Previous Page Table of Contents Next Page


3.5 Defoliation Management of Forage Tree Legumes

W.W. Stür, H.M. Shelton and R.C. Gutteridge


Introduction
Inherent Tolerance to Repeated Defoliation
Defoliation and Growth
Tree Size and Timing of Defoliation
Future Research
Conclusions
References


Introduction

The use of tree legumes for forage, fuelwood, mulch or green manure requires regular cutting or grazing of the trees. A sound understanding of the effects of defoliation management on tree regrowth is important in determining optimum production strategies. In general, regrowth of forage plants depends on:

· a the availability of active meristematic tissue (buds),

· a the amount and photosynthetic capacity of residual leaf area, and

· a the mobilisation of available carbohydrate and other reserves from plant material which remains after defoliation.

The rate of regrowth is also influenced by environmental conditions (e.g. availability of moisture) after cutting and the developmental stage of the plant (Humphreys 1981).

While there have been numerous experiments comparing different cutting heights and cutting intervals of tree species, there are few studies which have investigated the principles underlying the defoliation response of tree legumes (i.e. the relative importance of residual leaf area and carbohydrate reserves, or the role of meristematic tissue). The results of the various cutting trials are difficult to interpret because of the different conditions under which the trials were carried out, but major trends are apparent and these will be discussed.

This section will provide a framework for considering the effects of different management strategies on productivity of tree legumes, and for developing management practices appropriate to sustainable farming systems.

Inherent Tolerance to Repeated Defoliation

There are differences among forage tree legume species in their ability to cope with repeated cutting. Sesbania grandiflora, for example, does not tolerate repeated cutting of the main stem above a certain height (Home et al. 1986, Ella et al. 1989). Other tree legumes with a poor tolerance to regular cutting of main stems include Paraserianthes falcataria and Acacia cunninghamii (Gutteridge 1990). There have been no investigations into the reasons for the inability of some tree legume species to coppice satisfactorily. In herbaceous pasture species, poor branching close to the ground and a lack of lateral buds have been identified as causes for the poor persistence of regularly defoliated erect ecotypes of Stylosanthes guianensis (Grof et al. 1970). From practical observations, the same reasons may apply in the case of Sesbania grandiflora. It has little or no branching close to the ground and, when cut back, does not coppice profusely. This indicates a lack of buds on the main stem. Farmers in eastern Indonesia have developed a management strategy appropriate for this tree species, which they grow on paddy walls for wood and forage. They lop only side branches and leave the main stem of the tree uncut. The tree can cope with this type of defoliation which results in a tall tree with a small crown on top.

This example highlights the need to understand the effects of defoliation on forage tree legumes and to develop management strategies appropriate for each species. Research on tree legumes is still relatively new and, with the exception of Leucaena leucocephala, we only have a very limited knowledge of defoliation responses of individual species.

Defoliation and Growth

Defoliation can be broadly divided into cutting and grazing. Cutting of tree legumes is practised extensively in southeast Asia where tree legume leaves are used to supplement low quality, naturally occurring grasses and crop residues. On the other hand, grazing of tree legumes is common practice in large-scale cattle production systems such as in northern Australia.

Defoliation can be described in terms of frequency and intensity. Frequency is how often the trees are cut or grazed, while intensity describes the amount of leaf and stem remaining after defoliation. The latter can range from removal of all plant material above a certain cutting height (as is often used in experiments) to very lenient defoliation such as lopping of only some branches of trees. Grazing is often less severe than cutting in intensity, as animals remove mainly leaf while cutting removes whole branches. Defoliation intensity and frequency interact, with more severe defoliation intensity requiring longer intervals between defoliations to allow the trees to recover. Conversely, under lenient defoliation systems, trees can be harvested more frequently.

Forage trees require a number of attributes to survive and coppice under regular defoliation. The woody stem and branches of grazed trees need to have elasticity to withstand bending by the grazing animals. Brittle trees may break too easily and expose the tree to fungal attack. This happens in Sesbania sesban when grazed.

Figure 3.5.1 illustrates a regrowth curve for biomass production above cutting height for Calliandra calothyrsus (W.W. Stür and H.M. Shelton, unpublished data). Similar responses are expected for other medium sized forage trees such as Leucaena or gliricidia. Smaller trees or shrubs such as Desmodium rensonii or Codariocalyx gyroides may be expected to have a lower overall yield per tree and may reach their maximum leaf yield earlier.

The effect of defoliation on yield can be delineated into three distinct phases. The first is a commonly observed lag phase after cutting (weeks 0-4) when regrowth is slow due to low leaf area. This is followed by a period of maximum productivity (weeks 4-10) when leaf production increases markedly. The sigmoidal curve then plateaus as full light interception is approached and older leaves begin to senesce (weeks 10-24). Guevarra et al. (1978) reported that leucaena did not reach full light interception until 3 months after planting. This period may be shorter when cutting well established trees or very dense plantings. Wood growth is slow initially, but wood accumulation continues to occur over a much longer period than leaf. During the third stage, the trees increase in height and woody biomass increases, while leaf yield remains steady or increases only slightly.

In general, cutting interval has a more dominant influence on total yield than cutting height.

Cutting interval

Figure 3.5.1 illustrates the finding of many studies that, while total biomass yield continues to increase with longer cutting intervals, the additional yield beyond a certain interval consists mainly of woody stem (Guevarra et al. 1978, Ella et al. 1989).

By comparing change in weekly growth rates with change in mean weekly growth rate, it is possible to determine the best interval to cut, or graze, regrowth to maximise yield of edible forage (Figure 3.5.2). In this example, maximum mean growth rate occurred just after the point of maximum growth rate and was a flat peak, in contrast to weekly growth rate which showed a steeper response. This meant that maximum edible yield could be achieved by cutting at any time during a 3 week period from approximately 8.5 to 11.5 weeks after commencement of regrowth. This period coincided with an edible forage fraction of 50-60% of total regrowth biomass. These calculations, together with other data and general observations, indicate that leafiness may be used as an indicator of optimum cutting interval. Cutting at the time when the proportion of edible material falls to 50% maximises yield of edible forage. Longer intervals, with lower forage fraction, are recommended when higher wood yields are desired.

Fig. 3.5.1. Leaf (including edible stem to 5 mm diameter) and stem (wood) dry matter production (g/tree) of Calliandra calothyrsus after cutting (W.W. Stür and H.M. Shelton, unpublishe data).

Some experimental results reported in the literature are summarised in Table 3.5.1. Gliricidia had a lower proportion of wood than Leucaena and calliandra in the experiment of Ella et al. (1989), while the maximum edible yield of the shrub Codariocalyx gyroides occurred at short cutting intervals (Lazier 1981).

Cutting frequency also affects shoot number and size. Pathak et al. (1980) found that branch number was negatively related to length of cutting interval in leucaena plants cut every 6, 9 or 17 weeks. This agrees with our field observations that short cutting intervals result in a higher number of shoots or branches in Leucaena However, in the study of Guevarra et al. (1978), although branch size was larger at long cutting intervals, branch number was unaffected.

Fig. 3.5.2. Weekly growth rate, man weekly growth rate and proportion of edible forage of Calliandra calothyrsus during a 24 week regrowth period (W.W. Stür and H.M. Shelton, unpublished data).

In summary, the range of defoliation intervals for maximising leaf production from forage trees such as Leucaena or calliandra appears to be around 24 months in the humid tropics, but may be longer in drier areas or the cooler subtropics (Gutteridge and MacArthur 1988). It is probably shorter for smaller trees and shrubs. If wood is a desired by-product of forage tree legume production, then a longer cutting interval should be selected.

Defoliation intensity

Experimentally, forage trees are usually cut to a certain height. This often results in very severe defoliation with little or no leaf remaining. In such cases, the lag phase before high growth rates are again achieved is long as new leaf growth has to be supported initially by stored carbohydrate reserves and it takes some time before leaf area has recovered to support maximum growth. More lenient defoliation systems which leave some leaf area after defoliation, can be expected to have a shorter lag phase as growth is supported by the current photosynthesis from the remaining leaf area. Unfortunately, no critical studies have been published which compare the relative importance of available bud meristem, carbohydrate and other plant reserves, and residual leaf area after defoliation of forage trees.

Table 3.5.1. Examples of the effect of cutting interval on edible fraction and stem yield of various forage tree legumes.

Cutting interval (weeks)

Species

Edible
(t/ha)

Wood
(t/ha)

Edible
(%)

Reference*

6

Leucaena leucocephala

8.6

2.0

81

1

12

Leucaena leucocephala

10.5

9.2

63

1

11

Leucaena leucocephala

9.4

2.6

78

2

14

Leucaena leucocephala

11.5

6.4

68

2

18

Leucaena leucocephala

12.0

8.8

68

2

8

Leucaena leucocephala

9.2

7.8

64

3

16

Leucaena leucocephala

10.3

18.6

36

3

6

Calliandra calothyrsus

7.2

1.6

82

1

12

Calliandra calothyrsus

10.3

5.1

67

1

6

Gliricidia sepium

7.7

1.0

89

1

12

Gliricidia sepium

8.2

1.7

83

1

2

Codariocalyx gyroides

1.6

0.4

80

4

6

Codariocalyx gyroides

2.1

1.1

66

4

8

Codariocalyx gyroides

1.9

1.2

61

4

4

Sesbania sesban

2.7

0.3

90

5

6

Sesbania sesban

2.8

1.1

72

5

8

Sesbania sesban

2.7

1.8

60

5

* References: 1. Ella et al. (1989); 2. Guevarra et al. (1978); 3. Ferraris (1979); 4. Lazier (1981); 5. Galang et al. (1990)

Some researchers have found that higher cutting heights produced higher yields (e.g. Krishna Murthy and Munegowda (1982) with leucaena). Isarasenee et al. (1984) reported enhanced growth of leucaena cut at 120 cm compared with 60 or 30 cm. They further suggested that early regrowth was supported by movement of carbohydrate reserves from stem rather than from current photosynthesis. A low cutting height of 5 cm was detrimental to both yield and persistence of Codariocalyx gyroides (Lazier 1981). On the other hand, Ferraris (1979) found no difference in the yields of leucaena cut at 10 or 30 cm. Similarly, Pathak et al. (1980) found little difference in Leucaena yields when cut at 10, 20 or 30 cm. A wider range of cutting heights (30, 60 and 90 cm) for Leucaena also did not affect yield in an experiment conducted by Jama and Nair (1989). Cutting height was, however, positively related to shoot number per plant and this was also observed in the trial by Pathak et al. (1980).

In the absence of detailed studies on defoliation intensity on subsequent regrowth, only speculative comments can be made. Poor regrowth at low cutting heights may be related to a lack of regrowth sites (buds) and this can be expected to vary with species. If sufficient buds are available, initial regrowth must depend on the mobilisation of carbohydrate and other nutrient reserves. Increasing cutting height may result in greater available reserves and residual leaf on the stump and this may lead to a shorter lag phase.

Tree Size and Timing of Defoliation

Other factors which influence regrowth after defoliation are tree size and timing of defoliation.

It is general practice to leave forage trees uncut until they reach a height of at least 1-1.5 m. This establishment period can be greater than one year in many cases. The benefit of a long establishment period before first defoliation was demonstrated by Ella et al. (1991). They showed that the age of trees at the first harvest was positively related to yield at subsequent harvests (Figure 3.5.3). The positive effect of a long establishment period was more pronounced for Leucaena and gliricidia than for calliandra. 'Older' trees were larger than 'younger' trees at the first cut, and the increased growth may have been related to there being more reserves in the larger stumps and presumably to the larger root system (not measured) on the 'older' trees.

Cutting forage trees at different seasons of the year (dry vs. wet season) and at different stages of development (flowering vs. vegetative) may also influence subsequent regrowth. However, little has been published on these topics. It may be speculated that cutting at the beginning of a dry season or during the dry season could result in the exhaustion of reserves as growth and replenishment of reserves may be restricted by moisture availability. On the other hand, tree legumes are usually deep-rooted and therefore have access to moisture in the deeper soil layers. They may also be expected to have a large amount of reserves in stems and root system, which may not easily be exhausted. Guevarra et al. (1978) mentioned that a more pronounced lag phase during regrowth of leucaena was observed when trees were cut at the long interval of 18 weeks as compared with shorter cutting intervals. They attributed this to a 'sink' effect of flowering and pod development which may have restricted carbohydrate accumulation in roots and stems and vegetative growth.

Fig. 3.5.3. Effect of length of establishment period of tree legumes before first cutting on subsequent leaf yield (cumulative dry matter yield kg/tree over four 3-monthly harvests) (after Ella et al. 1991).

Future Research

There is a need for critical studies on the effect of defoliation on regrowth of forage tree legumes. Basic research is required on the role of residual meristem (buds), residual leaf area, and energy and nutrient reserves for regrowth. Such experiments could contribute significantly to our practical understanding of defoliation responses by investigating the appropriate defoliation management for the various forage trees.

Comparing different forage trees within an experiment is difficult. Many experiments have used a common cutting height and frequency for all species. As the optimum defoliation management will vary with species with contrasting growth habit, a common management system may advantage some species while disadvantaging others. Results from such experiments are difficult to interpret.

This problem can be partially overcome by harvesting each species individually at an appropriate interval or plant height to take into account seasonal variability. Criteria for cutting intensity may be specified levels of residual stem and leaf area after defoliation rather than a fixed cutting height. Until the optimum defoliation management for individual species is known, management decisions will be based on intuition or inflexible schedules rather than on physiological principles.

Conclusions

In summary, forage tree legumes differ in their ability to withstand repeated defoliation. It is suspected that this is related to the availability of bud meristem, but no work has so far been undertaken to confirm this aspect.

Regrowth after defoliation has to be supported from residual leaf area and stem and root reserves, and the relative importance of these two factors needs to be investigated. Leaving as much residual leaf area as possible can be expected to reduce the length of the lag phase after defoliation.

Cutting frequency has a major effect on the proportion of edible forage and wood production. Longer intervals are appropriate if wood is the preferred by-product, while shorter intervals are required to optimise leaf production. Common cutting intervals are in the range 24 months.

Cutting intensity affects the amount of leaf and stem remaining after defoliation. Although very low cutting heights may have a detrimental effect on subsequent regrowth of some species, the effect of cutting height on subsequent productivity is often neutral and sometimes positive.

Leaving forage trees uncut for a long period before the first cutting has been shown to have a beneficial effect on productivity.

References

Ella, A., Jacobsen, C., Stür, W.W. and Blair, G.J. (1989) Effect of plant density and cutting frequency on the productivity of four tree legumes. Tropical Grasslands 23, 28-34.

Ella, A., Blair, G.J. and Stür, W.W. (1991) Effect of age of forage tree legumes at the first cutting on subsequent production. Tropical Grasslands 25, 275-280.

Ferraris, R. (1979) Productivity of Leucaena leucocephala in the wet tropics of North Queensland. Tropical Grasslands 13, 20-27.

Galang, M.C., Gutteridge, R.C. and Shelton, H.M. (1990) The effect of cutting height and frequency on the productivity of Sesbania sesban var. Nubica in a subtropical environment. Nitrogen Fixing Tree Research Reports 8, 161-164.

Grof, B., Harding, W.A.T. and Woolcock R.F. (1970) Effects of cutting on three ecotypes of Stylosanthes guyanensis. Proceedings Eleventh International Grassland Congress, Surfers Paradise, Australia, pp. 226-230.

Guevarra, A.B., Whitney, A.S. and Thompson, J.R. (1978) Influence of intra-row spacing and cutting regimes on the growth and yield of Leucaena Agronomy Journal 70, 1033-1037.

Gutteridge, R.C. (1990) Agronomic evaluation of tree and shrub species in southeast Queensland. Tropical Grasslands 24, 29-36.

Gutteridge, R.C. and MacArthur, S. (1988) Productivity of Gliricidia sepium in a subtropical environment. Tropical Agriculture (Trinidad) 65, 275-276.

Horne, P.M., Catchpoole, D.W. and Ella, A. (1986) Cutting management of tree and shrub legumes. In: Blair, G.J., Ivory, D.A. and Evans, T.R. (eds), Forages in Southeast Asian and South Pacific Agriculture. ACIAR Proceedings Series No. 12, Canberra, pp. 164-169.

Humphreys, L.R. (1981) Environmental Adaptation of Tropical Pasture Plants. MacMillan Publishers, London, 261 pp.

Isarasenee, A., Shelton, H.M., Jones, R.M. and Bunch, G.A. (1984) Accumulation of edible forage of Leucaena leucocephala cv. Peru over late summer and autumn for use as dry season feed. Leucaena Research Reports 5, 34.

Jama, B. and Nair, P.K.R. (1989) Effect of cutting height of Leucaena leucocephala hedges on production of seeds and green leaf manure at Machakos, Kenya. Leucaena Research Reports 10, 46-48.

Krishna Murthy, K. and Munegowda, M.K. (1982) Effect of cutting frequency regimes on the herbage yield of Leucaena Leucaena Research Reports 3, 31-32.

Lazier, J.R. (1981) Effect of cutting height and frequency on dry matter production of Codariocalyx gyroides (syn. Desmodium gyroides) in Belize, Central America. Tropical Grasslands 15, 10-16.

Pathak, P.S., Rai, P. and Deb Roy, R. (1980) Forage production from koo-babool Leucaena leucocephala (Lam.) de Wit.). 1. Effect of plant density, cutting intensity and interval. Forage Research 6, 83-90.


Previous Page Top of Page Next Page