Grazing and pannage are still important to the Forest's ecology. Pigs can eat acorns without a problem, whereas to ponies and cattle large numbers of acorns can be poisonous. Pannage always lasts 60 days but the start date varies according to the weather – and when the acorns fall. The Verderers decide when pannage will start each year. At other times the pigs must be taken in and kept on the owner's land, with the exception that pregnant sows, known as "privileged sows", are always allowed out, providing they are not a nuisance and return to the Commoner's holding at night. Commoners must have backup land outside the Forest to hold these depastured animals if necessary.