Sorghum xdrummondii (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Sorghum ×drummondii (Steud.) Millsp. & Chase

Protologue: Publ. Field Columb. Mus. Bot. 3: 21 (1903); Flora of Trop. E. Africa. Gram. (3): 726 (1982).

Family: Gramineae

Chromosome number: 2n= 20 (counts of 40 are also reported, but require confirmation)

Synonyms

Sorghum sudanense (Piper) Stapf (1917), S. bicolor (L.) Moench ssp. drummondii (Steud.) de Wet (1978).

Vernacular names

• Sudan grass (En). Pasto Sudan (Sp)
• Philippines: batag (Tagalog), bukakau (Ilokano), layagah (Sulu)
• Thailand: ya-sudan.

Origin and geographic distribution

What is known commercially as Sudan grass is a segregate from a natural hybrid between S. bicolor (L.) Moench and S. arundinaceum (Desv.) Stapf. This hybrid combination is responsible for several other "species" which are, however, unstable, and revert to one or other of their parental types. They have all been included in the binomial S. × drummondii in the Flora of Tropical East Africa. The hybrid originated in the region from southern Egypt to the Sudan, but has rarely been collected there. It was introduced to the United States in 1909 and rapidly became popular as a forage, later being evaluated and sown in other regions with warm and dry growing season.

Uses

Sudan grass is used as an annual forage for ruminants, but more commonly as a parent in a wide range of inter-specific F₁hybrids in which S. bicolor, grain sorghum, is the alternative parent. Examples of such hybrids are "Sudax", "Zulu" and "Bantu". In Thailand, sorghum × sudan hybrids are utilized as fresh cut-and-carry forages or as hay for dairy cattle or water buffaloes.

Properties

Nitrogen concentrations can be as high as 3%, although levels fall with increasing maturity. Sudan grass is very palatable to livestock so the level of utilization is high and cattle are reported to consume 80% of the forage at panicle emergence. In common with other sorghum species, young herbage can accumulate quite high levels of HCN which may lead to prussic acid poisoning in livestock. This is rarely a serious problem, except where high levels of N fertilizer have been applied or where the grass is wilted. Prussic acid poisoning is considered to be less of a problem with Sudan grass than it is with either fodder sorghum or sorghum-Sudan hybrids. There are 90-120 seeds/g.

Botany

Annual with erect stems to 3 m tall, 3-9 mm thick. Leaf-blade lanceolate, 30-60 cm × 8-15 mm. Inflorescence an open pyramidal panicle with secondary and sometimes tertiary branches which end in short fragile racemes, which do not readily break up at maturity; spikelets paired; sessile spikelet 6-7 mm long; glumes loosely hairy, shiny and almost hairless when mature; upper lemma with an awn up to 16 mm long; pedicelled spikelet about as long as the sessile spikelet, but narrower. Caryopsis variable, enclosed by the glumes.

Seedlings emerge 5-6 days after sowing. Flowering in Sudan grass is to some extent photoperiod-sensitive, but in the tropics it flowers freely. As the racemes do not break up easily, seed production of Sudan grass is easier than it is with many tropical grasses, even though flowering within individual plants is poorly synchronized. A number of "cultivars" which are marketed as Sudan grass are, in fact, hybrids with other species. There are relatively few cultivars available within S. × drummondii as such, but considerable numbers of open-pollinated and F₁hybrids have been developed. Examples, and their parentage, are: "Tift" (Sudan grass × "Leoti" sweet sorghum) × Sudan grass; "Piper" ("Tift" × Sudan grass); "Greenleaf" ("Leoti" sweet sorghum × Sudan grass); "Lahoma" (Sudan grass × "Leoti" sweet sorghum); "Sucro"(perennial) ( S. × almum × perennial sweet Sudan grass); "Sudax"(F₁) (male sterile grain sorghum × Sudan grass); "Zulu"(F₁) (male sterile grain sorghum × "Greenleaf") (see above); "Bantu"(F₁) (male sterile grain sorghum × "Piper") (see above).

Ecology

Sudan grass is not adapted to the humid tropics, but is suited to warm conditions with low humidity and an average annual rainfall of 600-900 mm. It is intolerant of waterlogging but has reasonable tolerance of salinity.

Agronomy

Sudan grass should be sown in a well-prepared seed-bed, either broadcast or preferably drilled in rows 25-50 cm apart (wider in drier climates) and not deeper than 2.5 cm, at seeding rates of 8-12 kg/ha when drilled and 12-16 when broadcast. Higher rates have been recommended in more humid areas. It is normally grown as a pure crop, although companion legumes such as Glycine max (L.) Merrill and Vigna unguiculata (L.) Walp. have been used. It responds well to irrigation in dry climates. Relatively low levels of fertilizer are normally used, but it responds to N, P and K fertilizers where these nutrients are deficient in the soil.

Cultivars differ in resistance to leaf diseases. These diseases are of greater concern where the crop is grown under conditions of higher humidity and rainfall than it is normally suited to.

Sudan grass may be grazed, chopped and fed directly, or made into hay or silage. As stems are thin, compared with the perennial sorghum species, it dries quickly, which is an advantage when hay-making. Yields of green fodder average 20-40 t/ha but may be twice as high with optimal fertilizer use. Milk production may be low if sulphur is limited in Sudan grass forage.

Genetic resources and breeding

Genetic resources within S. × drummondii are limited, but there is a considerable genetic resource associated with closely related species with which it can be hybridized. Limited germplasm is maintained under the binomial S. sudanense at ATFGRC (CSIRO, Australia). More extensive collections of S. × drummondii and related hybrids are held by the USDA (Fort Collins, Colorado and Beltsville, Maryland) and ICRISAT (India).

Prospects

In a number of countries, commercial companies have a major interest in production of F₁hybrid forages, and it can be anticipated that there will be a continuing flow of new hybrid cultivars with improved characteristics. As with existing cultivars, these are likely to be better adapted to the less humid parts of South-East Asia.

Literature

• Bogdan, A.V., 1977. Tropical pasture and fodder plants. Longman, London. pp. 275-280.
• Clayton, W.D. & Renvoize, S.A., 1982. Gramineae (Part 3). In: Polhill, R.M. (Editor): Flora of tropical East Africa. A.A. Balkema, Rotterdam. p. 726.
• Oram, R.N., 1990. Register of Australian herbage plant cultivars. CSIRO, Australia. pp. 61-66.
• Skerman, P.J. & Riveros, F., 1990. Tropical grasses. FAO, Rome. pp. 690-694.
• Stobbs, T.H. & Wheeler, J.L., 1977. Response by lactating cows grazing sorghum to sulphur supplementation. Tropical Agriculture, Trinidad 54: 229-234.

Authors

J.B. Hacker