I assume you are asking about fodder for cattle?
Mulberry, poplar, alder, black locust, elder are just a few appropriate species, with white mulberry having the highest nutritional value and hardiness (it is toxic to horses like many trees are to them). Mulberry also is high in micronutrients and of course has a very valuable fresh fruit.
This wiki has the fodder value of many tree species listed out:
http://www.voederbomen.nl/nutritionalvalues/?geslacht=0
I am in the process of writing a white paper on the potential of mulberry silvopasture in mediterranean climates. Alfalfa uses the most water of any crop in Ca and mulberry uses about 1/4 the water. If graziers planted mulberry we would have a lot more water to go around!
Also called "tree hay" or "fodder banking". Tree hay can be harvested (pruned) mid summer and baled for feeding out over winter, while fodder banks can be browsed in summer as a standing crop.