2. ENVIRONMENT AND FACTORS IN THE PASTURE-CATTLE-COCONUT SYSTEM (contd.)

Table 24. - Shade tolerance of some tropical forages

Source: Wong, 1991 and Shelton et al., 1987a; (adapted and modified from Reynolds, 1978f; Eriksen and Whitney, 1982; Evans et al., 1992; Humphreys, 1981; Chen and Bong, 1983; and Wong et al., 1985b).

Shade imposes a limitation to biological productivity in plants although the extent of the limitation varies with shade tolerance of the species and the nitrogen supply.

Eriksen and Whitney (1981) found that in high-yielding grass species, well supplied with nitrogen, yield increased almost linearly with increasing light up to 75% of full sun, and then tended to plateau as light transmission increased to 100% of full sun. At low nitrogen, yield of the same species maximised at lower light levels. For species reputed to be shade-tolerant (Axonopus compressus and Paspalum conjugatum), yield maximised at much lower light levels. Wong concluded that both potentially high-yielding species, which are limited by nitrogen availability, and low-yielding shade-tolerant species, are light-saturated at low levels of solar radiation.

Table 25 - Growth characteristics of tropical grasses and legumes grown at four light levels (mean of 12 species) Wong, 1991

LWR = leaf weight ratio;
LAR = leaf area ratio;
SLA = specific leaf area.

Table 26. - Percentage composition of plant parts, specific leaf area and leaf area ratio of some tropical forages grown in shade (mean of 64, 30, 18 and 9% of light transmission). Wong, 1991.

SLA = specific leaf area;
LAR = leaf area ratio.
Source: Wong et al., 1985a, b.

“For both grasses and legumes, species differences were greater under moderate to high light transmission than under low light. The low yield potential of all species in low light remains a major constraint to forage productivity in plantations which close their canopies with age. However, in plantations with open canopies such as coconut, species with medium shade tolerance can be exploited to obtain higher yields”, Wong (1991).

Ranacou (1972) found centro (Centrosema pubescens) to be more shade tolerant than Siratro, Tinaroo glycine (Neonotonia wightii), or stylo (Stylosanthes guianensis) and Cori grass (Brachiaria miliiformis) was found to be more shade tolerant then Brachiaria brizantha (see Table 27) in Sri Lanka (Santhirasegaram and Ferdinandez, 1967).

Table 27. - Comparative performance of B. brizantha and B. miliiformis under shade in Sri Lanka (Santhirasegaram, 1976 b)

Note: dry matter yield g/m² at two levels of N.

Gregor (1972) carried out screening trials under 46 percent artificial shade to identify useful grasses and legumes for shaded conditions. Sillar (1967) compared the yield of S. humilis grown under shaded and open conditions, while Egara and Jones (1977) found some seedling shade tolerance in L. leucocephala. Eriksen and Whitney (1977, 1981 and 1982), Chen and Bong (1983), Chen and Othman (1984), other researchers in Malaysia (MARDI, 1980) and Wong et al. (1985a, 1985b) measured the performance of various tropical forage grasses and legumes under different light intensities, and Reynolds (1978f), Smith and Whiteman (1983a) and Steel and Whiteman (1980) demonstrated the effects of coconut shade on the yields of different tropical grasses (see Table 24 and Figure 32). When light transmission values fall below 40 or 50 percent then both production values and the range of species are severely reduced. Smith et al., (1983) indicated that in the Solomon Islands growth of all grass species in plots under clipping has shown a linear relationship between light transmission percentage and dry matter production. For every 1 percent decrease in light transmission (over the range 100 to 40 percent) pasture yield declined by about 1.2 percent of yield at open sites. In Vanuatu the yield of Panicum maximum var. trichoglume was reduced by 50 percent under shade where coconuts were planted at a density of 124 ha^-1 (Anon. 1978c).

Wong et al. (1989) determined the effects of shading (100, 60, 34 and 18 percent of sunlight) on the dry matter productivity, forage quality and mineral composition of six grasses (Axonopus compressus, Brachiaria decumbens, Panicum maximum, P. maximum var trichoglume, Paspalum conjugatum and Setaria sphacelata cv Kazungula) defoliated at six and ten weekly intervals. Common guinea and Signal were the best yielders across all shade levels and mean dry matter yield reduction of the six grasses was 23.1 percent and 37.6 percent of the control on the 34 and 18 percent sunlight plots.

Mohd. Najib (1989) studied the growth performance of P. maximum and Pennisetum purpureum under rubber over a five year period starting when the rubber was six months old. Up until the rubber trees were about four and-a-half years old, light transmission declined by only about 30 percent and there was only a modest decline in dry matter yields. In the fifth year there was a sharp decline in light transmission (to less than 20 percent) and herbage dry matter yields declined dramatically to lower than 30 percent of those obtained in the first year (Waidyanatha et al., 1984, found a similar decline in dry matter yields under rubber after five years in Sri Lanka). Dry matter yields of the two grasses were highly correlated with light transmitted through the rubber tree canopies:

* Reported by Lane (1981) to grow well and support useful liveweight gains in shade down to <20% of open conditions in Solomon Islands, although pasture appears to have consisted mainly of weeds and Pueraria phaseoloides

Figure 32. - Grass species and percentage light transmission.

It was noted that Wong et al. (1985a) had found that the critical level of shade for Common Guinea was about 52 percent. N concentration in the forage dry matter increased significantly with increasing shade, but the percentage of napier and guinea declined to 21 and 23 percent respectively as weeds and native species invaded the plots when light intensity was less than 18 percent in the fifth year.

Chong et al. (1991b) demonstrated that forage dry matter yield under rubber was strongly related to light transmission (see Figure 33), with yields falling from 4.5 – 6.5 t ha^-1 year^-1 at high light transmissions of 60–100 percent to below 1 t ha^-1 when light transmission fall below 50 percent. Chen and Othman (1983) and Wan Mohamed (1986) reported similar standing biomass reductions when rubber canopies closed. Sanchez and Ibrahim (1991), Wong (1991) and Sophanodora and Tudsri (1991) reviewed and demonstrated the effect of decreasing PAR (photosynthetically active radiation) percent on forage productivity, and Benjamin et al. (1991) investigating shade tolerance of six tree legumes found that dry matter yield declined as the light level declined with mean yields in g pot^-1 of 42, 39, 32, 31 and 28 for the 100, 70, 50, 30 and 20 percent PAR treatments respectively. However, the extent of these effects varied among the species. Yields of Acacia villosa, Sesbania grandiflora and Albizia chinensis were significantly reduced by shade whereas yield reductions in Leucaena leucocephala, Calliandra calothyrsus and Gliricidia sepium were not significant (see Figure 34).

When yield performance under all shading treatments was expressed as a percentage of yield at 100% light transmission, the relative order of shade tolerance was G. sepium (94%), C. calothyrsus (85%), L. leucocephala (84%), S. grandiflora (76%), A. villosa (70%) and A. chinensis (66%). When yield under very low light was examined, the relative yield performance of the species compared to 100% light transmission was G. sepium (92%). C. calothyrsus (78%), L. leucocephala (68%), S. grandiflora (62%), A. villosa (54%), and A. chinensis (48%).

Figure 33. - Dry matter productivity of cover crops and naturally occurring forages under different light levels in rubber. (Chong et al., 1991b).

A recent report by Samarakoon et al. (1990a) indicated that in an experiment under O (full sun), 42, 59 and 68 percent shade, Axonopus compressus, Pennisetum clandestinum and Stenotaphrum secundatum showed the somewhat unusual response for tropical grasses of an increase in dry matter yield of tops under shade compared with full sun. This response was expressed by S. secundatum and A. compressus up to 68 percent shade while maximum yield of P. clandestinum was reached at 42 percent shade (see Table 28).

Generally, herbage yield of tropical grasses decreases with increasing shade (Wong et al., 1985a) but under some circumstances some species show higher yields of tops under moderate shading than in full sun (Eriksen and Whitney, 1981). These circumstances seem to be especially associated with low N supply to the plants (Wong and Wilson, 1980; Eriksen and Whitney, 1981). The subject is examined in more detail in Section 2.4.

Table 28. - Effect of shade on the growth (dry weight of plant tops in g pot^-1) of three grasses (Samarakoon et al., 1990a)

¹ Standard error of mean values for shade x species (4 replications; residual D.F. = 24).

Figure 34. - Yield (DM g pot^-1) of tree legumes grown at five light levels (Benjamin, et al., 1991).

One grass which appears to perform better under shade than in open conditions is Cori grass (Brachiaria miliiformis). Observations in Western Samoa and Zanzibar by the author showed that while it thrives under coconuts with light transmission values ranging at midday between 50–85 percent, under open conditions Cori quickly became weed infested and at one site was competed out by the more aggressive Batiki grass (Ischaemum aristatum). Supporting evidence is found in the work of Eriksen and Whitney (1977, 1981) who demonstrated, in pot trials, that under zero nitrogen the yield of Cori grass was highest at 27 percent daylight and under 365 kg N it was highest at 70 percent daylight.

In Malaysia, Suparjo et al. (1991) found that for Asystasia intrusa the optimum rate of photosynthesis was 11.9 muMolCO₂/m²s^-1, with best growth at one third full sun (i.e. in the shade of other crops such as oil palm or rubber).

Recently Sophanodora (1993) carried out a laboratory experiment to determine light response curves for Brachiaria mutica, B. humidicola, Stenotaphrum secundatum, Panicum maximum cv. Hamil and P. maximum cv. TD58 and assess these as criteria for species selection under plantation crops.

Most of the literature from temperate zones reports a negative relationship between tree basal area and herbaceous forage produced on the same unit of land (Torres, 1983). Under the coniferous forest of North America, McConnell and Smith (1970) and Grelen et al. (1972) reported a linear relationship, where canopy percentage or tree basal area accounted for 94 and 58% of the variation in herbage yield respectively. Pase (1958) established a logarithmic relationship and Jameson (1967) an exponential one. The negative effect of tree basal area on herbage production also negatively affects the range carrying capacity (Clary et al., 1975).

Beale (1973), Pressland (1976) and Whiteman (1980) demonstrated a negative (curvilinear) relationship between herbage growth and tree density (see Figure 35 and Table 29). According to Atta-Krah (1993), Penning de Vries and Djiteye (1982) showed that increasing tree canopy cover leads to increasing tree foliage production but declining understorey herbaceous production. In the Solomon Islands and Papua New Guinea close relationships were demonstrated between estimated carrying capacities of pastures, animal growth rates and stand densities of five-year old E. delgupta (see Table 30). In Australia, Cameron et al. (1991) indicated that four years after planting Eucalyptus grandis into Setaria spacelata cv. Kazungula, the optimum tree density (in terms of both tree and pasture production) was about 300 trees ha^-1; producing a tree canopy cover of about 20 percent.

Table 29. - Relationship between herbage yield, tree foliage yield and tree density in South-west Queensland (Beale, 1973)

Figure 35. - Relationship between herbage yield (gm^-2) and basal area (m²ha^-1) of mulga (Acacia aneura) in south-west Queensland (Whiteman, 1980; Beale, 1973).

In Costa Rica, Somarriba (1988b) found that pasture growth under Guava (Psidium guajava L.) was only 49–63 percent of that in the open. Light intensity under the mature guava trees (at 264 trees ha^-1) ranged between 13–33 percent of direct sunlight in the open, with a mean of 24 percent. Floristically shaded and non-shaded pastures were very similar but the dominant graminae complex (Axonopus compressus and Paspalum conjugatum) suffered a 60 percent reduction in total biomass under shade as compared to open places.

Table 30. - The estimated animal unit (AU) carrying capacities of pastures and animal growth rates under various stand densities of 60 months old E. deglupta (Anon., 1981a)

Note: Based on animal performance on puero/T grass pastures in Solomon Islands and data from Papua New Guinea. Data suggest AU ha^-1 decline linearly with light transmission down to 50%. Below 50% it is suggested carrying capacity declines more rapidly.

Bazill (1987) evaluated 25 tropical legume species under Pinus caribaea var. hondurensis (spaced at 2.5 × 2.5 m with 18 percent full sunlight) in Turrialba, Costa Rica. After 19 months species showing good shade tolerance were Centrosema sp., Desmodium sp., Flemingia congesta and Galactia striata. Also doing well were Canavalia ensiformis (an annual), Lablab purpureus and Vigna unguiculata. The Centrosema species included C. plumieri, C. pubescens and C. macrocarpum while the Desmodium species included D. heterocarpon, D. ovalifolium, D. heterophyllum, D. intortum and D. uncinatum.

Anderson et al. (1988) reported a negative (linear) relationship between relative carrying capacities of pastures and Pinus radiata tree density and between light reaching pastures and tree density (see Figures 36 a and b). Although a number of other mathematical relations have been proposed, there is general agreement with Halls (1970) that herbage production is inversely related to tree density (and light transmission percent). However, MacFarlane (1993a) suggests that the response of pasture growth under silvo-pastoralism to tree density is not uniform and varies according to competition for available moisture and nutrients, levels of transmitted light and the pasture canopy and the presence of allelopathic effects exerted by trees. Thus Cameron et al. (1991) reported that three year old Eucalyptus grandis (growing in a 1,000 mm rainfall environment in Southern Queensland) did not reduce pasture production at 305 stems ha^-1 whereas MacFarlane and Whiteman (1993) reported that E. deglupta (in a 3,000 mm rainfall on Kolombangara Island in the Solomons) had 50% light transmission with 207 stems ha^-1 at three years of age and animal production a maximum of 40% of fully illuminated pastures. In this environment Caiger (1982) reported that T-grass (Paspalum conjugatum) production under shade as a percentage of open production was described by the equation:

% Total Yield = - 5.43 + 1.06 % PAR (photosynthetically active radiation) transmission.

Figure 36a. - Measurements of light reaching pastures under seven year-old P. radiata trees pruned to 4 m (x) and 20 year-old trees pruned to 6 metres (.), at various densities. (Anderson et al., 1988).

Figure 36b. - Regressions of relative carrying capacities of pastures on numbers of trees per hectare in five to eight year-old stands of P. radiata (Anderson et al., 1988).

Ludlow (1978) considered that differences in shade tolerance exert little control over the outcome of competition for light, the primary factor being the capacity of a plant to overtop its neighbours and expose its leaves to high levels of irradiance. The subsequent growth advantage enjoyed by the dominant plant then reinforces the unfavourability of the light environment in which the leaves of the suppressed species grow. Humphreys (1981) suggests that grazing management is an important factor because, for example, leaves of low-growing legumes such as Stylosanthes humilis receive sufficient illuminance if taller companion grasses are removed by heavy grazing; also, leaves of twining legumes such as Macroptilium atropurpureum may be adequately displayed in sunlight if grazing pressure is sufficiently lenient to permit the plant to climb up and overtop companion grasses. While this is a particularly significant factor under open conditions, under tree crops such as coconut competition is unequal because the pasture has no opportunity to shade the trees and thus species recommended for planting must have shade tolerance (Humphreys, 1981).

It is generally true that herbage production and tree density are closely related (see Figure 35 and Tables 29 and 30) and reduction in pasture yield is likely in shaded conditions such as under coconuts (and rubber, oil palm and forest species).

Wong (1991) stresses that “an important character in the selection of shade-tolerant species is their ability to persist and compete with the shade-tolerant weeds under continual defoliation…. The term persistence includes both the survival of individual plants (longevity and vegetative propagation) and seedling replacement. Indigenous shade species such as A. compressus, S. secundatum, B. miliiformis and P. conjugatum have been the most persistent and productive under low light levels. Any new shade-tolerant genotypes that are identified must be able to out-perform these species in dense shade.

Persistence of forages is affected not only by their tolerance to shading but also by their ability to tolerate regular defoliation. A longer cutting interval has enhanced persistence of a number of grasses grown under the closed canopy of oil palms (Table 31). The shade-tolerant species Axonopus compressus and Paspalum conjugatum had a higher plant density at the end of the experiment than at the beginning, while less shade-tolerant grasses persisted poorly. This may have been related to either poor shade tolerance or damage from pests and fungal diseases in dense shade. The persistence of legumes under the closed canopy of oil palms was, with the exception of Calopogonium mucunoides, generally poor in a trial reported by Chen and Othman (1984).

In other trials the sown grasses and legumes Brachiaria decumbens, Brachiaria mutica, Brachiaria humidicola, Centrosema pubescens and Calopogonium mucunoides did not persist under regular grazing (Chen et al. 1978, Smith and Whiteman 1985). Ultimately they were replaced by naturalised species of lower productivity.”

Shelton et al. (1987a) stressed that a better understanding of the mechanisms by which plants are adapted to shade will assist with the recognition of improved genotypes for different plantation situations and may modify the management of mixed pastures. They suggested that further research was needed to identify the most successful adaptive characteristics which are compatible with other desired attributes such as nutritive value or nitrogen accretion. Better understanding of the basis of grass/legume competitive relationships under shade is required so that these may be manipulated by defoliation and fertilizer policies to suit management objectives such as soil fertility increase or enhanced animal intake.

Wong (1991) suggests that the identification of characteristics of shade-tolerant species, that render them persistent under frequent defoliation, may help our understanding of stability of forages in integrated plantation production systems and lead to more rational species evaluation procedures for shade tolerance.

Table 31. - Persistence (expressed as % of initial plant density) of some tropical grasses as affected by defoliation frequency under the closed canopy of oil palm

Source: Wong (1991) from Chen and Bong (1983).

2.4 Nitrogen economy of shaded pastures

The growth of pasture species is markedly dependent on the light environment, and usually, growth and quantity of light energy available are closely and positively related (Black, 1957; Shelton et al., 1987a). Herbage yield of tropical grasses usually decreases with increasing shade (Burton et al., 1959; Ludlow, 1978).

However, according to Humphreys (1991) there are now well-documented instances of pasture grasses, such as Panicum maximum var. trichoglume, exhibiting higher total biomass production under moderate levels of shade than under full sunlight (Wong and Wilson, 1980; Wong et al., 1985a; Wilson et al., 1986). Recently, Samarakoon et al. (1990a) showed that the highest yields of Stenotaphrum secundatum, Axonopus compressus and Pennisetum clandestinum occurred under shade rather than in full sun. This has occurred where N availability was limiting and is associated with higher concentrations of tissue N and higher total N and K uptake (Deinum, 1984; Eriksen and Whitney, 1981; Fleischer et al., 1984; Stritzke et al. 1976; Wilson et al. 1986). Also Chen et al. (1991) reported that at low light intensity, signal grass production was lower at a high nitrogen rate than at a lower rate (see Table 32).

Table 32. - Mean dry matter yield (kg ha^-1) of signal and guinea grass under various shade levels at 100 and 400 kg N ha^-1 yr^-1 (Chen et al., 1991).

Light Intensity	Guinea grass		Signal grass
	100 kg N	400 kg N	100 kg N	400 Kg N
100% sunlight	32,770	34,428	40,633	44,407
40 %	8,353	9,961	25,777	18,457
10 %	652	1,029	7,173	1,473
	13,925	15,139	24,528	21,446

The source of this extra nitrogen does not appear to be from additional root nitrogen fixing activity (Smith et al., 1984; Eriksen and Whitney, 1981), or from redistribution of root nitrogen to plant tops; nor is it obviously related to improved moisture status of soils under a low light environment (Wilson et al., 1986). Rather, it is hypothesized by Shelton et al., (1987a) that the available soil nitrogen levels are increased by positive effects of shade on the rate of nitrification from soil organic nitrogen sources. This theory appears to be supported by work which shows that the effect is most apparent in low nitrogen status soils and is negated when adequate fertilizer nitrogen is supplied (Eriksen and Whitney, 1981; Samarakoon, 1987; Samarakoon et al., 1990a).

Following earlier reports of better grass growth and soil fertility under tree canopies than in the adjacent open grassland, where the trees were leguminous species such as Acacia and Albizia (Kennard and Walker, 1973; Ovalle and Avendano, 1987; Lowry et al., 1988; Lowry, 1989) Wilson et al., (1990) compared the growth of the grass Paspalum notatum, under shade in a plantation of Eucalyptus grandis trees with that in full sun in an adjacent area. Dry matter yield in the summer period was 35 percent greater under the tree canopy (55 percent light transmission) than under full sunlight (even though there was a substantial reduction in radiation received by the grass understorey). During winter when the shading by the trees was more intense, the herbage yield under the trees was similar to that in full sun. Throughout the year grass under the trees had a higher proportion of green leaf, a higher concentration of nitrogen and potassium and a lower dry matter content than the grass in full sun. Wilson et al. (1990) indicated that this should have a positive influence on its nutritive value for grazing animals. They concluded that the positive growth response of P. notatum under the tree canopy is probably linked to the influence of shade on soil nitrogen availability. Under a tree canopy there may be an additional benefit to soil nitrogen from accumulated leaf drop, but this is likely to be more important under leguminous trees (Lowry et al., 1988) than under eucalyptus because of the low nitrogen content of their litter. The positive growth response of P. notatum under the trees may also be linked to longer periods of adequate water supply to the understorey grass, but if the shade response is linked to better moisture availability then it would suggest that the greatest positive effects of shade should occur in dry periods, but this was not found by Wilson et al., 1986. That this increase in grass growth under tree canopies is directly associated with the shade provided, is apparent from the fact that similar yield responses have been demonstrated under artificial shade cloth (Eriksen and Whitney, 1981; Samarakoon et al., 1990a, Wilson et al., 1986; Wong and Wilson, 1980). Recently Wilson and Wild (1991) carried out a number of experiments to test the hypothesis (Wilson, 1990) that shade (tree or artificial) increases the availability of soil nitrogen and that this leads to better growth of grass under shade than in full sun when nitrogen is a limiting factor. The data (see Table 33) clearly show that under some circumstances grass growth under shade can be significantly increased. The response occurs under conditions where growth in full sun is restricted by nitrogen deficiency. Shade increases the availability of soil nitrogen and this stimulates plant growth. That the effect resides in the soil is shown by the lack of similar response when plants were grown in solution culture.

More recently, Hongyantarachai et al., (1993) demonstrated that shade can increase the yield and N concentration of a run-down setaria (Setaria sphacelata cv. Kazungula) pasture. It was hypothesized that shade may have improved soil moisture conditions, thus increasing N availability and uptake, and also that shade may have improved water use efficiency of setaria through reduced plant water deficit.

Cruz et al., (1993) showed that nitrogen uptake and conversion efficiency of solar radiation were twice as high in shaded Dichanthium aristatum. P and K uptake were also improved. It was suggested that this was due to the better water status of the grass leaves under shade due to a reduction in the evaporative demand on the grass due to the presence of the trees.

Table 33. - Effect of shade on dry weight yield of tops (t ha^-1), leaf nitrogen concentration (%) and soil nitrate-N concentration (ppm of oven dry soil)

^a I (irrigated); NI (non-irrigated).
^b Based on cumulative yield totals over five harvests.
^c Response mainly due to increased weed growth.
^d Nitrogen concentration in the youngest fully expanded leaf of each grass speciesat harvest on 22 May 1989 (from Wilson and Wild, 1991).

Current research was summarized by Wild et al. (1993) where the “shade response” (enhanced shoot growth and higher N yield observed in N deficient tropical grass pastures when moderately shaded) was explained in terms of “a more favourable micro-climate at the soil surface/litter interface, particularly soil moisture, appeared to be responsible for improved litter break down and possibly soil mineralisation activity”.

However, that the effects of tree cover on pasture production are complex, and depend upon particular circumstances, is clear from a recent study reported by Robinson (1991) where rhodes grass (Chloris gayana cv. Samford) was grown within and adjacent to a plantation of young flooded gums (Eucalyptus grandis) in South-east Queensland. The trees reduced the production of pasture (see Table 34) thus giving a very different result from that reported by Wilson et al. (1990). It was suggested that differences in the density of tree cover, length of pasture measurement periods (cutting interval) and shade tolerance of the pasture grasses were likely sources of differences in the results. The results of Robinson agreed with those of Cameron et al. (1989) rather than Wilson et al., (1990). Part of the answer may lie with the cutting interval; Wong and Wilson (1980) found that green panic production was decreased by shade when cutting interval was short, but increased when cut after a longer interval (Robinson used a much shorter cutting interval than Wilson et al.). Clearly this is an area where more research is required.

Apart from soil enrichment under leguminous trees, Torres (1983) mentions reduced net radiation and soil temperature as well as canopy precipitation and concentration under the tree as reasons why pasture growth may be improved under tree canopies compared to surrounding open areas.

Table 34. - Pasture growth, tree canopy cover and PAR levels over two periods along transects within, bounding, and adjacent to a young flooded gum plantation (Robinson, 1991)

¹ Pasture growth values followed by different letters are significantly different (P = 0.05) within each period.
PAR = Photosynthetically active radiation.

2.5 Nutritive value of shaded pastures

There is evidence that low light intensities may adversely affect the nutritive value of forage species (Shelton et al., 1987). Deinum and Dirven (1974) reviewed the effects of temperature and light intensity on forage quality. Wilson (1982) examined light as one of the environmental and nutritional factors affecting herbage quality and summarised the effects of shade on nutritive quality as:

1. a lowering of plant soluble carbohydrate level with, usually, an accompanying increase in cell wall content (Deinum, 1966, 1984; Hight et al., 1968; Masuda, 1977; Myhr and Saebo, 1969; Samarakoon, 1987; Wilson and Wong, 1982);

2. higher silica content and lignification (Deinum and Dirven, 1972);

3. lower cell wall digestibility (Garza et al., 1965; Wilson and Wong, 1982; Wong, 1978; Deinum, 1984);

4. a decrease in the proportion of readily digested mesophyll tissue relative to the less digestible epidermis (Chabot and Chabot, 1977; Wilkinson and Beard, 1975a; Wilson, 1984);

5. accentuated stem elongation and reduced tillering;

6. an increase in tissue percentage moisture content which may reduce herbage intake by animals; and

7. crude protein may sometimes actually be higher in shaded plants.

The effect of light level on the dry matter digestibility of green panic (Panicum maximum var. trichoglume) is shown in Figure 37. However, in the same experiment Wong found no effect of shade on the dry matter digestibility of the legume Siratro (Macroptilium atropurpureum). Navarro-Chavira and McKersie (1983) determined the effect of maturity and irradiance on the nutritive value of guinea grass and Wilson and Wong (1982) and Wong and Wilson (1980) have further studied the effect of shade on the nutritive quality of green panic and Siratro. Wong et al. (1989) carried out further studies on the effects of shade (100, 60, 34 and 18% of sunlight) on dry matter production, forage quality and mineral composition of six tropical grasses in Malaysia. Common guinea and Signal grass ranked top in DM production at all shade levels and there was no significant decline in vitro dry matter digestibility (IVDMD) of the whole plant tops for all grasses except for T grass. This finding agrees with that of Deinum (1981) but is contrary to the big reduction in IVDMD in green panic reported by Wong and Wilson (1980) in Australia. Wong et al. (1989) suggest that the lack of a consistent inverse relationship between shade and IVDMD augers well for the integration of livestock with plantation crops. In addition, it was noted that the grasses under shade had a higher nitrogen/crude protein content as already reported elsewhere by Deinum et al., 1968; Eriksen and Whitney, 1981; and Wilson and Wong, 1982. A longer cutting interval reduced IVDMD.

Figure 37. - Effect of light level on dry matter digestibility of an 8-week old green panic canopy divided into 10 cm strata (Wilson, 1982 adapted from Wong, 1978).

Recently Samarakoon et al. (1990a) found that the dry mater digestibility of Axonopus compressus, Pennisetum clandestinum, and Stenotaphrum secundatum grown under shade was higher than that of herbage grown in full sun, a result contrary to much of the published literature (Wilson, 1982). However, although the increase in dry matter digestibility was up to a maximum of 5 percent units, in most instances it was only of the order of 1–3 percent units.

Norton et al. (1991) suggest that while shading reduces the total non-structural carbohydrate of grasses, it may have variable (positive and negative) effects on cell wall content and composition, lignin and in vitro digestibility of plant dry matter (Wilson, 1991), Shelton et al. (1987) quote the work of Fleischer et al., (1984) Henderson and Robinson (1982) and Samarakoon (1987) as examples of studies where the effect of decreasing light intensity on in vitro digestibility varied with grass species tested and temperature.

In the southeastern USA Burton et al. (1959) showed that reduced light (in a comparison from 100-28.8 percent available light) decreased the herbage yields, production of roots and rhizomes, nutrient reserves for regrowth and total available carbohydrates in the herbage of Cynodon dactylon. Most significant for animal nutrition was the reduction in total available carbohydrates in herbage, particularly when less than 50 percent sunlight reached the grass canopy. The resulting energy value of grass could limit rumen flora activity and affect animal output. Shade significantly increased the lignin content of the herbage thus decreasing digestibility. Therefore animals consuming forage produced under cloudy or shady sites could be expected to make less liveweight gain (Crowder and Chedda, 1982). In an early study of the effects of reduced radiation levels on forage quality, Mayland and Grunes (1974) suggested that reduced radiation levels in Idaho, Nevada and Utah would probably result in a reduction in the amount of magnesium being made available to the grazing animal (resulting in grass tetany).

There have been few studies in the past where shaded and unshaded forages were evaluated as feed for animals, but this is an area presently receiving attention.

Hight et al. (1968) in New Zealand compared shaded ryegrass (Lolium perenne) at 22 percent light transmission with unshaded ryegrass and found that shading decreased soluble carbohydrate content by 3.7 percent units, dried forage digestibility by 0.6–3.6 percent units and voluntary feed intake by 9–15 percent. Liveweight gains were reduced by 38 percent compared to sheep fed on pasture grown in full sunlight. Norton et al. (1991) suggest that the shading period (of 2–3 days) was probably too short for the results to have much relevance in terms of the interpretation of the longer-term effects of shading on tropical pastures grown under plantation crops.

Samarakoon et al. (1990b) studied the effects of much longer periods of shade (50 percent light transmission) on the nutritive value of buffalo grass (Stenotaphrum secundatum) and Kikuyu grass (Pennisetum clandestinum) for sheep. There were no significant effects of shading on digestibility (in vivo and in vitro) or cell wall composition but there was a marked depression (28–33 percent) in feed intake of sheep given shaded Kikuyu. It was suggested that the decreased intake was associated with the increased stem content of shaded Kikuyu grass, but as this effect was found in only one of the harvests Norton et al., (1991) suggest that an alternative explanation for the reduced feed intake may be decreased palatability of the feed. However, the higher yielding capacity and maintenance of nutritive quality of shaded S. secundatum (compared with shaded P. clandestinum) confirms its potential usefulness for plantation agriculture. Samarakoon et al. (1990b) suggest that its quality is not as poor as generally believed.

Norton et al. (1991) undertook further experiments to investigate the effects of shading on the voluntary feed intake and digestibility of several tropical grasses by sheep. Grasses examined were setaria (Setaria sphacelata cv. Kazungula), green panic (Panicum maximum var. trichoglume cv. Petrie), guinea grass (Panicum maximum cv. Riversdale), Signal grass (Brachiaria decumbens cv. Basilisk), buffalo grass (Stenotaphrum secundatum), bahia grass (Paspalum notatum) and a mixture of mat grass (Axonopus compressus) and sour grass (Paspalum conjugatum) grown in full sunlight and under shade ranging from 68 and 50 to 30 percent light transmission. While there was no significant effect of shading to 50 percent on the intake and digestibility of grass species, there were changes in chemical composition (especially an increase in N concentration of shaded herbage) and sheep given feed from shaded pastures had significantly higher concentrations of ammonia in rumen fluid than did sheep fed herbage from non-shaded pastures. Fermentation patterns in the rumen of sheep fed shaded pastures also changed with propionic acid levels increasing and acetic acid levels decreasing (consistent with the fermentation of more protein in the rumen).

It was expected in an on-going (incomplete) experiment, where grasses were subject to very low light levels (30 percent light transmission), that detrimental effects could be produced.

Perhaps as suggested by Samarakoon et al. (1990b) only shade-intolerant species have their quality reduced by shade, because of greatly reduced total soluble carbohydrates, greater culm elongation (increasing their comparative ‘steminess)’ and perhaps their greater susceptibility to fungal attack. This hypothesis needs further investigation through additional feeding trials with a greater range of species.

2.6 Reduction in pasture area and yield

Forage production is likely to be less under a coconut stand compared with open conditions not only because of the obvious effects of shading and competition for nutrients and moisture (see Figure 38) but due also to three other factors:

1. The space occupied by the coconut palm basal trunk and the surrounding root mass reduces the area available for pasture growth;

2. Coconut areas cultivated for sown pastures are restricted because of fear of damaging the root system; and

3. Fallen fronds may negatively affect pasture growth unless removed and burnt.

Figure 38. - Relationship between the mean dry matter yield of eight grass species and light transmission percentage under coconuts, Solomon Islands. (Based on data from Smith and Whiteman, 1983a).

Clearly these factors relate to tree density (see Figure 35). In Western Samoa measurements on a number of plantations (Reynolds, 1975) where coconut palms of approximately 20–23 years of age were spaced at 9.1 m demonstrated:

1. The ground area occupied by coconut palm trunks was 44.6 m² ha^-1. The diameter of trunks varies with age (e.g. trunk diameters at ground level were 68–69 cm for trees 16–20 years old while for those 30 years or older were 91–92 cm). Thus assuming a standard tree diameter for all palms of the same age there is a linear relationship between tree density and the area occupied by coconut palm trunks (see Figure 39).

2. The area occupied by coconut palm trunks and the non-cultivated zone on improved pasture was 327.6m² ha^-1 or about 1/32th of one hectare. Where bunch grasses like Guinea grass (Panicum maximum) are sown, the non-cultivated area remains with native species, whereas stoloniferous types like B. brizantha may spread over the zone. It has been recommended that cultivation should not be performed closer than 1.5 m from the base of the palms in order to avoid root damage (Guzman and Allo, 1975; Plucknett, 1979). This would result in an area which was either occupied by basal trunk or non-cultivated of 1,290 m² ha^-1 or about 1/8th of each ha. However, if stoloniferous species are used (they may quickly cover the non-cultivated area) and if disc ploughing turns soil towards the coconut palm from two sides the area is much reduced. Figure 40 shows the influence of cultivation method on size of non-cultivated zone.

   

   Figure 39.- Relationship between coconut palm density and basal trunk area per hectare, Western Samoa.

3. The area affected by fallen fronds (see Figure 41) was difficult to assess. Although decay and fall of fronds are a continuous process, numbers can be influenced by high wind, rain, nut collection (collectors climb palms thus dislodging fronds) and whether or not the fallen fronds are frequently collected and removed from the pasture. Fronds appear to be more of a problem in pasture areas where there are local rather than improved grasses (e.g. Guinea grass) because with the latter the frond is quickly covered and little grazing pasture is lost. Fronds not collected on native pastures could result in loss of grazing area, however this has to be accepted as part of the nutrient circulation process with a small portion of the pasture being tied up in frond decay, nutrient return and soil rebuilding. Frond burning results in nutrient loss especially of sulphur and nitrogen and slight damage to the pasture if fronds are burnt in the paddock, however, piling of fronds in lines along the centre of each interrow area and burning is a practice used in Vanuatu to provide a seed bed into which to plant improved grasses and sow or plant legumes. Measurements taken in December 1974 and February 1975 demonstrated that the area lost to grazing varied from 120– 180 m²ha^-1 on native pastures and from 75–108 m²ha^-1 on Guinea-Centro pastures. The minimum area lost to grazing could thus vary from about 120 m²ha^-1 or 1/83rd of one hectare on native pastures to about 75 m² ha^-1 or 1/133rd of one hectare on improved pastures.

   Some idea of the amount of litter likely to fall to ground can be assessed from the figures produced by Nelliat et al. (1974) for the total annual biological production ha^-1 of a coconut plantation (see Table 35).

   

   a. Two-way cultivation prior to seeding with a Panicum maximum - Centrosema pubescens mixture.

   

   b. One-way cultivation prior to the establishment of Brachiaria brizantha cuttings and sowing Centrosema pubescens seed with a hand spinner.

   Figure 40.- The influence of cultivation method on size of non-cultivated zone.

   

   a. Frond patterns on native pastures.

   

   b. Grass growth through a decaying coconut frond.

   Figure 41. - Fallen fronds, Western Samoa.

Table 35. - Total annual biological production of coconut plantation (ha^-1)¹⁾

¹⁾ Nelliat et al. (1974). Based on average yield of 100 nuts palm^-1 yr^-1, 175 palm ha^-1 and 13 leaves palm^-1 yr^-1.
²⁾ No estimate included for root production.

Thus in the average coconut plantation in Western Samoa approximately 1/60th of each hectare under native pasture and from 1/80th to 1/25th of each hectare under improved species might not be available for pasture production (see Table 36). As stoloniferous species are usually recommended under coconuts, then the appropriate figure for area lost to pasture growth is probable 120 m²ha^-1 for improved pasture or about 1/80th of each hectare).

The reduction in pasture yield by tree litter is also a problem where pastures are established under pine trees (Silva et al., 1986). In New Zealand equations have been developed (Paton, 1988) to predict the area covered by pruning and thinning slash under radiata pine (Pinus radiata). Conversely, in Queensland Australia, where Lowry (1989) reported that the tree canopy of Albizia lebbek had a positive effect on grass dry matter production, the natural seasonal fall of leaves (approx. 60 kg tree^-1), flowers (30 kg tree^-1) and pods (30 kg tree^-1) produced a significant feed resource.

2.7 Trampling and soil compaction

The problem of soil compaction has been mentioned by various authors, among them Arganoza (1991), Chen (1984), de Silva (1953), Faylon (1982), Felizardo (1973, 1975), Guzman and Allo (1975), McPaul (1964), Payne and Smith (1975), Plucknett (1979), Thomas (1978) and Whiteman (1980). According to Anon. (1982d) a 300 kg cow with a total hoof area of 194 cm² resting its weight on two hooves at any one time exerts a pressure equivalent to 1.5 kg per cm². Pearson and Ison (1987) report that the pressure exerted on grassland by cattle is 1.2–1.6 kg per cm² and that by sheep 0.8–0.95 kg per cm². It has been estimated that grazing animals tread on 0.01 ha per day although, of course, the actual area affected depends on the availability of feed, animal behaviour and the weather. Therefore cattle may cause damage to soil structure if grazed on heavy soils under very wet conditions and if high stocking rates are maintained. Hartley (1977) mentioned that in Latin America, on heavy soils in high rainfall areas, grazing under oil palm caused puddling of the soil and adversely affected the root system of the palms which require a good surface structure. In Indonesia (Thomas, 1978), where heavy textured soils and a high water table are to some extent characteristic of conditions in oil palm plantations, grazing cattle may depress fruit yields by 1–1.5 tonnes per hectare. In Malaysia Chen et al. (1991) suggest that current observation shows that the soil compaction by grazing animals does not impose a serious problem on root development of tree crops as long as there is ground cover to cushion the trampling effect of the animal and as long as the soil is not heavy clay with a high water table. However, Majid et al. (1989) noted that 15 months of grazing by sheep slightly compacted soils under rubber as a result of trampling or treading.

Good herd management with periodic cultivation are necessary requirements on such soils (whether or not coconuts are present). To avoid soil compaction during wet periods cattle should be moved to higher ground or grazed on lighter soils (Humphreys, 1978). When cattle are tethered to individual palms trampling may result in localized soil compaction.

Table 36. - Reduction of pasture growth area in coconut plantations

2.8 Cattle dung and the rhinoceros beetle

Apart from the effect of dung and urine on pasture palatability and the return of nutrients to the soil, cattle dung is an excellent breeding place for the rhinoceros beetle (Oryctes rhinoceros), one of the major coconut pests (Anon., 1982d; Chen, 1984; Payne, 1985; Sivapragasam, 1989). The beetle thrives only in undisturbed cattle dung, however, this was not a problem in Western Samoa where the beetle was present. Introduction of the dung beetle (e.g. Onthophagus gazella) assists in a more rapid amalgamation of the dung and soil (Anon. 1978d). Arganosa (1991) suggests that measures such as night corralling and periodic ploughing of the plantation are recommended to minimize the likelihood of the beetle breeding in the manure. Night corralling would reduce manure deposition in the pasture and periodic ploughing would scatter the manure. While periodic chain harrowing might be tried the other options would (a) prevent the nutrient return to the pasture which is one of the advantages of running cattle under coconuts, and (b) be expensive. While in theory cattle dung could be a breeding place for rhinoceros beetle the evidence available to the author suggests that it is not a major problem.

2.9 Coconut harvesting

In some parts of South-East Asia coconuts are harvested from the coconut palm using long poles, with a knife attached to a long bamboo pole, with bamboo ladders, by harvesters climbing the trees (Harries, 1994; Woodroof, 1970; Thampan, 1994) or trained monkeys. For example, in southern Thailand the pig-tailed macaque (Maca nemestrina leonina) is used on very tall trees where poles cannot be used (Arnold, 1984). An efficient monkey can pick 800–1000 coconuts daily (Ohler, 1984). In the Pacific Region only the green drinking nuts are usually hand harvested from the tree. Nuts for copra are allowed to ripen, fall to the ground and are periodically collected and carried to the nearby village or central processing point. On the WSTEC estates in Western Samoa (see Figure 203) donkeys are used to transport the nuts, a method which is now being introduced at smallholder level (Anon. 1986). It is generally agreed that the best quality copra is obtained from nuts which fall at maturity (Plucknett, 1979). The intercrop may reduce the actual production of coconuts because of nutrient or moisture competition, but once the nuts are produced the intercrop may also reduce the pick up percentage.

Where pastures are established under coconuts care should be taken so that coconut harvesting operations are not hindered. Locating fallen nuts in tall high yielding grasses such as Guinea (P. maximum) or Napier (Pennisetum purpureum) before they are consumed by rats, sprout or rot is very difficult. Nut collection is easier if synchronized with the rotation of cattle, so that they are collected when cattle are moved to a new paddock. Since the recommended grazing height of bunch grasses such as Guinea is about 40–60 cm (leaving a good carbohydrate reserve for rapid regrowth), it may still be difficult to locate fallen nuts in Guinea pastures. In Vanuatu some plantation managers indicated that nut collection (and access with tractor and trailer to collect nuts) was hampered when silk forage sorghum (Sorghum spp. hybrid cv. Silk) was used to provide early grazing while other species established. (See Figure 42). The preferred situation to locate fallen nuts would be a low sward formed by creeping grasses (Humphreys, 1978; Reynolds, 1978f).

Figure 42. - Collecting coconuts on Malekula in Vanuatu under 60 year old coconuts on recently established signal/legume pastures where silk sorghum was used to provide early grazing.

2.10 Cattle damage to young trees

When pastures are grazed and the coconut palms are still short (particularly when less than 5 years of age), the trees may suffer physical damage from the grazing cattle in the form of chewed and damaged fronds - see Figure 43 (Lane, 1981; Payne, 1985). When the growing point is damaged the young tree may die (Eden, 1958; Ferguson, 1907; Hugh, 1972a; McPaul, 1964; Ohler, 1969; Plucknett, 1972; Thomas, 1978; Weltje, 1966). Similar problems have been reported for oil palm, pines, E. deglupta and rubber where damage may take the form of chewing or peeling of tree bark, rubbing and nibbling of fronds and the fresh fruit bunch (Anderson et al., 1985; Chen et al., 1991; Pillai and Seeveneserajah, 1988; Pillai et al., 1985; Samuel, 1974; Shelton et al., 1987b; Tustin et al., 1979). Chen et al. (1991) noted that in a recent cattle grazing trial under oil palm at MARDI fresh fruit bunch (FFB) production was affected at the high stocking rate of 3 animals ha^-1. However, FFB was only affected when more than half of the fronds were nibbled and damaged (Othman et al., 1985a). Animals were grazing in young palms and overstocking (3 cattle ha^-1 in 4 year-old oil palm) led to 57.1 percent frond damage. Dahlan (1989) suggested that a suitable time to commence cattle grazing under oil palm is when the palms are 1.5 to 2 years old.

Figure 43. - Young coconut with fronds chewed and damaged by grazing cattle.

Experimental work on cattle repellents to protect small coconut palms has generally proven inconclusive (Anon., 1971a; de Silva, 1953), although Gregor (1972) indicates that browsing can be effectively and inexpensively discouraged with a foliar application of copper carbonate. Chen (1984) suggested using a slurry of cattle dung which Payne (1985) indicated was sprayed on young palms in Sri Lanka with some success. De Silva (1953) also suggested fencing of individual palms and in Australia and New Zealand various tree guards, polythene tubes, fencing and repellents have been suggested for protecting Monterey pines (Pinus radiata) and other trees from livestock during the early years (Knowles, 1991; Pearson et al., 1990; Reid and Wilson, 1986), but guards of the type used for temperate trees (see Figure 44) would clearly not be an economic proposition.

Guzman and Allo (1975) note that if a post and wire protection is build around each young tree, it proves to be quite a costly matter. The heaping of (coconut) husks around freshly planted trees, although inexpensive is not good for plantation hygiene. If stocks of suitable bamboo are available, the canes can be used to make a protective framework around each palm.

Figure 44. - Individual tree guard to protect the young tree from grazing cattle.

As similar problems are experienced by foresters with animals browsing young tree seedlings, various animal repellents have been developed in different countries. If severe frond damage is a problem it is suggested that reference be made to appropriate Forest Research and Extension Publications (e.g., Anon., 1988; McAdam, 1991) and work undertaken by ICRAF (Von Carlowitz and Wolf, 1991).

However, once the terminal growing point is above grazing height and adequate forage is available, then damage should be slight (Whiteman, 1980). Tree damage may be minimized by establishing a good forage stand and by moderate grazing pressure to avoid overgrazing and pasture degeneration (Chen, 1984). Lane (1981) suggests grazing sheep under coconuts after two years and then introducing cattle when the palms are about five years of age and the fronds and growing point are out of reach. Alternatively, cut-and-carry operations overcome the problems associated with grazing. In Western Samoa according to Plucknett (1979) weaner cattle are grazed successfully in four year-old palms and in Solomon Islands well fertilized young palms can be grazed after three years.

Guzman and Allo (1975) suggest that stock must not be grazed until the palms are at least 8 years old and the fronds out of reach of grazing animals. A farmer may adopt a cut-and-carry system in which the herbage is harvested and carried to the animals. In many cases, farmers do not establish pastures in a young plantation, preferring to intercrop the palms with some type of cash crop until the trees are sufficiently tall to be safe from stock.

In the dairy region of Costa Rica where alder (Alnus acuminata) is grown in grazed pastures (mostly Kikuyu grass - Pennisetum clandestinum) the young alder seedlings are usually protected from cattle for a few years by wire mesh (Budowski, 1983).

2.11 Tree spacing

The effects of plant population on the yield of tree and herbaceous crops have been reviewed by Cannell (1983). As the maximum functional leaf area in a plantation is strongly influenced by the intensity and duration of the seasonal soil water deficit (Foale, 1993) then the optimum density of palms is primarily a function of available soil water (Coomans, 1974). Although Foale (1993) does not specify actual palm spacing he indicates that to achieve high production from the long-lived coconut in monoculture “there must be a trade-off between the maximum allowable density in young palms which is set by the constraints of water supply and harvest index (i.e., the amount of copra dry matter as a proportion of the total vegetative dry matter) and the density required to achieve a reasonable yield beyond the age of 40 years.”

The coconut grower wishing to maximize pasture growth as well as yields of copra should use the least number of palms per hectare required to realize this goal (Whiteman, 1980). In Jamaica (Smith, 1972; Smith and Romney, 1969) and in the Solomon Islands (Whiteman, 1980) the reported numbers are:

200 palms ha^-1 for tall varieties (i.e. spacing about 7 × 7m),
285–300 palms ha^-1 for dwarf varieties, and
250 palms ha^-1 for hybrids.

Romney (1987) indicates that new plantings of Dwarf × Tall hybrids are usually planted at 8.5 m triangular and Dwarfs at 7 m triangular.

The subject of planting density was reviewed by Child (1974) who noted that in Jamaica triangular planting at 9 m (approx. 140 palms ha^-1) and 10 m (approx. 115 palms ha^-1) has been used as well as the closer 7 × 7 m spacing mentioned by Whitehead and Smith (1968). In Africa 160 palms ha^-1 is considered the maximum with the optimum for yield being 143 palms ha^-1 (i.e. 9 m triangular).

The decrease in light transmission resulting from a dense growth of coconut trees has led Guzman and Allo (1975) to suggest that 8 × 8 m is about the minimum spacing that should be considered for the economic establishment of improved pastures. Anon. (1982d) noted that planting density in Philippines ranges from 103–172 trees ha^-1, compared with a recommended density of 146–160 trees ha^-1. In the Solomon Islands copra production on the local tall variety declined at densities above 170 palms ha^-1 and Litscher and Whiteman (1982) suggested that high density stands should be thinned to the minimum of about 160 palms ha^-1. Steel et al. (1980) indicated that only coconut sites with light transmission above 60 percent should be planted with pastures although in Western Samoa grazing trials have demonstrated continuing good liveweight gains where light transmission was around 50 percent (Reynolds, 1981).

In Malaysia Chen et al. (1991) suggest that the optimal planting density for coconut is between 148–197 trees ha^-1.

In Tanzania spacing trials suggested that the recommended 9 × 9 m triangular planting pattern was too dense and wider spacings with corridors for food crop cultivation were adopted in trials in 1990 (Diehl, 1993).

According to Liyanage and Dassanayake (1993) the present system of planting coconuts at 8 × 8 m and the favourable environment, particularly rainfall and soil, in the Low Country Wet and Intermediate Zones in Sri Lanka makes it possible to “grow a range of crops, trees and raise livestock in association with coconut”. De Silva and Tisdell (1985) using tall cultivar coconut data from the Matara district of Sri Lanka estimated yield density functions using regression analysis. The quadratic functions showed that yield-maximizing densities were in the range 73–76 palms acre^-1 (or 180–193 palms ha^-1). The relationship between yield and density is shown in Figure 45.

In Vanuatu spacing of coconuts has varied from 7 m × 7 m to 11 m × 11 m (80 – 200 trees ha^-1) but many have used 9 m triangular spacing giving 143 trees ha^-1 (Berges et al., 1993). It was suggested that production per tree decreases at planting densities above 150 trees ha^-1.

Figure 45. - Observed Yields and Densities of Coconut Palms together with the Estimated Yield-Density Function for Coconut Palms (De Silva and Tisdell, 1985).

Duhamel (1993) mentions that production of the improved local tall planted at 143 palms ha^-1 (and well maintained) is about 2.5 t of copra ha^-1 yr^-1. “In the same conditions, several Dwarf × local tall hybrids planted at 160 palms ha^-1 are producing 3.5 to 4 t of copra ha^-1 yr^-1.” Duhamel indicates that coconuts cannot be planted above a relatively low density ha^-1 (e.g. 205 or 210 trees ha^-1 for Dwarfs and 143 to 160 trees ha^-1 for Talls).

De Taffin et al. (1992) suggest coconut densities ranging from 143 to 160 trees ha^-1, (i.e. 9.0-8.5 m triangular) in Ivory Coast, but where the grower is interested in intercropping lower densities should be used.

Friend (1990) in reviewing various planting densities used with tall and dwarf varieties concluded that the present range used for monocrop coconuts (i.e., of 150– 200 palms ha^-1) is about right and variations within this range would have little long term significance in terms of yield. These figures suggest that if traditional planting layouts are used then at around 150–180 palms ha^-1 good copra and pasture yields should be possible. However, Child (1974), Liyanage (1955) and Plucknett (1979) describe ‘group’, ‘bouquet’ and ‘hedge’ planting systems to allow space for intercrops and Manthriratna and Abeywardena (1979) have suggested that wider inter-row spacings than intra-row spacing are preferable where intercrops are grown. While traditional coconut spacings continue to be used in many areas (see Figures 46a and 46b) there is now considerable evidence to suggest that to maximize intercrop yield different planting layouts should be used. This subject is dealt with again in Chapter 10 where details are given of on-going research with different planting layouts.

What is clear is that at higher coconut tree densities, light transmission, forage production and carrying capacity will be considerably reduced. Thus the goal should be to balance the system so that maximum returns per hectare in terms of copra and cattle products are obtained on a continuing basis.

Friend (1990) concluded that in intercropping there is usually a compromise (in spacing) to optimize returns from the various combinations.

Minimum recommended coconut spacings and maximum coconut planting densities for pasture establishment are summarized in Table 37.

Table 37. - Minimum recommended spacing and maximum planting density for coconut palms (tall variety) for pasture establishment

Figure 46a. - Widely space 60 year old coconuts on Malekula, Vanuatu (with young coconuts inter-planted).

Figure 46b. - 60 year old coconuts at 9 × 9 m spacing, Malekula, Vanuatu.