Facts about Itchgrass

Rottboellia cochinchinensis commonly known as Itchgrass, Raoul grass or corngrass, is a species of grass belonging to Poaceae / Gramineae (Grass family). The plant is native to Old World tropics, and is thought to have an origin in Indo-China in the area that is now Vietnam. It has been introduced into tropical America, as a potential pasture grass in the USA in the early 1900s and since the 1960s has been spread widely by contaminated rice seed, agricultural equipment and along transport routes in Central and South America, the Caribbean, and in the Gulf Coast region of the USA. It is widespread in tropical Asia and the Pacific Islands, tropical Africa, and Australia. An aggressive, significant weed in more than 40 countries, R. cochinchinensis is listed as a Federal Noxious Weed in the USA, and is suggested by Vibrans to be possibly the most harmful invasive plant in Mexico. It is a weed of warm-season crops around the world, preferring tropical and subtropical climates. It is a major weed of a wide range of crops including maize, rice, sorghum and sugar cane.

Few of the popular common names of the plant are Guinea-fowl grass, Kelly grass, Itch grass, Raoul grass, Kokoma grass, Itchgrass, Guinea fowl grass, Kokomo grass, Corn grass, jointed grass, Lisofya, Shamva grass, Sugarcane weed, Prickle grass, Rice grass, treadmill and lisofya. The genus name Rottboellia was named in honor of the Danish botanist Christen Friis Rottbøll (1727–1797) by Carl Linnaeus the Younger in 1781 in the publication Supplementum Plantarum. The species epithet cochinchinensis traces its etymology back to the basionym Stegosia cochinchinensis, which was used by João de Loureiro to represent the specimen he described being from Cochinchina, now part of Vietnam. The common name itchgrass is derived from the presence of rigid penetrating hairs on the leaf sheaths.  Although attempts have been made to use the plant for pasture in many areas, these hairs will break off in flesh and cause severe irritation, making the species a poor fodder. It is tolerant to most herbicides that are applied in cotton and maize fields. Management and removal of R. cochinchinensis requires the use of many man hours and the application of several techniques to ensure control.

Itchgrass Facts

Plant Description

Itchgrass is an erect, profusely tillering, tufted annual grass that normally grows about 3 meters (9 ft. 10 in) in height. The plant is found growing in hill thickets or teak forests, along roadsides, sunny or moderately shady localities, dry cultivated fields, grasslands and in other open, well-drained sites. The plant thrives in moist, penetrable heavy-textured soils. Roots are fibrous and hard, with stilt developing from lower nodes of the plant. Culm (Stem) is cylindrical, sparsely covered with stiff hairs and with scabrous dark nodes. It is robust at the base, with a diameter reaching up to 1 cm.

Leaves

Leaves are simple and alternate. The sheath is cylindrical, and slightly keeled and hispid over its entire length. It has white stiff hairs (tuberculate and perpendicular to the surface of implantation). The ligule is membranous, 1 mm high, barely cilliated at the top. The lamina is linear, flat, drawn obliquely acute apex, up to 1.5 m long and 1 to 2.5 cm wide, grooved white midrid. Blades are 25 to 40 cm long and 10 to 20 mm wide, flat, with hairs. Although larger dimensions of about 80 cm long and 45mm wide have been suggested as upper limits.  The margin is scabrous. The upper surface is sparsely covered with long tuberculate hairs, the underside is glabrous.

Flowers

The racemes of Itchgrass are 3–15 cm long, cylindrical, hairless (glabrous), erect and are found on terminal and axillary culms. Each raceme is either embraced at the base by, or projects beyond, a leaf arising from an inflated leaf-sheath. The peduncle at the base of each raceme is rough and widens as it approaches its apex, where the raceme is divided by fragile nodes between each rachis. The rachides are strongly inflated at around 2–3.5mm wide and 5-7 mm between each node and bear pairs of spikelets abaxially: one sterile and attached with a pedicel fused to the internode, the other fertile and attached directly (sessile). The raceme tapers off with a number of reduced, sterile spikelets at the apex, which form a false panicle as they appear to be much-branched.

Spikelets

The sterile, pedicelled spikelet in each pair is well developed at about 3-5mm long and is egg-shaped and dorsally flattened. The spikelet is enclosed by bracts called glumes which are hardened, smooth, obtuse, blunt, distinctly veined, glabrous and winged on the margins. The sterile spikelet is deciduous with the fertile spikelet, meaning they break off together with a central peg where the pair was attached transversely at each rachis node. The fertile, sessile spikelets are very similar to the sterile spikelets, being around 3.5-5mm long, ovate and dorsally flattened, although they are attached differently to the rachis. The glumes of these spikelets are dissimilar as the lower one is wider than the upper and is not winged, whereas the upper is V-shaped. The lower glume has 13 veins and is 2-keeled, unlike the upper which is 11-veined and singularly keeled. Both reach the apex of the florets but the lower glume apex is notched (emarginated) whereas the upper is acute.

Grain

Oblong grains are 2 to 2.5 mm long and surmounted by a short point. It remains included in spikelet, itself secured to the article of the spike that disarticulates at maturity.

History

The natural distribution of Itch grass is somewhat unclear as the species seems to have been very successful at growing its range. Most sources assume the species is native to South-East Asia, as it was first described from specimens found in this area, although some sources cite India. It is now pan-tropical in distribution, being found across the Old World tropics from southern Asia, Sub-Saharan Africa, Madagascar and Indonesia to New Guinea and the Solomon Islands, and Australia as far as Queensland, with one geo-referenced record from New South Wales. In addition, it has been widely introduced across the tropical Americas, possibly first to Cuba or Brazil but rapidly spreading across the Caribbean, southern United States and across tropical Central and South America.  Introductions are speculated to have originated from the transportation of crop products and agricultural and forestry machinery, or even intentionally introduced for grazing in the Caribbean.

Some Facts

• It is an extraordinary seed producer, sometimes producing over 2000 seeds per plant (although not all would be viable) and over 650 kg per hectare.
• The hairs on the stems and leaves can cause severe irritation, and the plant is unpalatable to livestock and wildlife.
• Individual plants produce 2000 to 16,000 seeds that are shed as soon as they mature.

Itchgrass Management

Successful management of itchgrass depends on the exhaustion of its soil seed bank and preventing production of seed. No single control method is able to achieve this goal, thus a truly integrated strategy is required to decrease itchgrass populations steadily. Available and promising tactics include mechanical, cultural, chemical and biological options. Eradication as a strategy does not seem entirely feasible even when a new, much localized infestation at the farm or country level is detected early. For example, eradication efforts were initiated at Wales Estate, one of the major sugarcane- producing units in Guyana, soon after itchgrass was first identified in 1991. Areas affected were designated as restricted. All agriculture-related movement required previous approval, including that of workers and equipment, and the use of planting material from infested sites was banned. Several tactics were implemented to eliminate the weed: roughing, herbicide application, legume cover cropping or flooding during fallow periods (under their production system, flooded fields remain under fresh water for 6-12 months after tillage but before planting). Although the eradication program did not eliminate the weed after five years, it reduced the density of the infested sites and limited new infestations.

Mechanical control

Shallow tillage can be used to promote itchgrass germination prior to planting. Emerging seedlings could then be controlled by additional mechanical means or with herbicides. Failure to control itchgrass seedlings after soil preparation, however, may result in extremely high densities that would substantially reduce crop yields. In small-scale farming inter-row slashing or cultivation are frequently used but the practice results ineffective as seedlings growing within the crop rows escape control and will then add to the seed bank and reduce crop yield. In-crop cultivation may damage crop roots, bringing seed to the surface where it easily germinates and increases the risk of erosion.

Cultural control

Since itchgrass is easily dispersed with crop seed, an important tool for preventing its introduction to new fields and spread is the use of certified crop seed.

Several agronomic practices also can help in decreasing itchgrass densities and depleting the soil seed bank once the weed is established. Crop rotation could help in disrupting the close association between itchgrass and some crops (such as maize and sugar cane) by allowing the use of alternative control tactics such as other herbicides (selective graminicides) and flooding. Maize monoculture facilitates the rapid establishment of itchgrass as a dominant weed.

One of the most successful and researched tactics to smother itchgrass plants is the use of cover crops. Cover crops have been developed in Mesoamerica, Africa and Asia to improve soil characteristics and control weeds. They can be used either as part of a rotation scheme or as intercrops. Preference has been given to nitrogen-fixing legumes, including Cajanus cajan, Calopogonium mucunoides, Canavalia spp., Crotalaria spp., Dolichos lablab, Pueraria phaseoloides, Mucuna spp. (velvetbean), and Stylosanthes spp.

Selection of the cover crop species should take into account local conditions and cropping systems as well as farmers’ needs, including consideration of negative competitive effects on the crop, additional problems related to pest control, management costs, and the positive value of the cover crop as a food supplement and in the prevention of soil erosion. Legume covers also increase soil organic carbon and phosphorus levels and improve the cation exchange capacity and Ca contents. Some of these cover crops also are allelopathic. Allelochemicals present in velvetbean are inhibitory of weed growth.

Biological control

Because itchgrass thrives in exotic ranges, a very promising and complementary management alternative is classical biological control. Of several itchgrass pathogens screened as possible bio control agents, a head smut, Sporisorium ophiuri (P. Henn) Vanky (Ustilaginales), has been thoroughly studied. The smut is a soil-borne, systemic pathogen, infecting itchgrass seedlings before they emerge from the soil. S. ophiuri has been recorded as a head smut of itchgrass in Africa and Asia and is restricted to the Old World. Infected plants grown individually in pots grew similarly to healthy plants but when gown under competitive conditions (8 plants per pot), smut-infected plants produced less tillers than healthy plants. Experimentally, infected plants produced substantially less seed than healthy plants. In the endemic range of the weed, natural epiphytotic of the smut are common, often with a high percentage of plants infected within a population. Isolates of the smut were found to be itchgrass-biotype specific but one from Madagascar was found to infect a wide range of biotypes including a number from Latin America, and hence selected for a comprehensive host range screening. The smut was found to be extremely host specific; none of 49 species/varieties of graminaceous test plants other than itchgrass became infected. Screened species included pastures, weedy grasses, graminaceous crops (rice, sugar cane, maize, sorghum) and teosinte (Zea (Euchlaena) mexicana), the maize ancestor.

Chemical control

Labrada compiled a list of conventional herbicides for itchgrass control. Typically, selective itchgrass chemical control has been achieved with some triazines (e.g. dimetamethrin), dinitroanilines (e.g. pendimethalin) and acid amides (e. g. diphenamid). Pendimethalin has proved very effective against itchgrass and can be easily included as a tactic for the integrated management of this weed in maize. In Trinidad, pendimethalin (1.5 kg ha-1) and inter-row cultivation at 14 and at 28 days after planting effectively controlled itchgrass in maize during the critical period of interference. More recently, selective systemic graminicides that inhibit acetyl-CoA carboxylase (aryloxy-phenoxy propanoates and cyclohexanediones) have also been used to eliminate itchgrass. The weed, however, has evolved resistance, in one instance, to fluazifop-p-butyl in soybeans in Louisiana, United States. Total herbicides, especially paraquat and glyphosate are also widely used to control itchgrass

References:

https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=504836#null

http://www.hear.org/pier/species/rottboellia_cochinchinensis.htm

https://www.cabi.org/isc/datasheet/47782

https://www.feedipedia.org/node/388

http://www.iucngisd.org/gisd/species.php?sc=772

http://www.theplantlist.org/tpl1.1/record/kew-439578

http://issg.org/database/species/ecology.asp?si=772&fr=1&sts=&lang=EN

https://en.wikipedia.org/wiki/Rottboellia_cochinchinensis

https://gd.eppo.int/taxon/ROOEX

https://plants.usda.gov/home/plantProfile?symbol=ROCO6

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