I have been practicing grass based dairying since I first got into goats in 2007. I do provide my goats with minerals, and alfalfa pellets and during bad weather a bit of grass hay. Today I came across an article that really goes into depth on the benefits to everyone involved with goats are given the opportunity to browse.
The younger the goat the greater the success in transitioning them from a life of leisure to a life of having to rustle up their own grub. And a pampered goat isn't going to get out there unless they don't have an alternative. Also the younger the goat the more likely they will develop the rumen necessary to process that forage into tasty and healthier milk.
So take a moment to read up on this at the Goat Dairy Library.