Currently doing work on Farmers' Groups in Trinidad, West Indies. This type of study has been limited in the Caribbean in contrast to the Asia and Africa.
In general farmer groups in Ethiopia are not very much successful as they are supposed to be, largely due to poor internal management and member commitment.
Not much, as it is very difficult to bring the farmers together and make them work as a group. It requires a lot of capacity building and training initially to form a group of like minded farmers. Even after the groups are formed, they require a prolonged period of hand holding support by the agency promoting the group. Unfortunately, there are not many sincere and qualified agencies working in this area on a long term basis. Having said that there are of course some good examples here and there, but up-scaling the concept would require a sound public policy and support on a long term basis, because group approach to farming is the need of the hour in countries like India, where average size of land holding is going down sharply.
I was in a conference when some one presented on a very successful farmer group. The techniques they adopted is that, you must be farming for at least 4 year before you can join the group and they hired professionals to manage their affairs. These group also created their own credit union (10% savings by members from sales of their produce and they give loans among themselves without interest) which eventually serve as rural bank in their community. I don't remember the country but that was the model used.
In Trinidad, anyone can form a farmer's group once they have people interested in coming together. There are cases where even non-farmers belong to farmers groups. The persons who are the founding members believe that they were more entitled to benefits or privileges that the group may get. In the majority of cases farmers form groups in an attempt to hold onto state lands.
Here are some cases of this from our local newspaper :
“Comprising members of the Endeavour Farmers’ Association, they called for the PM’s intervention in the matter of the displacement of farmers who occupy land at Soogrim Trace West, Chaguanas. The protesters were led by co-ordinator of the Trinidad Unified Farmers’ Association, Shiraz Khan.” Khan said close to 45 farmers from both Egypt Trace and Dass Trace, Chaguanas, could not earn a livelihood, since their crops were bulldozed a year ago to make way for the construction of houses by the Housing Development Corporation (http://www.guardian.co.tt/news/2012-04-20/farmers-protest-outside-pm%E2%80%99s-office-warn-food-shortage)
and
"The Estate Management Business Development Company (EMBDC) was justified in prohibiting farmers from entering lands at Orange Grove, Minister of Food Production, Land and Marine Affairs, Vasant Bharath said yesterday."
(Nadaleen Singh, Trinidad Guardian Newspaper. 8th June 2012. http://www.guardian.co.tt/news/2012-06-07/bharath-embdc-right-block-farmers)
There is a vast literature on this. Look at the World Agricultural Economics and Rural Sociology Abstracts WAERSA published by CAB International (Commonwealth Agricultural Bureau. Probably classified under the Cooperative section, but you can try other searches. There may well be other classifications for informal farm groups.
Farmer's groups have been involved in many agricultural development projects in Nepal, a country in South Asia. Most projects feels that social mobilization group or team make farmers groups and help in projects related to rural development. So far, successes have been achieved.
Farmers groups are encouraged on commodity basis and also growers group which are being facilitated public and credit lending institutions.We have many success stories like grape growers association and farmer producers company
But you must read the literature for failures. It is too easy to pick out the successes (whether for cooperatives or any other intervention), to ignore the failures and say you have solved the world's population. The review articles, for instance suggest coops are great for wealthy capitalist farmers, have a high failure rate for illiterate farmers and a very high failure rate for politically motivated coops with illiterate farmers. In Albania, after the fall of communism, I saw the farmers pull down every building of the hated cooperative and divide the bricks equally between them.