Ageratum conyzoides (herb)
Chemical: Pre-emergence application of simazine, atrazine, diuron, oxadiazon, oxyfluorfen, methazole or metribuzin provides excellent control of this weed. Post-emergence application of 2.4-D controls established infestations (Rao 2000).
On the other hand eco-based, environment-friendly strategies for the effective control of A. conyzoides are suggested. Plant extracts of parthenin and eucalyptus (volatile essential oils) may hold promise in controlling A. conyzoides (Batish et al. 1997, Singh et al. 2002, in Batish et al. 2004) and some success has already been achieved using these and other natural plant extracts. For example, a study on the allelopathic effect of two volatile monoterpenes (cineole and citronellol) on A. conyzoides has revealed their potential for future weed management. Both the monoterpenes severely affected the germination, speed of germination, seedling growth, chlorophyll content and respiratory activity of A. conyzoides and after two weeks of exposure, the weed plants wilted. Cineole was the more toxic of the two monoterpenes (Singh Batish and Ravinder 2002).
The addition of activated charcoal, an inert material with high affinity for organic biomolecules, partly ameliorates the negative effects of A. conyzoides phenolic allelopathic root residues on rice (Oryza sativa) growth (Batish et al. 2009).
Field and crop management: Increased fallow length in slash-and-burn rice (Oryza sativa L.) production systems of northern Laos decreases weed pressure (Roder et al. 1998). Compared with continuous rice treatments treatments with fallow or cowpea (Vigna unguiculata) in the previous year had 32% less herbaceous weed biomass and 90% fewer A. conyzoides.
The timing of weed seedling emergence relative to the crop is important in planning and optimizing the time of weed control. Ekeleme and colleagues (2005) have predicted seedling emergence of tropical weed species, specifically in low-input and small-scale farms. To control weeds adequately, especially with limited use of herbicides, farmers need to know the timing and extent of weed seedling emergence before and during the growing season. Knowledge of when weeds emerge is equally applicable and beneficial to all forms of weed-management technologies (see: Ekeleme et al. 2005).
Physical: Physical methods tried with little success include: mowing (at tender stage), manual uprooting, cutting with swords, use of shrub-masters for heavily infested areas followed by burning or burning live plants. These methods are useful only at the vegetative stage and are labor intensive and pose a risk to human health. Once the plant matures (starts producing seed) physical removal becomes only a short-term solution (Batish et al. 2004).
Chemical: A number of synthetic herbicides have been tried for the management of these weeds such as atrazine, alachlor, paraquat, glyphosate, simazine, 2,4-D, and 2,4,5-T. The effective dosage required varies with the growth stage. Although herbicides are quite effective in providing immediate control the weeds soon reappear because of quick regenerative potential. Repeated use of herbicides comes with toxicological concerns (Batish et al. 2004).
Integrated Land Management: Batish and colleagues (2004) advocate integrated land management as the best strategy forward. After clearing infested areas land should be put to some use, with community input as a priority. Motivation should stem from education, with people becoming aware of the hazards of these weeds. Identification methods should be taught to school children, farmers and the general public. The involvement of non-governmental organizations and eco-task forces is also important. Educational awareness should focus on weed identification and life-cycle, motivation for the effective removal of the weed at an early growth stage and management of waste through vermi-composting.
Efforts to manage A. conyzoides through eco-friendly means tried are also discussed in Kohli and Batish (1996).
Medicinal plants, including Acyranthes aspera, Dichanthium annulatum, Murraya koenigii, Adhatoda vasica, Carissa spp. and Colebrookea spp., declined in areas invaded by A. conyzoides (Singh Undated). There is an urgent need for the management of indigenous or medicinally valuable plants in the invaded areas under their natural habitats (Dogra et al. 2009).
Factors considered to contribute to the invasiveness of A. conyzoides are (Singh Undated): Fast growth rate; Unpalatable or bitter tasting - rich chemistry; Long flowering and fruiting periods; Very high seed production; Adaptability and wide ecological amplitude; High regenerative potential; and Absence of natural enemies / competitors.
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