TWIN FALLS, Idaho (AP) — Firefighters say a large stack of alfalfa that went up in flames in south-central Idaho will burn for almost a week.

The 480-ton, $100,000 stack spontaneously combusted Thursday. Craig Giles, who grew the alfalfa, says he hired a custom operator to bale it. He says the bales were stacked with enough distance between them to allow moisture and heat to escape.

Agricultural officials say spontaneous combustion is common in haystacks because they often have trapped moisture in them. Once the moisture mixes with enough heat, the stack will combust. Officials recommend taking moisture samples rather than feeling by hand when bailing hay or alfalfa.