbed. The technical aspects of direct seeding of L. leucocephala Sesbania spp. and G. sepium are provided in Brewbaker et al. (1985), Evans and Macklin (1990) and Glover (1989) respectively and will not be repeated here.

Planting seedlings

Most tree legumes are readily established from transplanted seedlings. Seedlings are first grown in greenhouse nurseries in polythene bags or in small plastic dibble tubes until they reach a height of 30-50 cm. After a short period of 'hardening' in the open air, seedlings are directly transplanted into the field into moist soil. Weeds need to have been previously controlled either mechanically or chemically. Watering and protection of seedlings from predators will be necessary until trees become well established.

Some nitrogen fixing trees can be planted from stump cuttings which are easier to transport into the field. The NFTA Establishment Guide (Anon. 1989) reports that Albizia lebbeck, Calliandra calothyrsus, Dalbergia sissoo, Enterolobium cyclocarpum, Gliricidia sepium, Leucaena spp., Paraserianthes falcataria and Pterocarpus indicus can be planted in this way.

Stump cuttings can be made from seedlings which reach 60-90 cm in height and 10-20 mm in diameter in nursery seedbeds. They are first carefully removed when the seedbed is thoroughly wet and stems and roots cut 15-20 cm above and below the crown. Gliricidia stump cuttings can survive several weeks if kept moist although survival is best when transplanted promptly.

A recent comparison of planting methods shows the superiority of transplanted seedlings over direct seeding methods especially when weed growth is not adequately controlled (Figure 3.3.2) (B. Woodhead, unpublished data). Survival of transplanted seedlings was also higher than that of plants from direct seeding methods.

Vegetative propagation

Vegetative propagation of tree legumes is commonly practised with some species. Its advantage is more rapid establishment of new stands which are genetically identical to the parent lines without the need for seed collection. Disadvantages are that it requires more hand labour and the root development of cuttings may be shallow and devoid of a strong taproot compared with seedling grown trees. Shallow rooted trees are more susceptible to drought and wind damage.

Gliricidia sepium is commonly planted vegetatively and a full description of propagation methods is given in Glover (1989). Gliricidia establishes readily from cuttings or 'quick sticks' and is ideal for shade trees, support trees or 'living fences'. Cuttings should be mature branches >7 cm in diameter which are brownish-green in bark colour. The cutting is normally cut obliquely at both ends, discarding the younger tips, and the base inserted 20-50 cm into the soil depending on the length of the cutting. Cuttings for living fences may be up to 200 cm long whilst those for hedgerows may be 30-50 cm in length. In Indonesia, cuttings are sometimes planted as close as 10 cm apart with alternate cuttings bent sideways at 45° and plaited onto upright cuttings. This makes a surprisingly strong fence. In other areas, barbed wire is strained along the line of rooted cuttings and anchored on supported comer posts to make an equally strong fence. The fences can be periodically pruned to provide fodder, green manure, fuelwood or stakes for new fences. Frequency of pruning depends on the environmental conditions for growth and the end use of prunings. Living fences around agricultural fields need to be pruned regularly to reduce shading.

The Sesbania species seed prolifically and are normally planted from seed, although research suggests that some sesbanias can be established from cuttings (Evans and Macklin 1990). Sesbania species can also be propagated using in vitro methods (Harris and Puddephat 1989, Harris et al. 1989).

Fig. 3.3.2. Growth rates of six tree legumes established by planting seed or transplanted seedlings at Mt Cotton in southeast Queensland. Vertical bars represent LSD (P < 0.05) at days 30, 100 and 190. (B. Woodhead, unpublished data).

(a) Cultivation

(b) Seedling

Leucaena leucocephala is difficult to propagate vegetatively (Litzow and Shelton 1992) although Duguma (1988) and Bristow (1983) have reported successful establishment from cuttings. Both the University of Queensland at Brisbane and NFTA staff in Hawaii have recently reported success with grafting techniques onto suitable root stocks.

Similar techniques may be used for other species of tree legumes.

 

References

Anonymous (1989) NFTA Cooperative Planting Program. Establishment Guide. A guide to the establishment of research and demonstration plantings of nitrogen fixing trees. NFTA, Hawaii, 36 pp.

Atkinson, D. (1980) The distribution and effectiveness of roots of tree crops. Horticultural Review 2, 424-490.

Bowen, G.D. (1980) Coping with low nutrients. In: Pate, J.S. and McComb, A.J. (eds), The Biology of Australian Native Plants. University of Western Australia Press, Perth, pp. 33-64.

Bowen. G.D. (1985) Roots as a component of tree productivity. In: Cannell, M.G.R. and Jackson, J.E. (eds), Attributes of Trees as Crop Plants. Institute of Terrestial Ecology, Natural Environment Research Council, Abbots Ripton, United Kingdom, pp. 303-315.

Brandon, N. and Shelton, H.M. (1993) The role of mycorrhizas in establishment of leucaena. Proceedings of XVII International Grassland Congress. (In press.)

Bray, R., Palmer, B. and Ibrahim, T. (1987) Some Nutritional Constraints for the Growth of Shrub Legumes on Acid Infertile Soils in the Tropics. Proceedings of an international workshop, Thailand, pp. 100-104.

Brewbaker, J.L., Hegde, N., Hutton, E.M., Jones, R.J., Lowry, J.B., Moog, F. and van den Beldt, R. (1985) Leucaena Forage Production and Use. NFTA, Hawaii, 39 pp.

Bristow, S. (1983) Propagation of Leucaena leucocephala by cuttings. Nitrogen Fixing Tree Research Reports 1, 28-29.

Duguma, B. (1988) Establishment of stakes of Gliricidia sepium (Jacq.) Walp. and Leucaena leucocephala (Lam.) de Wit. Nitrogen Fixing Tree Research Reports 6, 6-9.

Evans, D.O. and Macklin, B. (1990) Perennial Sesbania Production and Use. NFTA, Hawaii, 41 pp.

Glover, Nancy. (1986) Herbicide screening for Gliricidia sepium. Nitrogen Fixing Tree Research Reports 4, 59-61.

Glover, Nancy. (1989) Gliricidia Production and Use. NFTA, Hawaii, 44 pp.

Harris, P.J.C. and Puddephat, I.J. (1989) In vitro propagation of Sesbania species. Nitrogen Fixing Tree Research Reports 7, 129-130.

Harris, P.J.C., Moore, T.H.M. and Puddephat, I.J. (1989) Micropropagation of Sesbania sesban. Nitrogen Fixing Tree Research Reports 7, 123-125.

Lesleighter, L.C. and Shelton, H.M. (1986) Adoption of the shrub legume Leucaena leucocephala in Central and Southeast Queensland. Tropical Grasslands 20, 97-106.

Litzow, D.R. and Shelton, H.M. (1992) Establishment of Leucaena leucocephala and Gliricidia sepium from the stem cuttings. Leucaena Research Reports 12, 3-6.

Maasdorp, B.V. and Gutteridge, R.C. (1986) Effect of fertiliser and weed control on the emergence and early growth of five leguminous fodder shrubs. Tropical Grasslands 20, 127-134.

Manjunath, A. and Habte, M. (1991) Root morphological characters of host species having distinct mycorrhizal dependency. Canadian Journal of Botany 69, 671-676.

Moloney, R.A., Aitken, R.L. and Gutteridge, R.C. (1986) The effect of phosphorus and nitrogen applications on the early growth of Adenanthera pavonina, Albizia falcataria and Schleinitzia insularum. Nitrogen Fixing Tree Research Reports 4, 34.

Mzoma, N. (1989) Nursery and field performance of inoculated and uninoculated Gliricidia sepium in Malawi. Nitrogen Fixing Tree Research Reports 7, 101-102.

Ruaysoongnern, S. (1989) A study of seedling growth of Leucaena leucocephala (Lam.) de Wit cv. Cunningham with special reference to its nitrogen and phosphorus nutrition in acid soils. PhD thesis, The University of Queensland.

Ruaysoongnern, S., Shelton, H.M. and Edwards, D.G. (1989) The nutrition of Leucaena leucocephala de Wit cv. Cunningham seedlings. 1. External requirements and critical concentrations in index leaves of nitrogen, phosphorus, potassium, calcium, sulphur, and manganese. Australian Journal of Agricultural Research 40, 1241-1251.

Sanders, F.E. and Tinker, P.B. (1973) Phosphate flow into mycorrhizal roots. Pesticide Science 4, 385-395.

Sorensson, C.T., Shelton, H.M. and Austin, M.T. (1993) Seedling vigour of Leucaena leucocephala, L. diversifolia, L. pallida and their hybrids. Tropical Grasslands 26. (In press.)

Swasdiphanich, S. (1993) Environmental adaptation of some forage tree legumes. PhD thesis, The University of Queensland.

Withington, D. (1986) A request for information about pre-germination treatments for important NPT species. Nitrogen Fixing Tree Research Reports 4, 70-71.


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