A lord could try to refuse, but their serfs had the right to appeal to his overlord if he did so without good cause, and there were very rarely situations where you had good cause to not help feed a family who had very little food over the winter, barring a costly war. The feudal contract was taken very seriously under Griffonian law. You had clansworn, they worked your land and fought for you in war and paid your taxes, but whatever extra they made was theirs to keep and do with as they will.