Since, farm subsidies make farm products cheaper, poorer countries should welcome them. Poor countries should remove all import barriers against subsidized farm products without waiting for WTO negotiations. Subsidized farm products will reduce poverty and malnourishment and thus they should thank rich countries for the subsidies as they represent net transfer of wealth from tax payers in rich country to the poorest people in poorer countries.
Apart from solving the problem of poverty this will help poorer countries in other ways. Since, rich countries have already spent money on subsidies, less money is available for investment in producing other products. Increased demand for other products because of savings from food and freeing of resources employed earlier in production of subsidized farm goods should make it easy for poorer countries to develop competitive advantage in production of other products.
In short term farmers in the poorer countries will face losses as they have to discontinue production of subsidized farm goods. However, in poorer countries agricultural production is still done mostly manually. Thus, there is little capital investment to really go waste. Land can always be utilized for other purposes without loosing much invested value. Labourer in poorer countries are also unskilled as thus the transition represents a great opportunity to invest in developing alternate skills.
Developing countries cannot use excuse of farm subsidies to erect import barriers. That will not be retialiation against rich countries but actually against consumers in developing countries.
This is not to argue against free trade itself. I am against using one injustice to perpetuate another injustice against a third party (consumers in developing countries). And the point is that countries benefit by unilaterally dismanting trade barriers irrespective of behavior of other countries.