Why can't a newborn foal eat grass?

Horses like all mammals rely on their mothers milk initially and yes in the first few hours of life they receive vital antibodies which can stop them from picking up infections until their own immune system is up to the job.

Most foals are born with teeth and a lot of very young foals will attempt to graze and mess around with fibrous material like grass but are unable to eat or digest sufficient quantities to survive.

they don't have teeth and swallowing it might cause choking. They need the antibodies and nutrients from it.

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