Jamieson, Fausset and Brown on Genesis 41:18

18. there came up out of the river seven kine--Cows now, of the buffalo kind, are seen daily plunging into the Nile; when their huge form is gradually emerging, they seem as if rising "out of the river."

and they fed in a meadow--Nile grass, the aquatic plants that grow on the marshy banks of that river, particularly the lotus kind, on which cattle were usually fattened.