You need to first recognise what the bracken fern is telling you:
1. you soil where the bracken fern is growing is LOW in fertility: yours and all the neighbouring farms that have the bracken have probably been heavily overgrazed in the past and the land flogged of its fertility
2. the bracken fern is TOXIC to stock because it is one of the plants trying to recover and rebuild your fertility, NATURALLY. IF animals try and eat it whilst it is trying to do its job, it cannot succeed. That's why Nature made it toxic so that animals wont eat it. You may have to supplementary feed till the problem is fixed.
3. you need to raise the fertility with organic matter, manure anything.
4. you need to keep stock out until the recovery process has had time to restore fertility and you have increased the biodiversity.
Isabel, your take on this is a little far over toward the organic and naturalistic -- what you say is not untrue but it's coming from a certain more extreme perspective. Ferns are toxic -- because of their genetics, because of the kind of metabolism they have -- not because of some fore-ordained cosmic plan to keep animals from eating them. Because if you think about it -- by the time the fern has poisoned the horse that eats it, the fern is a "goner" too.
Plants that colonize damaged or overgrazed ground are called "early successional plants". Again, we find them there not because they have any conscious plan that they're going to help to make the ground fertile again. In fact, early successional plants sometimes suck up lots of the trace minerals and nitrogen in the soil because one of their characteristics is that they grow fast and that requires their roots to pull in a lot of nutrients. When they die and decompose, it's true that those nutrients go back into the soil but they do not return more in the way of nutrients and trace elements than they took up in the first place. Their decomposition may help to form humus, which can assist in creating a more loamy soil with better structure and capability for aeration, which may in turn attract worms and other subsoil fauna which DO put more nutrients into the soil by way of their poop.
Early successional plants are, as their name implies, eventually replaced by late successional plants, i.e. woody shrubs and the smaller, faster-growing sorts of trees. Or, depending upon the climate in the particular area, they may be succeeded by more lasting types of grasses to form a savanna or prairie in which there is a stable ratio of grass to brush. Of course this cannot happen fully on any farm, because the humans who manage the land also have needs and preferences -- although if they understand ecological succession, that will inform their management.
Of course horses and other livestock should always be removed from any area where there are toxic plants -- no matter what our take on why those plants are growing in pasture or paddock. Cheers -- Dr. Deb