Bracken fern (which we have an overabundance of) is very poisonous to livestock. You'll want to get it out of your pasture.
This is true. Fortunately, it isn't at all palatable to most animals. The largest danger (and this is true of *almost all* of the stuff I wrote above, especially the plants that grow naturally) is when pastures are depleted and the ferns are one of the few growing things available.
For people with livestock, one of the most important things you can do is to walk pastures and drylots at least weekly. For the pastures, you want to make sure that what looks like adequate feed (ie: it looks green from a distance) actually is adequate safe food plants, not just a bunch of barely edible (or potentially toxic) weeds.
Some things animals just don't touch. Jimson Weed is a narcotic plant that pops up in barnyards and waste areas around here every summer. I used to worry about it, and pull plants when I found them. But I noticed that even if it's the only green thing in the lower barnyard, the cows (and horses) simply never go near the stuff. I still yank most of it, just to keep it from going to seed and taking over instead of useful forage crops, but I don't worry about it anymore.
I also forgot to mention our current bane... poison hemlock! Again, it's not a plant livestock will willingly eat, but it doesn't lose much toxicity when it's dry, and they can't really pick it out of hay or silage. If you see a patch of what looks like Queen Anne's Lace, or Wild Carrot, blooming in June (at least a month too early) be VERY careful... It's probably Poison Hemlock.
Also, watch out for Wild Parsnip, and its even more dangerous relative, Giant Hogweed. Both contain an acrid sap or Judi e that will blister and burn the skin, and the damaged areas will be *severely * sun sensitive for years afterward. Hogweed can actually cause third degree burns!
St Johnswort, a useful medicinal herb, us considered a noxious weed in the West, because consumption of it will cause sever sun sensitivity, in hairless areas, or areas which have white hair (and less skin pigment). Again, it's a plant animals won't graze much by choice, but if it's a predominant pasture plant in an area, they will eat it.
Water hemlock is another truly toxic plant. And Locoweed is a problem in the American West.
Moldy.. I didn't realize that about amaranth! Since our son raises pastures pigs, it's a good bit of knowledge to have! The woods pasture where we plan to put his pigs when they mo've back to our farm doesn't have any, but it's something to keep in mind if we wanted to use some hogs to clean up a garden plot before planting.
Summerthyme