Avoid oaks trees at all costs. Acorns are toxic to cattle. A few acorns are okay but some cattle seem to love acorns and eat too many resulting in damaged kidneys. Affected animals may survive but they are thin and unthrifty.
I don't agree with this, actually I'll refrain from commenting further on it, as I don't want to get people upset over my words...
Anything is toxic if consumed in large ammounts. Hell corn is toxic if consumed in large ammounts, do people put corn in their cattle pastures?- grass can also be toxic! Water can be toxic, even a teaspoon of it down the wrong pipe. Acorns are not toxic to animals, but they can give humans a stomach ache. Actually, if you want to get down to it, apples are one of the most toxic things available to people--- YOU ever ate an apple?
Ever seen a thin, unthrifty deer eating acorns?
Cottonwoods would make very fast shade, but they are extremely prone to lightening strike. I'd guess you don't want that...
If you can grow maples in your area, which you should, they would be about the best option.. something like a silver, very widespreading, fast growing, and not tall.
In searching for a tree, you are going to want something of medium height, and wide spread. This increases shade, while still being able to keep your tree fenced in to protect and lengthen it's lifespan... and it reduces lightning strike potential. Like I already said, I'm not familiar with the trees of the west coast, but if you were in IL, I'd be able to give you a list longer than you'd like.