Hi Ramon.

Spraying using drone is faster, cheaper, more accurate, more economical consumption of fertilizer can be used on steep slopes and does not destroy the crops which traditional tractor... and is cheaper then plane...

http://www.dji.com/product/mg-1

Ramon-

I believe they will definitely be part of the future but the technology is not quite there yet.  In some cases it will be the least expensive option, but not all.  Sebastian makes good points regarding steep slopes and the avoidance of crop damage, so it might make sense in many of those situations.  The other area it may be useful is in selective application, however, the technology is not quite there yet at any scale.  There is a lot of work that still needs to be done regarding the analysis of the sensor data, crop specificity, data analytics for specific crops, and the selective application technology.

Adding to what is already pointed out by Sebastian Banaszek and Ed Greutert :

In order to have drones completely replace tractors, planes and such heavy sprayers / machinery in agriculture, drones need to be made just as reliable, not overly expensive, well supported aftermarket and most importantly easy to use by any lay man.

Yes, there is a lot of work that needs to be done on sensing, video processing, mapping, autopilot setup/s, ground station/s, controller/s, flight endurance, flight range, payload capacity, hours of operation before maintenance, etc. Also there is the question that in cases of requirements of manual operations will the user be able to pilot it, does he need formal drone pilot training - usually the legal answer is yes - but then the commercial questions are why, when, where, how, how much and is it all really worth it.

Drones as a whole have a long way to go; they do not have the international standards, procedures, safety-reliability, certifications, etc. like in the case of manned planes, tractors and such large scale manned life-size machinery. Then there are many brands, choices, types, capacity and application variations in manned machinery. This will only happen if multiple government and non government bodies agree on technologies, agree on rules and regulation, enforce them; all the while making multiple independent complete drone system products like in the case of the automotive industry so that the end use has the final choice.

Dear Ramon;Interesting discussion, I agree with the colleagues, whose comments successful based on their extensive experience in the field of drones, my humble opinion is that developing countries are very important because besides the qualities described above are the only posibility to increase food production, since the investment to machine using tractors would be immense, because besides the drones can be used to sow such situation that they are too expensive, should motivate experts in the design of these devices, and governments of poor countries should encourage their researchers to develop farming systems based on them