Seriously. They’re made from potatoes:
[E]co-friendly tent pegs … are to be made from biodegradable potato starch. The problem with the normal metal sort is that they can injure grazing dairy cows which inhabit the field once the festival is finished. The metal pegs are “a real problem for the cows”, said [organiser Michael] Eavis. The potato pegs, which organisers are buying in advance, will therefore be made compulsory for campers this year.
With around 175,000 visitors expected, that’s a lot of pegs. Apparently they’re already in widespread use in the turf industry, and they’ll hold up for the three days of the festival, after which they’ll start to break down. I have no idea if the cattle, once returned to the grounds, will actually try to eat the pegs.