They are much more efficient than our domestic buns at relying on a totally fibrous diet which allows their caecums to function optimally and means that during a whole day they can get sufficient nutrition from their caecotrophs.
Dr McBride told me that they will often not come up above ground in bad weather and will survive on their caecotrophs and on any grass they have dragged below ground into their burrows (that becomes hay when dried). They will surface for as long as necessary to graze if needed. Buns that live primarily on the surface will be more likely to be seen above ground in bad weather.
Cold weather also means a slower metabolism and thus not eating so much is not a problem if the metabolism is also slowed. The problem occurs when metabolism is normal but eating is less = stasis.