“You can’t necessarily milk an oat,” Fatima Disu, a food scientist at Hood, said in an interview, raising her fingers and pretending to tug the udders of a tiny oat grain.

But in January, after a six-month sprint, the company began shipping four varieties of its Planet Oat milk to national supermarket chains. It’s Hood’s first standalone plant-based brand, and at $3.99 per carton, it is no haute milk, like the artisanal pea, flax, or hemp varieties cropping up in natural food stores. This is the dairy-free nectar of the proletariat — free of all major allergens, low in added sugar, with a richer, creamier taste than other plant-based “alt-milks.” It’s so popular that Amazon can’t keep it in stock, company executives say.

That one of the nation’s major dairy producers considered it a coup to introduce an oat milk to America was yet another sign that the traditional milk market is at a crossroads. Americans bought $16.16 billion of cow’s milk in the year ending in late March 2015, the market research firm Nielsen found, but that number dropped to $12.13 billion at the same time this past year.