You'd have to make sure the tractor would be sturdy enough to protect the rabbits, in case the sheep freaked out. I don't think they'd just barrel through stuff, if there was a way around it, but bumping into it/ jostling it around would put the buns in a state of terror. If you have done the PPP research on the 3, have a good, sturdy tractor with shelter from the weather, and have plenty of shaded 'edge' areas to move them around to, I don't a see why not.
(PPP = possible parasite problems)
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My main concerns would be either parasite issues or trampling/jostling issues as Carla mentioned. I don't think there are many, if any, parasites that are likely to be an issue, but look that up.
For the jostling of the tractor by the sheep, I'd add making sure the tractor can handle the sheep standing on the roof. Sheep aren't as prone to that as goats, but I wouldn't be in the least bit surprised to find one or more standing on the tractor.
I assume you're doing rotational grazing? If so, I personally would have the rabbits either ahead of or behind the sheep by an appropriate amount depending on the length of grass that is idea for each (in other words, let the forage recover enough from one before the other is moved to that paddock). Who goes first should probably be dictated by parasite control needs followed by forage height requirements for each animal.
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