I don't own sheep, I own goats. Here's a link that will give you a start... http://poisonousplants.ansci.cornell.edu/php/plants.php?action=display&ispecies=sheep
Generally, sheep are grazers first and browsers second. They also "select" what tastes best to them so will be somewhat selective as they move through a pasture. Time will show what they like best and what they don't and the remainder will need to be tended to with a mower.
IMHO, unless you offer them only poisonous plants, they are pretty selective and know what they can and can't eat safely. Even if they do eat a few bites (sampling) of something that is "poisonous" to them, the little they eat in conjunction with everything else is not typically going to kill them. Maybe some of the Sheeple here can chime in with better guidance. @Sheepshape@mysunwolf@Mike CHS@Baymule@secuono@Roving Jacobs@farmerjan And there are others...
They are selective which is why we find it easier to do intensive grazing and move them fairly often. When they have the run of our almost 19 acres they will move quickly over the entire place eating only their favorite graze. When we restrict them they are less selective and eat the majority of things. There are lists of toxic plants some of which we have but it doesn't bother our flock. Even Milkweed is supposed to be mildly toxic but they go after that first to the point that we no longer have Milkweed. I got rid of all wild cherry trees since the leaves are bad for them when they start falling but even those they browsed on before I took them out.
My sheep are Dorper/Katahdin mixed. They LOVE elm leaves, they like oak leaves, but only the young, spring leaves. They adore giant ragweed, lambs quarters, poor joe and other weeds. They like Bermuda and Bahia grass, clovers and rye grass. They like pine needles and cedar also. Mine were instrumental in clearing masses of green briars up as high as they could reach. When we chopped the vines off at the ground and dragged them out of the trees, the sheep descended on than and ate the leaves off. They will kill honeysuckle, eating it down to the roots. They will strip off the leaves of dewberries and blackberries.
What are the grasses in your area? Sheep will browse, but graze more than they browse.