I saw this video from Bloomberg about the use of robots in dairy farms. It's always a challenge to work in an environment that constantly changes such as a farm. However, that didn't stop the company in the video to create 3 robots for tasks that used to be the domain of human workers:
Of the three tasks above, I consider milking the most challenging as it involves a moving creature with non-rigid body. Issues such as force control (to prevent cow discomfort or injuries) and dynamic visual servoing (to catch the moving udders) are some of the control issues that come to mind. Other engineering problems would be the robustness of the robots, battery life or power (for the mobile robots), and cost.
The robotic solutions doesn't come cheap (USD 200K per robot).
Maybe full automation is not really necessary? If the goal is to increase productivity, would providing farm workers with tools that could assist in executing his job at high speed be more cost-effective?