10. RECENT DEVELOPMENTS AND POSSIBLE FUTURE TRENDS (contd.)

10.6.1 Screening of new forage species for shade tolerance and persistence

Key problems identified by Stur and Shelton (1991) include:

1. The lack of species available for reduced light situations (especially <30% light transmission).

   “There is quite a range of species naturalised under the various environmental regimes of plantation crops. However, no species can be recommended for light levels of less than 30% because of the limited production potential of very low-light environments. In plantations with light transmissions of 30– 50%, species such as Axonopus compressus, Stenotaphrum secundatum, Ischaemum aristatum and Desmodium heterophyllum may be suitable. Only when light levels are higher than 50% do more productive species warrant consideration.”

2. The need for a greater range of grasses and legumes which will persist and contribute to animal production in low management and input situations.

   “At low management levels (high stocking pressure, no fertilizer, etc.), persistence and suppression of weeds usually requires an aggressive grass such as Stenotaphrum secundatum. Unfortunately, the ability to suppress weed growth usually means incompatibility with most useful herbaceous legumes. The most successful herbaceous legumes for combining with aggressive grasses are Desmodium heterophyllum and D. triflorum. Tree legumes may also play an important role in improving the feeding value of such pastures. Under higher levels of management, excellent levels of animal production can be achieved with highly productive sown grass/legume swards, particularly at light levels of 70% and above.

   Despite the plethora of naturally occurring species available for reduced light situations, a greater range of grasses and legumes is required which will persist and contribute to production in low management and input systems.”

Wong (1991) suggests that persistent shade-tolerant species are needed for the long-term productivity and economic viability of integrated land-use systems. Species that show high productivity and forage quality in a wide range of light levels (90 to 20% of full sunlight) are desirable but are presently not available.

As part of the ACIAR forage programme (No. 8560 and 9113) a total of 130 grass and legume accessions were screened for shade tolerance at the University of Queensland research station at Redland Bay. Grown under shade cloth at 5 light levels (100, 70, 50, 35 and 20%) plots were harvested every six weeks for three growth cycles for a period of more than a year. The yield and relative yield of the 30 highest-yielding accessions for the 50% and 20% light transmission treatments are shown in Tables 262 and 263. According to Stur (1991) the 50% treatment approximates the light level experienced in coconut plantations, while the 20% light transmission treatment is similar to the light level in mature plantations, while the 20% light transmission treatment is similar to the light level in mature rubber plantations. The tables also show the relative yield ranking of each species relative to all 46 grasses and 84 legumes in the experiment, not just the species presented in the tables.

Stur (1991) indicates:

50% light transmission

Grasses - “many of the commercially used tropical pasture grasses such as the Panicum maximum cultivars had the highest dry matter yields (Table 262). These were tall, upright species which would be expected to have high yields in full sun. The highest yielding species of the lower growing grasses was Paspalum wettsteinii. Grasses with an above average yield (>166 g) and an above-average relative yield (>68%) were Paspalum malacophyllum, Urochloa stolonifera, Paspalum wettsteinii, Digitaria natalensis and Paspalum conjugatum. With the exception of Paspalum conjugatum, these species have not been widely used under coconuts. The species with the highest relative yields (i.e., Axonopus compressus and Paspalum conjugatum) are known to be shade-tolerant pasture grasses and successful in coconut plantations (Shelton et al., 1987; Reynolds, 1988).”

Legumes - “As with the grasses, the highest-yielding legumes tended to be the more upright species such as Aeschynomene americana, Desmodium intortum and Mucuna sp. (Table 262). All of the 30 legume species listed in Table 262 had a higher than average yield (> 47 g) and many also had a higher than average relative yield (>60%). The five highest yielding species were Aeschynomene americana cv. Glenn, Desmodium intortum cv. Greenleaf, Vigna luteola cv. Dalrymple, Stylosanthes humilis cv. Gordon and Stylosanthes humilis (commercial). Stylosanthes guianensis cultivars have been regarded as having a low shade tolerance (Shelton et al. 1987a) but it appears that variation in shade tolerance exists between species of the same genus. Other legumes with a relative yield of more than 80% included Rynchosia minima, Centrosema macrocarpum and Desmodium heterophyllum cv. Johnstone. Of these species, Desmodium heterophyllum is known to be a shade-tolerant species (Reynolds, 1988; Shelton et al. 1987a;).”

20% light transmission

Grasses - “The yield ranking of grasses was similar to the yield ranking at 50% light transmission. The Panicum maximum cultivars remained the highest-yielding grasses at this low light level (Table 263). Species which improved their yield ranking substantially were Brachiaria humidicola cv. Tully, Dichanthium aristatum cv. Angleton, Axonopus compressus, Digitaria pentzii CPI 59772 and Acroceras macrum. Of the grasses with an above-average yield only Brachiaria humidicola cv. Tully, Paspalum malacophyllum, Digitaria milanjiana CPI 41192 and Axonopus compressus had a higher than average relative yield. Grasses which had a substantially lower relative yield at 20% than at 50% light transmission were Setaria sphacelata cv. Kazungula, Setaria sphacelata cv. Splenda and Urochloa stolonifera.”

Legumes - “The yield ranking of legumes changed to a greater extent than that of the grasses as light transmission was reduced from 50 to 20% (Table 263). Legumes which had a much lower yield ranking at 20 than at 50% light transmission included Aeschynomene americana cv. Glenn (see Figure 241), Aeschynomene americana CPI 56283, Stylosanthes humilis cv. Gordon and Stylosanthes humilis (commercial) indicating low tolerance of heavy shade.

Conversely, accessions which improved their yield ranking by 20 ranks or more were Arachis pintoi cv. Amarillo, Desmodium heterophyllum CPI 52420, Desmodum gangeticum, Centrosema pascuorum (see Figure 242), Neonotonia wightii cv. Tinaroo, Teramnus labialis cv. Semilla Clara and Desmodium heterocarpo cv. Florida. Many of the legumes with an above-average yield also has an above-average relative yield. The legumes with the highest relative yields and above-average absolute yields were Desmodium intortum cv. Greenleaf, Macroptilium atropurpureum cv. Siratro, Calopogonium mucunoides, Arachis pintoi cv. Amarillo, Desmodium heterophyllum CPI 52420, Centrosema pubescens (common) and Desmodium heterophyllum cv. Johnstone.”

Table 262. - Dry matter yield of forages grown at 50% (g plot¹) and their yield at 50% light relative to their mean yield at 100 and 70% light (%) - the 30 highest yielding accessions (After Stur, 1991).

Table 263. - Dry matter yield of forages grown at 20% (g plot¹) and the yield at light level relative to the mean yield at 100 and 70% light (%) - the 30 highest yielding accessions (After Stur, 1991).

Although it was noticeable that there was considerable variability between species of the same genus and also there were differences between accessions of the some species, the experiment identified a number of grass and legume accessions which have shown a high yielding and high relative yielding capacity at 50 and 20% light transmission. Many of these shade tolerant species have not previously been used or have not been widely used commercially and need to be tested under plantation conditions taking into account particular environmental and socio-economic conditions and likely management levels. This is one of the areas where future work is required.

Figure 241. - Aeschynomene americana cv. Glenn.

At the same time as the University of Queensland screening trials, a series of trials were undertaken under coconut in Bali (Rika et al., 1991; Mendra et al., 1995) and North Sulawesi (Kaligis and Sumolang, 1991; Kaligis et al., 1995) and under rubber in Malaysia (Ng, 1991) to evaluate the performance of 41 grass and 46 legume species selected for their assumed shade tolerance (Shelton and Mullen, 1994). The objective was to identify new forage genotypes which are superior in their persistence of yield compared to existing species. In Indonesia species are required for cut and carry and for intensively grazed sites and to extend the growing season. In Malaysia, species are needed which are adapted to the declining light environment of maturing rubber and which are suitable for grazing by sheep. Shade screening trials have been carried out by MARDI (see Figure 243). Light transmission was 58% in Bali, 73% in North Sulawesi and initial values at two sites in Malaysia were 90% and 65% declining to 50% and 19% respectively.

Figure 242. - Centrosema pascuorum (Photo D. MacFarlane).

Figure 243. - Shade screening of various grasses and legumes, MARDI, Malaysia.

Although a number of species gave high yields initially, their yields declined sharply in later harvests and medium and low-growing species tended to be more resistant to more regular defoliation than the more upright species. Species which showed both good growth and persistence (although initially low yielding) over the full trial period of up to 11 harvests in Bali and North Sulawesi were the legumes Arachis pintoi CPI 58113, Arachis repens CPI 28273, Arachis sp. CPI 12121 and 29986, Desmodium ovalifolium and D. heterophyllum, and the grasses Paspalum notatum cv. Competidor, P. notatoum CPI 11864, P. wettsteinii, Axonopus compressus (local) and Digitaria milanjiana CPI 59721. Kaligis and Sumolang (1991) noted that these species showed generally good seedling vigour and resistance to diseases and insects, provided a good ground cover and indicated an ability to compete with weeds. Because of these characteristics it was predicted that these species would be well adapted to the grazing systems practised under coconuts in North Sulawesi. For further details of both Paspalum notatum (Bahia grass) and P. wettsteinii (Broad-leaf paspalum) reference should be made to Bogdan (1977) and Skerman and Riveros (1990). It is interesting that Skerman and Riveros (1990) suggest that P. notatum does not grow well in shade.

In Malaysia over only 6 harvests species which showed good regrowth and persistence under the declining light environment of maturing rubber were the grasses Panicum maximum cv. Riversdale, P. maximum cv. Vencedor, Brachiaria brizantha, B. humidicola, B. dictyoneura and Paspalum notatum CPI 11864, and the legumes Stylosanthes scabra cv. Seca and S. guianensis CIAT 184. Other species which were lower yielding but which showed promise were the grasses Paspalum wettstenii and Stenotaphrum secundatum and some Arachis spp.

It was concluded that further testing in grass legume combinations and on farms was required. Some of the Arachis species looked particularly interesting. In fact small grazing experiments are underway at two sites in Indonesia and one in Malaysia to gather management and production data from a number of ‘best bet systems’ (Shelton, personal communication). Much of this work will be summarized in the proceedings of the round-up meeting of the ACIAR funded project (Shelton and Mullen, 1995).

Shelton (1993) suggests that criteria for selection of species to be included in well grazed pastures under coconuts would include:

• Shade tolerance in both grass and legume;

• prostrate habit for ease of coconut collection;

• vigorous sward-forming grasses to provide weed control and botanical stability;

• competitive and vigorous legumes to provide a protein supplement to grazing animals.

A series of recent trials demonstrated the following shade tolerant species.

Grasses:

Stenotaphrum secundatum demonstrates both shade tolerance and persistence and is the most suitable grass species studied to-date.

Paspalum notatum cv. Competidor (see Figure 244) shows promise as a shade tolerant and persistent grass but is slow to establish from cuttings and therefore initially is weedy. Mendra et al. (1995) noted that its yield persistence and sward legume content was variable. However, Firth and Wilson (1995) suggested that a species selection programme for orchard ground cover in New South Wales, Australia, “should empahsize competitive low maintenance legumes such as Arachis pintoi × Arachis repens hybrids and dwarf grasses such as short-leafed forms of Paspalum notatum”.

Paspalum wettsteinii (see Figure 245), Paspalum malacophyllum and Brachiaria decumbens though shade tolerant and initially productive, did not persist under sustained grazing. Mendra et al. (1995) found that legumes combined poorly with P. malacophyllum and that its erect habit resulted in bare ground between tufts. “This characteristic may make it unsuitable for use under coconuts”. Wong and Stur (1993) found P. wettsteinii to be prostrate and aggressive but intolerant of severe defoliation under heavily shaded conditions. P. wettsteini also demonstrated poor drought tolerance (Mendra et al., 1995).

Axonopus compressus shows good shade tolerance but has poor vigour and is therefore prone to weediness.

Figure 244. - Paspalum notatum with A. pintoi (Photo B. Mullen).

Figure 245. - Paspalum wettsteinii (Photo B. Mullen).

Legumes:

Arachis species are outstanding. A. pintoi and A. repens (see Figure 246) were quick to establish, vigorous, and maintained a high legume content provided pastures were regularly defoliated. Although not the highest yielders (see Table 264) they were sufficiently competitive even with more vigorous grasses to maintain a high legume content in frequently defoliated plots. A. glabrata (see Figure 247) while slower to establish, has performed well under coconuts in Bali. A. glabrata is a rhizomatous species which must be established vegetatively, but which is very persistent.

Desmodium ovalifolium and D. heterophyllum have also shown promise under shade.

Table 264. - Performance of some forage Arachis accessions under coconuts in Bali and Manado, and under rubber plantations in Malaysia (after Stur and Ndikumana, 1993, from Shelton, 1993).

LT = Light Transmission.

Figure 246. - Arachis repens.

Figure 247. - Arachis glabrata (Photo B. Mullen).

Calliandra calothyrsus has been established with Buffalo grass in Bali and North Sulawesi and is being grazed in plots. According to Yuhaeni and Ivory (1994) this tree legume is high yielding in several sites in Indonesia and Mendra et al. (1995) suggest that it shows promise as a productive cut-and-carry tree legume. However, according to Mullen (personal communication) in the grazing trials in Bali and North Sulawesi it has demonstrated intolerance to heavy grazing pressure.

Recently Wong and Stur (1993a, 1993b) reported on an experiment to examine the effect of 2- and 4-weekly cuttings at a 5 cm cutting height on the regrowth of two shade-tolerant grasses (Paspalum malacophyllum and Paspalum wettsteinii) grown in 100, 50 and 20% ambient light over a 12 week period (3 cycles of 4 weekly interval). Total dry matter was reduced significantly (P < 0.01) by shading in both species, the magnitude of reduction being directly proportional to the intensity of shade as illustrated by the highly significant r² of linear regressions of total dry water of the two grasses on light transmission percentage (see Figure 248) in the 2- and 4-weekly defoliation frequencies. Paspalum wettsteinii produced a higher root yield while P. malacophyllum (see Figure 249) had a higher shoot component. This differential allocation of dry matter resulted in a higher shoot/root ratio and tiller number in P. malacophyllum. Leaf area indices and light interception were not significantly different between the two species except in the 4-weekly cutting interval (Table 265). Relative growth rate of the two species declined with shading but was higher in P. malacophyllum. Frequent defoliation in increasing shade intensity reduced stubble total non structural carbohydrates in both species and increased plant mortality, particularly in P. wettsteinii. The ability of P. malacophyllum to tolerate frequent defoliation in shade was attributed to its higher tillering capacity and stubble TNC pool for regeneration.

In Thailand, a series of screening trials were undertaken under rubber (and coconut) near Pikun Thong Research Centre, near Narathiwat (see Figure 250).

In Vanuatu, Macfarlane (1993) stressed that while not ready for release promising research results were being obtained for non-commercial accessions of Aeschynomene villosa, A. americana cv. Lee, Leucaena leucocephala × L. pallida, Calliandra calothyrsus, Indigofera spicata, Arachis glabrata, A. pintoi, A. repens and Digitaria milanjiana.

Figure 248. - Linear regression of TDM (biomass) yield of P. malacophyllum (PM) and P. wettsteinii (PW) over three regrowth cycles on light transmission (%). (Wong and Stur, 1993a, 1993b).

Figure 249. - Paspalum malacophyllum (Photo W. Stür).