I live in the PNW, and full disclosure, all I have are chickens and two Nigerian Dwarf goats... so I'm no expert. But I can point you to some experts. Sepp Holzer is your man. This thread in Permies is a good start. The idea is that animals can self medicate and aren't quite a dumb as we think.
https://permies.com/t/1294/animal-care/Sepp-Holzer-animal-husbandry
My experience with Goats seems to corroborate this. We let them browse whatever they want, although we don't give them access to things we aren't willing to destroy. (Goats LOVE cambium, that bottom layer of tree bark has a higher amount of sugars and goats will gird trees and kill them. I'm pretty sure they leave evergreens alone, but leafy trees are gonners. Sepp recommends a bone paste reduction that smells like death, apparently the goats leave that smell alone. I haven't tried it yet. But ask any butcher for extra bones and you'll get a garbage can full if you want em. (Actually, that's a "cheat code" for life... you could darn near live on bone broth...)
https://permies.com/t/71424/Sepp-Holzer-bone-salve-sauce
I would add a consideration, that where heavy breeding has been done,
I think that animals are dumber. We're optimizing for something, something else has to give. Plus, where animals are not getting taught by their parents
I think they might also be dumber. All my theory and I'm looking to prove/disprove that with time.
Where I am West of the Cascades, we have Tansy and Bleeding Hearts. My aunt freaks out about Bleeding Hearts. She's even insisted Bleeding Hearts and Foxglove
must be harvested with gloved hands or "they'll stop your ticker!" But I've eaten Bracken fern heads. We have 5 fer varieties, and I believe 4 are edible (do your research). Elderberry is edible, albeit very very bitter, which is
why every book insists they be boiled. (Do your own research) Everyone seems to freak out when Tansy is in their yard or pasture because animals could eat it and die an instant and painful death! I find lots of these kinds of concerns to be founded, albeit greatly embellished due to inexperience. The worst part is, there's a confirmation bias when people are
overly careful. And everyone wants to hang their hat on
one singular reason why things work or don't.
But I've never heard of a live animal eating live toxic things of their own volition. What I have heard, is if toxic things get harvested and dried out (e.g., tansy), and if that is fed (e.g., bails/flakes) to animals then they can't smell it like they normally would and are then liable to eat it unknowingly, then get sick/die. So, if you're buying your winter bails, you're probably fine. If you're bailing your own, be more careful. If you're offering your animals plants that are alive and growing in soil, they're probably ok. I'd even think that if you pull Tansy and leave it in the pasture to rot, they're not going to eat it. We also have bleeding hearts in our goat pen and they seem to be fine. Maybe they are fine or they are getting a little high now and again...
Anyway, like with selective breeding or pulling young away from their parents... whenever we break a natural cycle there's going to be some fallout. In this case, it seems we have to do the "smelling" for them if we feed them dried grass. (In the NW, what grass survives dry into winter... that's not normal...).
For stuff like this I'd consider a more "load, shoot, aim" approach. Do a little research, consider what the actual downside is, and then go for it. I've learned a bunch more from iterating through change than from what my neighbors think. In permaculture we're trying to optimize, or freeze, a system in its most desirable state. In some ways that's done through natural means, but it can have unnatural implications... find those and most of your risks are usually related. (E.g., We plant a garden, maximize for production, that's an unnatural concentration of energy for pests, pests come, we optimize for predators of pests... etc...)
I'm sure someone will reply to this and say I'm an idiot because their friend's horse... yadda yadda... I'm sure that's happened. But consider that later generations of plants will optimize their genetic material (yes, some genes are not immutable but can be switched on/off in offspring). The longer a plant is in an area the more suited it is for that area. Suppose we gather our own seed, but we also ensured the 100% survival rates for all the starts... What would happen? Would the later generations have the same benefit from these changed genes? I'd think they wouldn't. So yeah, folks can show pictures of what happens to animals who's mothers ate something toxic. In the wild, that would be self selection... But we're optimizing for 100% survival rates, regardless of intelligence or anything else... genes be damned... Consider how the system is getting disrupted where we optimize, and figure out what the fallout will be... Our breeding practices usually doesn't much care if animals can taste, smell, be smart... they want them to not die, sleep in filth, and eat this corn. Animals like that let out to pasture with toxic things are going to have a debt of selection to pay...
That said, consider different plants may change how the milk tastes, maybe... I don't have milking goats, maybe someone else can talk about that.
Your animals. Your risk. You weigh your risks. All I know is my goats are doing fine. So the tl;dr: Everything is toxic, if you consume enough of it. Consider favoring heirloom breeds.
What can my goats eat? Almost anything that is currently alive, in whatever quantity they choose. If you starve them or they eat everything else in the area, Tansy probably looks pretty yummie... Have lots of healthy growing things, notice what they eat most of, move them around from time to time and give them more of what they want... except grains. Goats will convince you they will DIE without their grains. Grains are crack for goats... We only use them to get them to lap up their minerals.