A few other aspects to consider when grazing (amongst) bracken.
1. Ticks love bracken, and if there are deer about or you are using sheep, there will be ticks. So you would probably need to treat your stock with a tick and/or tick disease preventative. Plus if your disease is a Lyme Disease area, you will want to try to limit the contact between people and ticks.
2. I heard (but it could be incorrect) that the spores would be carcinogenic without being specifically eaten. They'd get onto stock that was moving through the bracken, and then be ingested when the stock lick themselves and each other.