North American weed spotted in India, could be major threat to agriculture and forests in future

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If the initial hints are anything to believe, the farmlands and forests in India have a brand new invasive threat from a rarely reported foreign plant species native to North America. A recent research correspondence published in the Current Science journal claims to report the plant for the first time from India and argues that the plant can have tremendous invasive potential.

Western ragweed (Ambrosia psilostachya)
Image courtesy: Krzysztof Ziarnek Kenraiz (Wikimedia commons)

 According to the researchers, the plant is presently reported from farm lands in the M. Bevinahalli village in Turuvekere taluk of Karnataka. On close examination and surveying the literature, the researchers were able to identify the new plant as Ambrosia psilostachya. Though the plant resembles its close relative Ambrosia artemisiifolia which is found in India, it is actually different, says the correspondence. 

Its Indian variety, Ambrosia artemisiifolia, was the only member of the genus hitherto reported from India. According to the correspondence, the new weed is native to North America and is known as perennial rag weed or western ragweed in North America.

New invasive plant threat in India

The research note warns that the weed has tremendous capacity to spread very rapidly and is known to cause allergic reactions in Humans. “If the weed is not controlled now, it may create a difficult to control situation once it proliferates from the location of present report”’ says the correspondence. 

According to the correspondence, the weed has not only invaded finger millet, mulberry, coconut and areca nut cultivation  but has also conquered the cattle pastures which have affected animal husbandry of these villages. The correspondence also notes that the plant has seriously affected local flora, by replacing local grass varieties, thus decreasing fodder availability. 

The plant is capable of spreading to large areas through extensive vegetative reproduction through underground rhizome like roots and stolon (part of the plant which makes new plant).  Though the plant has been present in some villages in Karnataka for the last 15 to 20 years, it is not yet clear what restricts it from spreading to other places.

 According to the researchers, since the plant does not have parts which supports seed dispersal through wind, the plant has not yet started spreading to other places. It is also not clearly known at this point if all the seeds produced by the plant are viable or if they exhibit any type of dormancy.

Though, there are not many issues reported about this plant expect its rapid invasion of farm lands in Karnataka Villages, reports from other countries indicate that the plant can cause allergic diseases in humans. “The species is said to be responsible for causing severe allergenic diseases in human beings in the United States”, says the correspondence.

The North American Connection

The plant is actually native to North America and the way in which it has reached Indian farmlands is still unknown. Though some speculates that the seeds could have been brought to India by migratory birds that use the seeds and other parts the plant for making nests, the connection is yet to be scientifically proved.

Though contaminated food grains are a usual way in which invasive plants reach farther places -as in the case of Lantana, a notorious weed in Indian forest- that may not be the reason for the spread of the present weed. “The introduction of the species through contaminated food grain may not be possible, as the weed is localized only in these villages in Karnataka”, says the paper.

The plant has already spread to parts of South America, Europe, New Caledonia, Japan, Taiwan and Australia. The researchers do not rule out the chances that the seeds reaching India from either of these places.

Weed control measures

 The presence of root buds, horizontally running rhizomes and dense root ramification makes the plant difficult to manage and eradicate. Periodical manual removal, shallow plowing before passing cultivator in farms has been recommended as a control measure. According to the note, if repeated control measures are taken, the weed can be controlled in 4 to 5 years from the country with its present restricted proliferation.

 M. T. Sanjay  and T. V. Ramachandra Prasad from the Directorate of Weed Science Research Centre, University of Agricultural Sciences, Bangalore, R. A. Sharma from Directorate of Weed Science Research, Indian Council of Agricultural Research, Jabalpur and R. R. RAO, from Bangalore co-authored the correspondence.

Read more on IBT's coverage on invasive species in India.

8 best practices to eliminate weeds in Conservation Agriculture method

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Conservation agriculture is a new method of agriculture which avoids the conventional methods of tilling the land before sowing the seed. Instead, it focuses on minimal soil disturbance and retention of residue to keep the moisture of the soil and to avoid erosion of manure and herbicides.

Maize under Conservation Agriculture in Malawi
Photo courtesy CIMMYT

 However, a major issue with the method is the presence of weeds. Following are some of the best practices to avoid weeds in conservation agriculture. However, they are modified to meet sustainable agricultural practices.

The methods avoid chemical herbicides as they cause environment pollution and turn less effective in long term.

1. Prevent weeds

·         Prevention is always better than herbicides. So make sure that you use clean crop seeds and machinery for seeding, harvesting and threshing, since many weed seeds actually reaches the seed bed in this way.

·         Keep field margins and irrigation canals free from weeds.

2. Stale seedbed techniques

·         Irrigate the seedbed lightly in advance so that the weed seeds which are often found in the soil surface germinate in two three days. Once they germinate in this period, first clear them off manually or using organic herbicides.

·         It is also advised to encourage seed predator insects or birds in the seed bed before sawing.

3. Adjust time of crop sowing

·         Make the crop sowing early in the season so that the crop gets a competitive edge over late germinating weeds.

·         This method has been proved successful in North India against certain weeds like Phalaris minor (Red Rice) when wheat crop was sowed early in the season.

4. Grow a cover crop which gives thick mulch

·         Grow crops like cowpea or sun hemp, which will produce thicker residue, between two main crops. Then cut down it before the main crop season.

·         The dead mulch of the cover crop not only helps retaining the moisture of the farm land, it also suppresses the weed germination by releasing allelo-chemicals and blocking sunlight which is needed for the seeds to germinate.

5. Use of narrow row spacing

·         Place crop rows closer so that they grow thicker, often blocking the sunlight to the space in between two rows, thus decreasing the chances of weed growth in between two consecutive crop rows.

Ground nuts under Conservation Agriculture
Photo courtesy CIMMYT

·         The space between two rows is a vulnerability of the conventional method which actually helps weeds, especially in maize and soya bean cultivation. Narrowing the gap between two consecutive rows can control weeds to a great extent.

·         15 centimeter row spacing is found very successful against P.minor.

·         Long term adoption of the method will bring down the availability of viable weed seeds in a seed bed.

6. Higher seeding rates

·         Increased crop density sometimes helps the crops compete better against weeds.

·         Higher seeding rate is proved to help in weedy or partially weedy seedbeds, though the method may not bring good yield in weed-free environments.

7. Use of weed competitive cultivar

·         The wild or traditional seed varieties which produce long shoot plants are more weed competitive though their yield is not on par with the short stature, high yielding modern seeds.

·         A continuous use of conventional weed-competitive cultivars coupled with narrow row spacing and other methods will bring down the weed presence and number of available weed seeds in a seed bed considerably if the farmers are ready to afford the decrease in production during a short term.

8. Adopting crop rotation

·         Crop rotation helps decrease certain crop specific weeds like P. minor in the case of rice or wheat crops.

·         Rotating the rice crop with a non-rice crop like maize or soybean can disrupt the lifecycle of such weeds.

·         Rotation the management practices also disrupts weed life cycle.

·         Crop rotation improves the soil quality too.

These methods are proved as effective weed management methods in conservation agriculture practices. Many of them are effective in other types of agriculture practices too. However, anyone of these methods when practiced alone may not bring a good result in eliminating weeds. But a brilliant coupling of different methods will surely produce a good result.

If you know any other weed management methods, share it for our readers in the comments.

Reference:
 Chauhan, B. S. & Mahajan, G (2012), Role of integrated weed management strategies in sustaining conservation agriculture systems, Current Science Journal, Volume 103, Issue 2, July 25, 2012, Indian Academy of Sciences, Bangalore, Retrieved from http://www.currentscience.ac.in/Volumes/103/02/0135.pdf on July 25, 2012.