I am rather curious where you read that willow is supposedly bad for horses -- I have always been told by vets and extension agents that it is a SAFE pasture tree, and a quick google seems to bear that out.
Willow does have a small amount of salicylic acid, related to aspirin, in the tissues, but not to the point of bothering a horse. OTOH the horse may bother the willow a bit, esp. in the sense of pruning low branches. But that's all. In my experience horses mainly graze willow branches in late winter when there is nothing else to do, or in an Aug-Sept drought if you haven't got sufficient normal grazing in your pasture and they're bored with hay.
All you can really say against willow as a pasture tree is that they have really crappy wood and catch every disease and pest coming thru the neighborhood, and therefore they tend to self-disassemble in every good storm or blow, and you don't want large chunks falling on fences. (You may not like the cleanup chores associated with large downed limbs even if they're not near any fences).
Unfortunately this is true of pretty much all other fast-growing trees as well (poplars, basswood, manitoba maple, etc). So if you want shade in any reasonable time frame you are probably going to have to accept the likelihood of frequent fallen branches, and just pick whatever tree best suits the soil, exposure, etcetera.
Trees to avoid with horses (in Northeastern north america anyhow) are:
--red maple (leaves can be toxic at some times of year, and after symptoms appear the horse may not be able to be saved) - I have heard it suggested that any maples that may hybridize with red maple, such as silver maple, should be avoided as well, although I do not believe there is empirical evidence that *typical* silver maples cause problems and I do have them in my own pastures)
--any kind of cherry, including black cherry, pin cherrry, chokecherry, etc. Compounds in the wilted leaves convert to cyanide when eaten (fresh leaves can be eaten without general harm, but you never know when a branch will blow down or etc and produce wilted leaves that some genius horse decides to eat...)
--black locust; the seeds are poisonous but I've never head of horse poisoning because of them - the main issue is those horrible long steel-hard thorns which can go through feet and hide and so forth and make really nasty wounds
--really, fruit trees should not be around horses, as you can get various colic etc problems if they consume to much fruit, esp. if it is underripe. Also of course horses are not good for the fruit trees, either <g>
--black walnut should *probably* be avoided too - I am not aware of conclusive PROOF that it is a danger in pasture (as opposed to in bedding) I have heard enough vets say they suspect it as a cause of pastured horses' colic and laminitis that I'd stay away from it, as well as related trees such as butternut and heartnut.
(edited to add, for completeness: you also prolly don't want significant numbers of acorns, or any fruits of things from the horsechestnut/buckeye family, getting where horses can eat them -- although AFAIK it is rare that a horse will eat 'em and in most cases not actually fatal).
Pat
