Brian Wang of Next Big Future blog posted links and a video about robot agriculture. Current advanced tractors posses a surprising amount of automation. The difference between current robotics in agriculture and what is being advocated here is the suggestion of using swarms of small autonomous robots. While I am an enthusiastic proponent of robotics, this idea strikes me as overly ambitious and it misses a simpler solution.
Let us consider the real world environment that would be encountered by an agricultural robot. Light conditions change constantly. Vision systems will have to deal with rain, fog, and dust. Assessing the health and growth state of plants is an extremely difficult problem for a computer. Human farmers use judgement, based on years of observations, to accomplish this task. Machine learning algorithms can certainly do this, assuming that the data is relatively clean and non-noisy. However, due to the conditions cited above, the input data will be ambiguous and messy. While I have no doubt that eventually robots will become the norm for farm labor, current technology is not up to the task.
A simpler solution is to use telepresence robotics. The main reason for using robots to replace human labor in agriculture is that such labor is physically onerous and monotonous. However, as described above, it does require some human judgement and decision making. A telepresence robot would allow the operator to use his knowledge and judgement, while being in a physically comfortable location and allowing the robot to perform the actual labor. The operator would essentially be playing a video game. An outstanding example of telepresence robotics is the da Vinci surgical robotic system. The technology currently exists to build precision telepresence robots for agriculture.
I should note that robots for indoor hydroponic farms are currently in use. Since this is a controlled environment, an automated assembly line is being used. Given limits of current robotics technology, this is a better solution than autonomous robots.