Any question answered...

What nutrients are they lacking then? They certainly will me low in calories, hence the high volume required but not rich in nutrients compared to?

Grass doesn't lack the nutrients that grazers need, it's just not rich in them, so that a grazer needs to firstly eat a lot of grass, and also have a mechanism to digest grass, which is almost entirely indigestible due to the high cellulose content. Digestion takes a long time, so the digestive system needs to be large to allow a large quantity to be digested slowly, so the animals tend to be large, which in turn means they need a lot of nutrition, so they basically spend most of their waking time grazing.

And then they need to have teeth that keep growing because chewing silica-rich plants is not something that your dentist would recommend.