Hmm. According to
this source, cows can live up to sixty days without food. Being conservative, since these cows probably have less fat reserves to burn and we still want them to have enough meat to eat, that's still up to four weeks worth of extra food storage.
Said storage would involve 240 animals, and I think Mathilde would be best served breaking the casting into chunks to reduce the chain casting risks. Call it 60 casts per day spread out over four days, thirty casts an hour for two hours?
Obviously that many casts over the course of the expedition is well into dice roll territory, but the risk shouldn't be that bad if the time is managed correctly.