Family: Poaceae
Synonyms: Andropogon halepensis (L.)Brot.
Common names: Johnsongrass, Aleppo, Milletgrass, Sorgo de Alepo, Zacate Johnson, Grama China, Cañuela, Don Carlos, Sorgho d'Alep, Herbe de Cuba, Gumai.
A tufted perennial grass with tall erect culms up to 2-2.5 m height . Its purplish inflorescence is an open panicle with dissimilar spikelets, 4-7 mm long.
Plants develop long rhizomes bearing buds, which germinate readily.
Rhizome growth of johnsongrass is more abundant than the shoot growth and in some cases rhizome fresh weight reaches 90% of the whole-developed johnsongrass plant. Apical dominance is broken with rhizome fragmentation, which consistently stimulates lateral bud germination and each bud produces one shoot.
It is present in many countries of the world. It prevails in hot climatic conditions and tends to be more productive during the rainy season in tropical areas.
Reproduction is by seed and rhizome production. It grows and develops intensively in warm habitats, with air temperatures above 15C.
Rhizomes are distributed mainly in the top 20 cm of the soil profile. Most are located in the first 15 cm, but up to 10% are found below 30 cm. Rhizome initiation usually takes place a month or 45 days after johnsongrass emergence and coincides with tillering or the 6-7 leaf stage of shoot growth. This process is commonly faster with plants developing from long rhizomes.
Seed production is variable and dependent on several environmental factors. High seed production is found in plants with high tiller production. Production of seed may vary from 540 to 1440 kg/ha, but not all of them are able to germinate immediately after ripening. The optimal air temperature for seed germination is within the range 25-30oC. The seeds can remain viable in soil for periods of up to 6 years. In Mississippi (U.S.A.), germination of Johnsongrass decreased from 62% after being buried for 2.5 year to 37% after 5.5 year.
Vigorous growth requires high light intensity and photoperiods of 12.5 hours are optimal for johnsongrass growth and development.
Prevention of seed dispersal and rhizome production is an important measure to avoid high johnsongrass infestation in crop areas. Seeds are normally dispersed through wind, water, animals and contaminated crop seeds, therefore slashing the plants at the beginning of the flowering stage in field margins, ditchbanks, canals and crop land and/or avoiding animal grazing in infested areas help to prevent new johnsongrass infestations.
Mechanical control of johnsongrass aims to prevent new rhizome production and must be implemented within the first month after shoot emergence.
It is considered that cultivation is most effective when the grass is about 36 cm tall. This helps prevent plants from forming rhizomes or seeds. In arable land johnsongrass infestation may be reduced by dragging the rhizomes onto the soil surface with a sweep or spike-tooth tiller during land preparation as this facilitates desiccation by sunlight. Desiccation of rhizomes can be accelerated by cutting them into fragments less than 5 cm length. Rhizomes dried to 20% of their original weight completely lose their regenerative ability. In fruit tree orchards regular mowing when the shoots are 20-25 cm tall tends to deplete rhizome food reserves and limits further rhizome production.
Crop competition. In annual crops such as potato and vegetables, sweet potato is considered an effective preceeding crop which smothers johnsongrass and other grassweed infestations.
Pasturing over a period of several seasons is advocated as an effective method to reduce johnsongrass stand. Geese have also provided excellent control of johnsongrass and other grasses in cotton, but required a high level of management.
Herbicides. Johnsongrass is not easily controlled with herbicides but several systemic foliarly-applied compounds can be effective. Glyphosate is useful in pre-planting treatment, applied to well-developed foliage 2-3 weeks before planting or seeding, and may also be used in fruit tree orchards. Post-emergence application of fluazifop-butyl, haloxyfop-methyl, fenoxaprop-ethyl, quizalofop-ethyl and sethoxydim are selective in broadleaf crops and provide control of seedlings and suppression of established plants.
A single application of these herbicides is not usually enough to reduce heavy johnsongrass infestations below the economical threshold in the affected crops.
In sugar cane, post-emergence treatment with compounds such as asulam, dalapon and MSMA have been selective in the crop. These compounds are usually applied when the shoots are 20-25 cm high. Dalapon is less selective to sugar cane than asulam or MSMA, and application should be directly onto the weeds, avoiding any contact with cane foliage.
Trifluralin as a soil-incorporated pre-planting treatment is effective for the reduction of johnsongrass stand from rhizomes. This treatment is most effective against short rhizomes (less than 10 cm) (McWhorter 1974). Good to excellent effectiveness is provided by a second treatment in the second year of a johnsongrass control program (McWhorter 1989). This treatment is selective in crops such as sugar cane, beans, soybeans, groudnut and cotton (McWhorter 1974; McWhorter 1977; Millhollon 1978; Kleifeld et al. 1986; Labrada et al. 1987). However, this treatment is not selective in all sugar cane varieties. Another useful dinitroaniline herbicide for the control of johnsongrass seedlings is pendimethalin.
Pre-seeding soil-incorporated EPTC+ dichlormid is also effective for johnsongrass control in maize crop.
It is important to stress that chemical control measures alone do not usually control johnsongrass. Furthermore, chemical treatments are not always economically feasible for smallholder farmers. Therefore, the best control option for this perennial weed is the integration of land preparation, crop rotation and rational chemical treatments.
Johnsongrass is one of the major weeds in 30 different crops in 53 countries. In infested areas, johnsongrass competes severely with crops such as maize, sorghum, sugarcane, soybeans, groundnut, cotton, vegetables, fruits, tobacco, small grains, pastures and alfalfa. S. halepense is widespread as a weed in North, Central and South America, and in the Near East.
Countries: Belize, http://www.fao.org/agriculture/crops/core-themes/theme/biodiversity/weeds/db-countries/l/en/#lebanonChile, Cameroon,Colombia,Cuba, India, Iran, Jordan, Lebanon, Mexico, Syria, Togo,
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