Artificial vitamins for maintaining your body?

Can any scientist say that this is the vitamin value in dry grass? If there is no vitamin value in dry grass, how the cow is producing so much milk, full of vitamins A and D? They prescribe some artificial vitamins for maintaining your body. Now, what is the vitamin there in the dry grass so that the cow is eating dry grass and giving you nice milk full of vitamins. So these are wrong theories.

~Srila Prabhupada, 66/05/23 N.Y.