Do you need to feed grain to lowlines if feed good hay?

No. Grain should only be fed if a) you are wanting to finish them on grain for beef, or b) they are loosing weight and condition on hay, or if the hay is not adequate nutrition enough for lactating cows and/or growing cattle. If you're doing none of the above or your cows and growers are doing just fine on hay alone, then don't bother wasting your money on grain.