Stylosanthes scabra Vog.

Leguminosae

Synonyms

Diarthra Blake; S. gloiodes Blake; S. plicata Blake.  

Shrubby stylo (Australia), Capitan Juan and pata de terecay (Venezuela).

A tall (up to 2 m), shrubby, perennial legume, erect to suberect, with strong, woody stems, usually densely hairy and viscid, making them sticky under hot, dry conditions. Stems vary in colour from pale green to dark blue-green, brown or reddish depending on the strain. Leaves trifoliate. Leaflets hairy on both surfaces, varying in shape from elliptic to oblong-lanceolate and in colour from pale green to dark green and dark blue-green. Stipules obovate, bristly hairy, 15 to 25 mm long including teeth. Inflorescence a broad, loosely formed and shortly elongate spike, occurring singly in the leaf axils. Fruit a biarticulating pod, the upper articulation carrying a short, slightly curved beak (residual stigma), and short viscid hairs. Seeds small (up to 2 mm long), pale to light brown, unsymmetrically reniform, the radical ends prominent (Mohlenbrock, 1957).

S. scabra is a South American species, widely distributed across Brazil, especially in Minas Gerais, Pernambuco, Ceara, Piaui and Goias, to eastern Bolivia, and in a band across Venezuela and Colombia to Ecuador (Williams et al., 1984).
'T Mannetje (1984) regards S. scabra as a problematic species that possibly should also include the presently separate S. tuberculata Blake and S. nervosa Macbr., and perhaps several others.

S. scabra is a summer-growing perennial with some strains possessing slight frost tolerance, giving them a long growing season. The tops are tolerant of light frosts and the crowns of quite heavy frosts.

Grows from sea level to c. 600 m.

Requirements have yet to be fully determined . Upper limits will be determined by disease spectrum and resistances; the lower limit could be as low as 500 mm. S. scabra is currently planted to the 600-mm isohyet in Queensland.

S. scabra is very drought tolerant; plants that have received no rain since the previous April will shoot with the rising temperatures in September in northern Australia's dry tropics. The very deeply penetrating taproot stands it in good stead in this regard. Leaf tissue is also able to survive much lower water potentials than other species before it dies (Williams and Gardener, 1984).

Cv. Seca, at least, is not very tolerant; its dry-matter production was only 38 percent of the control when subjected to artificial waterlogging (Williams and Gardener, 1984). Some variation between accessions exists (Edye, Grof and Walker, 1984).

While best adapted to infertile, acid, sandy-surfaced soil, especially solodics, shrubby stylo is more versatile in its soil requirements than most stylos and at times will grow in heavier textured, less acid soils. Such results have been variable and planting on such soils is currently not recommended.

Spreads readily and widely on some soil types. especially sandy surfaced solodics, but spread on soils with hard-setting surfaces has, at times, been disappointing.

In the seasonally dry tropics of northern Australia, shrubby stylos can be established satisfactorily by surface broadcasting into the burnt native grass swards. On these predominantly infertile soils, the competition to establishing seedlings is weak. On better soils, limited seed-bed preparation is necessary to reduce grass competition. Shrubby stylo seedlings tend to be weak and slow growing during the first season.

Suited to aerial seeding or ground spreading, depending on the sophistication of the farming system.

In the dry tropics, S. scabra is sown just prior to the break of season at 1 to 2 kg./ha. Further south, in Queensland, it is generally sown somewhat later (into January), nearer the high-rainfall periods, and at 2 kg./ha. In central Queensland, Wildin and Middleton (1984) recommend burning in November-December and oversowing after a fall of at least 30 mm rain.

400 000 to 500 000 in the pod and 600 000 to 800 000 clean seed.

Hard-seed levels are invariably high in freshly ripened seed. Little germination can be expected before these seeds either soften naturally or are treated.

In the seasonally dry tropics of northern Australia, seed planted on the surface of a recently burnt area will soften rapidly under the influence of a hot sun. It is generally ready to germinate after the first rain. Further south, the rate of softening is slower, and it is normal to scarify half the sample by depodding prior to planting. This gives some seed ready for immediate germination while preserving some hard seed as an insurance against false germinations.

While able to establish and persist on quite low phosphate soils, the shrubby stylos respond readily to applied phosphorus. Not only is dry-matter yield increased but phosphorus content of the forage available is also increased substantially. Sulphur and potassium responses are also regularly obtained.

Once established, S. scabra is a very strong competitor, able to persist with most companion species, including buffel grass (Cenchrus ciliaris), on the texture-contrast brigalow soils.

Herbicides are not used in extensive field stands, but for seed production, trifluralin and 2,4-D are safe and widely effective in establishment. The shrubby stylos have the wide tolerance of herbicides common to most Stylosanthes spp.

Seedlings are very slow growing for the first season, but are particularly hardy and able to establish unless grass competition is strong.

Regrowth following grazing is normally from buds along the aerial stems, which, being hard and woody, are rarely removed. When mown, strong and rapid regrowth occurs from crown buds at or even slightly below ground level.

Normally, S. scabra is continuously but lightly grazed. Under extensive conditions, animals tend to ignore the shrubby stylos until late in the growing season.

Fire kills most aerial stems, but regrowth rapidly occurs from the low-set crown or from root tissue down to 38 mm below the soil surface at the beginning of the next wet season (Gardener, 1980). Mott (1982) found that three-year-old plants in northern Australia failed to survive at all.

Dry-matter yields in cutting trials have varied from less than 1 tonne/ha on a solodic soil to more than 9 tonnes/ha on deep red earths and yellow earths in high-rainfall areas (Edye, Grof and Walker, 1984).

S. scabra is not currently made into silage or hay, but regrowth on regularly harvested experimental plots suggests that, if cut early, before the stems begin to harden, useful hay could be made.

Few analyses are available, but Little, McIvor and McLean (1984) recorded a dry-matter digestibility of 70 percent for a 59 g/day intake of S. scabra grown with irrigation and 250 kg./ha superphosphate. They also record N content of 1.5 to 2.0 percent and P of 0.08 to 0.11 percent, both of which were little affected by superphosphate fertilization in mature herbage.

There is no record of toxicity to livestock, but acaricidal properties have been recorded for a number of accessions, including 'Seca' and 'Fitzroy' (Sutherst, Jones and Schnitzerling, 1982).

A perennial, the seed stands are retained for as long as possible, but usually require renovation after about four years as old plants tend to become moribund. Harvesting is by direct heading after the ripening crop has begun to shed seed. At this stage, the inflorescences are dry and beginning to disintegrate and the crop is less sticky than it would have been earlier. Suction harvesting is possible, but rarely used (J.M. Hopkinson, personal communication).

Mean seed yields for cv. Seca from nine crops at Walkamin, north Queensland, between 1973 and 1978 were 180 kg./ha off the header (yields ranged from 550 kg./ha to virtual failure) (Thompson and de Medeiros, 1981).

The collection available in Australia in the early 1970s contained four morphological/agronomic groups (Edye, Grof and Walker, 1984). Representatives of two of these, cvs Seca and Fitzroy, have come into use. Seca is a late flowering, strongly erect, rather open bush with complete initial resistance to anthracnose (Colletotrichum gloeosporioides).
'Fitzroy', a denser, bushier and agronomically more attractive mid-season-flowering type, proved highly susceptible to anthracnose when it appeared and was restricted in use to lower-rainfall (less than 750 mm) habitats and higher latitudes (greater than 22°S). However, this is now breaking down. Earlier-flowering types are also available if anthracnose-resistant types cannot be located.

Anthracnose is endemic in the natural habitats of the shrubby stylos in South America, but was first identified in Australia only in 1974, as cvs Seca and Fitzroy were about to be released. The present situation is extremely dynamic. In Australia, the only other serious disease is Botrytis head blight (Botrytis cinerea), which can be a problem in seed crops in years with overcast weather during flowering.

S. scabra is widely adapted and hardy, well suited to low-fertility soils and extensive stock-raising conditions, and possesses good drought, adequate fire and some frost resistance. The shrubby stylos were showing considerable promise before anthracnose arrived on the scene. There were still a number of unanswered questions, including their effect on livestock performance on low-fertility soils in the absence of applied phosphate fertilizer.

The most serious defect of S. scabra is proving to be its susceptibility to anthracnose. It has also proved, on low fertility soils at least, to provide a very poor quality forage that is especially low in phosphorus. Palatability has also been low. The value of S. scabra is restricted to low-fertility soils and extensive land-use situations.

S. scabra nodulates readily with a wide spectrum of rhizobium strains. Specific inoculation is not required in northern Australia.

Burt et al. (1983); Edye, Grof and Walker (1984).

S. scabra occurs naturally at least as far south as the Tropic of Capricorn, while cv. Fitzroy has performed well as far south as 24°S, near Bundaberg in Queensland, Australia. Earlier-flowering lines should extend the limit further south if released there. At Lansdown Research Station, north Queensland, flowering dates varied from mid-January to mid-May (Edye, Grof and Walker, 1984).

Once established, S. scabra is well able to hold its own against most weed species, especially on the less fertile soils where shrubby stylos are most suited for use.

The larvae of a native Cerambycidae, (Platyomopsis pedicornis) (F), tunnelled into the lower main stems and killed all plants, usually after they had been weakened by fire, in several plantings in north-western Queensland (Hall, 1980). In Colombia, stem borers of Caloptilia spp. attack the species (CIAT, 1978).

The flowering response of S. scabra to photoperiod is apparently unclear. 'Seca' was classified as a short-day plant in controlled environments (Cameron and 't Mannetje, 1977) but other factors also may operate in the field (Ison and Humphreys, 1984).

According to Stace (1982), shrubby stylo is normally a true breeding fixed heterozygote, an allotetraploid with 2n = 40 chromosomes, 98.2 percent self-pollinating and 1.8 percent outbreeding.