I don't know if they are poisonous. My horses do not eat them, but the real problem is they are very invasive and hard to get rid of, so you may end up fencing that area off and trying to spray them to kill them off, otherwise you quickly end up with more docks than grass.
Spray with grazon and keep horses off for 10 days. It will be a long term effort to get rid, but worth it. My horses don't touch them don't think they are poisionous, but they spread and squeeze the grass out if left.
P>S. nip the seed heads off when they appear if any get missed when spraying.
horses seem to be avoiding them at mine, i cant use sprays, so am weeding them out by hand, luckily there arent many and if i get the seed heads before they go over i'll be oon to a winner
Problem is the seeds spread in the wind and contaminate the rest of your grazing, so I would still get rid if poss. Sorry to be so pessimistic about the topic but I am battling against the dam docks, they are driving me mad, but I will get the b......ts , I back pack spray areas at a time, but new ones keep popping up. Then there is the ragwort, don't start me on them!!! I will get them too, if it kills me.!!!!