I’m also trying to get rid of the scrub oaks out of the pasture. Because I know they aren’t great for them. But they are used for shade so I can’t just remove them.  I would also love some wind block trees around a future arena.  Something like they use in orchards. But I’m not sure what they are or if they are safe for horses. 

You are correct that scrub oaks are not the best for horses, they can be toxic if the horses are hungry enough to eat any part of them.

Any tree that you plant will need to be fenced away from the horses, either through a small fence around the tree, or even better place trees outside the arena or pasture fence if possible. Horses can girdle any tree due to their habit of cribbing trees – though not all horses do this, read more here: https://ceh.vetmed.ucdavis.edu/health-topics/cribbing, basically they chew the bark around the tree and can kill the tree.

Most of the orchards have planted poplars or cottonwood trees as windbreaks. These windbreaks overall do a good job of breaking the wind and creating shade, but when these trees start getting old after a few decades the wind easily starts breaking them causing trees to fall, along with branches breaking loose. So don’t plant them right next to a barn or other structure. These trees do not pose any risk to animals from direct toxicity. Also consider fire risk depending on where you live, pine and fir trees pose more of a fire hazard than poplars or cottonwoods. Hopefully you can get a drip irrigation line set up to help get trees established when you first plant them or be prepared to water them with a hose on a frequent basis in the summer time.

In Oregon mainly oaks, walnuts, and maples are the trees that pose the greatest risk to horses. This article from Penn State Extension clearly lists trees you should avoid due to toxicity concerns with horses:

https://extension.psu.edu/trees-in-horse-pastures

This is a great website for looking up any plant to see if it is toxic to horses or not:

http://poisonousplants.ansci.cornell.edu

In terms of information on trees to consider, I would look at these two sites:

https://static1.squarespace.com/static/5a849d4c8dd041c9c07a8e4c/t/5a9f15e1652dea8c739715c4/ data hidden>84/pnw0005.pdf

https://www.trees4you.org/from-plants-to-planting/2016/12/29/windbreak-trees

Hopefully this information is helpful, if you need additional assistance feel free to contact me at<personal data hidden> at the Wasco County Extension Office.