Various types of methods are involve. Please check the following links it may be helpful

http://caws.org.au/awc/1978/awc197811271.pdf

http://oar.icrisat.org/9093/1/Chapter.pdf

https://books.google.co.in/books?id=hNIZihxXTwsC&pg=PA68&lpg=PA68&dq=weed+management+in+tropical+mountain+area&source=bl&ots=FzZ4dYduND&sig=qhaRpbdY7PITpSttfXbnNblMHWQ&hl=en&sa=X&ved=0ahUKEwivjtbu9bDWAhVJP48KHem5AS0Q6AEIODAG#v=onepage&q=weed%20management%20in%20tropical%20mountain%20area&f=false

In tropical mountain area , you can't afford to advocate chemical weed control methods, so course would be to adopt various cover crops ..

Thank you very much, sir. Currently, farmers have to use a lot of herbicide to control weed. The fact that they have to maintain food crop (maize, cassava...) for food security. Cost for weeding is on of the biggest investment and barrier for sustainable agricultural cultivation in tropical slope land. 

These herbicides not only destroy the microbial environment of the soil but pollute the nearby water bodies since Alfisols and Ultisols are known for high rate of leaching with relatively poor retention capacity  

The use of herbicides in mountainous area may not be totally out of place. Considering that it is difficult to achieve weed control with mechanization on mountainous terrain, it will be a double jeopardy to rule out herbicides too. What one can say is that in selecting herbicides, the persistence and the drift potentials of the herbicides should be seriously considered. 

Alternatively, other methods such as geomat, plant spacing, mulching, and cover crops may be considered but the fact remains that there is no better alternative to chemical weed control and every method has its own demerits.

Of all the  ecosystems, ecosystem prevailing in hils/mountains is considered to be most fragile  , and application of any herbicides is usually not  advised , since herbicides seriously  cut down  the carrying capacity of ecosystem  ( Some PDFs enclosed for further reading ) , sometimes cause severe eutrophication of  nearby water bodies and adversely affecting the aquatic life as well. So , as far as possible , efforts should be other than herbicides , always as thumb rule in such mountain  ecosystem 

Dear Pham Huu Thuong,

Please check the following links, hope you will find these links helpful.

regards. -- Amit

http://www.westmauiwatershed.org/UserFiles/Weed%20Mgmt%20Plan%20FINAL%2012-11-13.pdf

http://m.mekonginfo.org/assets/midocs/0002124-farming-weed-management-for-developing-countries.pdf

https://www.environment.gov.au/biodiversity/invasive/weeds/publications/guidelines/wons/pubs/l-camara.pdf

http://caws.org.au/awc/1978/awc197811271.pdf

Anoop Kumar Srivastava - Sir, it will be appreciated if you can shed more light on your opinion. Your description of herbicide is general and it is not perculiar to mountainous areas alone.  However, it is not all herbicides that are significantly  mobile.  The emphasis here is weed control in mountainous area, the major associated risk of herbicide use on elevations is drift. The demerits of herbicide has been emphasized many times with little attention on its benefits. Some herbicides  promote plant growth at low dosage (hormesis), and chemical control  is the most effective weed control measure.

Deer Pham Huu Thoung, with the observations made over 25 years in plantations, many other field crop systems and home gardening models at different sites, fully agree with Anoop Kumar Srivastava. When high concentrated synthetic plant growth hormone derivatives such as MCPA and 2.4.D are concerned they may promote plant growth at very low concentrations as mentioned by Olatunde Ayodele theoretically. However, the actual experience is that such herbicides promote secondary weed populations with rapid growth in all crop systems, causing more trouble resulting severe economic losses while promoting violent growth in algal and aquatic plant communities leading to eutrophication.

Thus, following the concept of ecosystem immunity could be a better solution for tropical agriculture in mountain areas since such landscapes are also blessed with higher native biodiversity.

Below references could be useful to you in this regard.

# Weed Management in the Commercial Tea Industry: The Case of Hapugastenne Estate, Maskeliya

# Diversity and Behaviour of the Naturally Regenerated ...

http://114.242.223.200 › auto › detail

# HERBICIDES FREE INTEGRATED WEED MANAGEMENT

https://aslm.lk › 2018/08/15 › herbicidesfree-weedmana...

# Ecosystem immunity: A climate smart solution to manage weeds in commercial tea.

# https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiKjsDyl6H0AhVSJHIKHUvODTkQFnoECA4QAQ&url=http%3A%2F%2Fwww.cgl.org.cn%2Fauto%2Fdb%2Fsearch.aspx%3Fdb%3D706211%26agfi%3D0%26cls%3D0%26uni%3DTrue%26showgp%3DTrue%26prec%3DFalse%26md%3D93%26pd%3D6%26msd%3D93%26psd%3D6%26mdd%3D93%26pdd%3D6%26count%3D10%26wrd%3DNatural%2Bvegetative%2Bregeneration&usg=AOvVaw2F9DSOuQhLruJ0VaVbaS3Z