What did Black Beauty live on before he could eat grass?

Black Beauty lives on his mother's milk before he's old enough to eat grass. Like all young horses, Black Beauty needs to wait for his teeth to develop before he can eat solid food. Feeding on his mother's milk allows the frail young foal to form a very close bond with her, running alongside her during the daytime and lying down next to her at night. The mare and her foal are inseparable. They stand by the pond in the shade of the trees when it's hot; when the weather gets cold, they snuggle up together in a nice warm shed near the grove.
When Black Beauty's finally old enough to eat grass, he starts spending less time with his mother. She goes out to work in the morning, returning to be with her foal in the evening.