Sporobolus africanus
(rat’s tail grass)

Sporobolus africanus is an invasive tussock grass native to sub-Saharan Africa. It is a grass of low palatability and regarded in Australia as a very serious and declared weed. It is dispersed very easily by several mechanisms and once established it can quickly dominate existing pastures, causing loss of productivity and reduced land values. Its presence may also be an indicator of reduced soil fertility and pasture mismanagement. Control on extensively grazed properties is problematic and every effort must be made to prevent its introduction to clean properties.

S. africanus originates in Africa, where it is found from South Africa, through East Africa to Ethiopia (van Oudtshoorn, 1999). It has been introduced into North and South America, southern Asia, Indonesia, Hawaii, the South Pacific islands, New Zealand and Australia (Parsons and Cuthbertson, 2001; USDA-ARS, 2016). In Australia it has a mainly coastal distribution stretching from Brisbane (Queensland) to Adelaide (South Australia), also occurring in Tasmania, coastal areas around Perth (Western Australia) and inland Northern Territory (Mallett, 2005). In Hawaii, S. africanus is naturalized on the main islands of Kauai, Oahu, Molokai, Lanai, Maui, Kahoolawe and Hawaii (Wagner et al., 1990, 1999).

The risk of introduction of this plant should be considered high because there are several effective dispersal mechanisms for it. International movement would be more likely as a contaminant in pasture seed but could also occur through movement of machinery, fodder, livestock or animal skins. Regionally, as well as these mechanisms, dispersal can be effected by water, wild animals, vehicles and even people’s clothing.

S. africanus is generally found in the tropics and subtropics, occasionally in temperate regions, with annual rainfalls of at least 700 mm (Campbell et al., 1999). In East Africa it has been found in medium altitude and montane woodland and forest, always in disturbed areas, roadsides and in grazed areas (Friis and Vollesen, 2005). In South Africa, it is found in moist, cool-temperate grasslands (Werger and Coetzee, 1978).

At least two bioclimatic models have been generated: a BIOCLIM model based on New Zealand data (Campbell et al., 1999) and a CLIMEX model reported by Walton (2001) and Palmer (2012). S. africanus is reported from a wide range of soil types, including alluvial and volcanic soils (Walton, 2001). In New Zealand it is associated with steeper land and lower fertility soils (Campbell et al., 1996). In Australia it has become a widespread and common weed of roadsides, recreation areas and wasteland (Australian Weeds Committee, 2016).

Genetics

The genus Sporobolus is cytogenetically complex and basic chromosome numbers of x = 6, 9 and 10 are present. Reported haploid chromosome numbers of n = 12, 18, 24, 24, 30, 36 or 48 for S. africanus (Spies et al., 1991; Simon and Jacobs, 1999) suggest a basic chromosome number of x = 6 for this species. Using a novel RAPD-PCR technique for generating genetic markers to differentiate Sporobolus species in Australia, Shrestha et al. (2005) found that S. africanus exhibited low within-species genetic diversity compared to S. natalensis, S. fertilis, S. sessilis, S. elongatus and S. laxus, which had high genetic diversity.

Reproductive Biology

This grass is a prolific seeder and is capable of producing up to 3600 seeds/m² per year. The mucilaginous pericarp of the seed, a feature common to all Sporobolus spp., is an important adaption for dispersal. S. africanus can flower throughout the year but mostly spring to autumn (Simon and Jacobs, 1999).

Physiology and Phenology

In common with the other weedy sporobolus grasses, S. africanus has many traits within its life cycle which make it highly adapted for successful invasion and establishment: long plant lifespan, seedlings and plants unpalatable and difficult to kill, high seed production, large seed banks, drought tolerance and effective seed dispersal.

S. africanus has a C4 photosynthetic pathway of the phosphoenolpyruvate carboxykinase subtype (Campbell et al., 1999). Species falling into this subtype are most abundant in tropical and subtropical areas of intermediate rainfall (Wand et al., 2001).

Associations

In a region of moist, cool-temperate grasslands in South Africa, S. africanus was found associated with the grasses Eragrostis curvula, Cynodon dactylon and Hyparrhenia hirta, as well as several forbs, including Walafrida densiflora, Solanum spp. and Wahlenbergia caledonica [Wahlenbergia undulata], in secondary grasslands on previously cultivated lands that had been left fallow (Werger and Coetzee, 1978). The plant communities (i) Sporobolus africanus - Hyparrhenia hirta - Eucalyptus camaldulensis woodland, (ii) Sporobolus africanus - Hyparrhenia hirta - Cynodon dactylon grassland and (iii) Sporobolus africanus - Hyparrhenia hirta - Senecio isatideus grassland were identified in a study carried out in Gauteng province, South Africa, by Tuckett (2013).

Environmental Requirements

S. africanus grows mainly in tropical and subtropical areas but can also be found in temperate regions with moderate to high rainfall (Parsons and Cuthbertson, 2001). It favours sites with compacted soil, such as road verges and tracks, but will also invade pasture and sandy coastal sites (Eurobodalla Shire Council, 2016), especially those sites receiving water run-off (Parsons and Cuthbertson, 2001). The species is well suited to growing in soils of low fertility and its occurrence in pastures may indicate that the level of soil fertility has fallen below that required by the more palatable and nutritious grasses (Burbidge, 1970).

Surveys of natural enemies of Sporobolus species in southern Africa were undertaken to identify potential biological control agents for controlling alien Sporobolus in Australia. While around 70 phytophagous insects and 23 plant pathogens were identified, only the leaf smut pathogen Ustilago sporoboli-indici and the stem wasp Tetramesa sp. showed potential. Both, however, were discarded as the smut proved pathogenic to native Australian Sporobolus species, while the wasp could not be reared in captivity (Palmer et al., 2008).

In Hawaii, the grass webworm Herpetogramma licarsisalis was found in 1968 feeding on pasture grasses, including S. africanus on Maui (Davis, 1969). 

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Wildlife habitat

Medicinal, pharmaceutical

• Traditional/folklore

There are several Sporobolus species that are very similar and difficult to distinguish. In Australia, S. africanus can be confused with the native S. elongatus (Burbidge, 1970), as well as the other weedy sporobolus grasses; it differs from S. indicus in its narrow, denser inflorescences and larger spikelets, and from S. fertilis in its larger spikelets and by the inflorescence branches being more appressed at the base (Mallett, 2005).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Detection and Inspection Methods

There are no easy methods for detecting S. africanus in the field. In areas likely to become infested, landholders must remain vigilant to grasses with the general characteristic of this species and seek early expert identification of any possible incursion. Similarly, it is very difficult to distinguish its seeds from those of other grass species. The recommended approach is to ensure that machinery and vehicles are completely cleaned of all plant matter before being allowed onto clean land.

Diagnosis

A laboratory technique was developed to create genetic markers to differentiate seed samples of S. africanus from those of other Sporobolus species present in Australia (Shrestha et al., 2005). This RAPD-PCR technique, however, has not been employed for routine screening.

Prevention

It is important to prevent infestation of clean pastures with S. africanus. The seeds are very easily moved on animals, vehicles, people and fodder so strict property hygiene is essential. Landholders in areas likely to become infested must be alert to new infestations and eliminate these as quickly as possible. It is also important to maintain existing pastures in a vigorous and dense condition to increase competition for any weed seedlings that might establish.

Viable seeds of Sporobolus spp. can take several days to pass through a cow’s digestive system (Andrews, 1995) and a minimum quarantine period of 5 days is recommended before cattle which have grazed in infested pastures are introduced to new pastures; a quarantine field should be especially reserved for such cattle (Elphinstone, 2013).

Government can assist in preventing S. africanus infestations by declaring the plant noxious and regulating to prevent its movement. In Australia, S. africanus is a declared weed in Queensland and New South Wales so that landholders are legally required to suppress and destroy the weed and/or prevent its spreading (depending on region). Government, regional groups and producer organizations can also mount effective extension campaigns to ensure that landholders are aware of the issues.

Eradication

Eradication of this grass is extremely problematical even at a property level. There are no documented cases where S. africanus has been eradicated from an area of any appreciable size.

Cultural Control and Sanitary Measures

Herbicide-treated pastures can be oversown with suitable species to prevent reinfestation by S. africanus and to restore pasture quality. Although not native to Australia, Elphinstone (2013) recommends stoloniferous grasses and other pasture species, such as creeping bluegrass (Bothriochloa insculpta) cv. Bisset, Rhodes grass (Chloris gayana) cv. Katambora and cassia (Chamaecrista rotundifolia) cv. Wynn, for pasture recovery.

Physical/Mechanical Control

Where S. africanus infestations are relatively small, plants can be hand chipped, bagged and removed from pasture for burning or similar destruction.

Movement Control

Movement Control and property hygiene are critical elements in mitigating the effects of this weed. Every attempt must be made to prevent its introduction into clean areas. Measures include being certain that any seed or fodder brought on to a property is not infested, thoroughly cleaning machinery and vehicles, or refusing entry onto clean areas of machinery that has been in infested areas.

Biological Control

Although the Australian grazing industry is keen to have biological control for S. africanus, prospects are not good (Palmer, 2012). Grasses are difficult targets for biological control for several reasons. Australia also has some 15 native congeners necessitating a very high degree of host specificity of any prospective agent. One agent, the smut Ustilago sporoboli-indici was fully screened but rejected because of attack on Australian native Sporobolus species (Yobo et al., 2009).

Chemical Control

Two active substances are available for chemical control of S. africanus, flupropanate and glyphosate. These can be used by spot spraying, by boom spraying or by pressurized wick wiping. Spraying should be carried out in low rainfall months.

Flupropanate can be applied as granules to sites where plants have been grubbed out or as a liquid solution spot spray where population densities are low to medium (1-2 plants/10 m²). Scout plants at the periphery of population clumps should be targeted first before progressing inwards. Where weed populations are dense (>2000 plants/ha), (i) arable land should be cultivated and cropped for several years, with spot spraying of headlands; (ii) marginal arable land should be cropped for fodder for a couple of seasons using reduced tillage to minimize soil erosion, with spot spraying of headlands; (iii) non-arable land should have aerial application of flupropanate granules or boom spraying with flupropanate liquid formulation; and (iv) steep or broken land needs precision aerial application of flupropanate granules. There are grazing withholding periods for both dairy and beef cattle when flupropanate is applied to pasture: at least 14 days after spot spraying and at least 4 months after boom spraying or aerial application of granules (Bray and Officer, 2007; Elphinstone, 2013).

IPM

Integrated weed management operations minimize the detrimental effects of S. africanus (Dyason, 1988). These options include strict hygiene to prevent initial infestations, early detection of infestation, maintaining competitive pastures of non-Sporobolus species, minimizing overgrazing, grazing strategies to prevent S. africanus maturing and appropriate herbicide application.

Despite considerable advancements in the taxonomy and phylogeny of the genus Sporobolus over the past two decades, there are still issues in identification that could be clarified with further work using the most recent techniques of DNA analysis and relating these studies to morphological characters suitable for field identification. These studies need to be undertaken at the global level. One benefit of definitive identification is that knowledge from various countries can be pooled.

There is very little information appearing about this species in the peer reviewed scientific literature.

25/01/2016 Original text by:

W. A. Palmer, Consultant, Queensland, Australia