Electromagnetic driven actuators will be off the table for agricultural robotics because of their high current draw and low force/current ratios. On the other hand, because of their high force/current and high force/size ratios, piezoelectric driven hydraulics has the potential to be a good candidate for "robot muscle" for portable and field deployed robotic systems. The other nice thing about hydraulics is they are very abuse tolerant, work in the mud and dirt, and even under water.
This is going to be an ongoing area of research for me. As I find new information, I'll post it here and then bump this post to the head of the queue again.
This isn't hydraulics, but it is an interesting use of piezoelectrics to drive a linear actuator. It uses a ‘walking” motion to generate 8-Nt of force with a 20-cm/sec velocity. http://www.linear-actuator.net/Ultrasonic-Motors_Linear_Actuators.php
Here is a piezoelectric driven hydraulic pump capable of 2500-psi output pressure and a flow rate of 40-cc/sec: http://www.kineticceramics.com/piez_fluid.html
Also see: http://aml.seas.ucla.edu/research/areas/piezoelectric/pump-non-mech-valves/2002-12_lee.pdf