It's really not a silly question
In my view (and of course I might be entirely wrong), most 'normal' grass is just grass, with minimal nutritional differences between them. Alfalfa is different, but I suspect you're not growing Alfalfa.
I would just feed them a small selection of all of it from day one. I presume you don't have vast quantities anyway and so the amount they will get each time won't be massive. If you were to go into a meadow and pick a handful of 'grass', there would be several different types. I know this as we have a small 'hay patch' in our back garden and there are probably around 15-20 different grasses. Slightly off subject, but it always surprises me how hay companies can sell just one species of grass hay. How, in an enormous field, do they prevent other species arriving?