I fed my guinea pigs on forages only and raise my horses on grass 24/7 during most of the year. Grass is only too rich when an animal is not used to it or has a health problem. I had one horse with a metabolic disorder that would founder on new spring grass, after a drought (grass grows faster then), clover, and other situations. If done smartly though the more greens the better for most grazing herbivores.
Grazing animals also evolved to have a constant supply of food. I was told if a guinea pig goes more than 4hrs or a horse more than 6hrs without food their digestive tract can begin to shut down. Such animals also don't sleep all night but take naps between eating. Unless they are on or given enough grass and forages for the entire night they need hay. With guinea pigs being so small I could just keep a grocery bag of forages in the fridge and dump the whole thing in the cage in the evening. It would be completely gone by morning and then I'd put them out in the double fenced herb garden all day where they would keep the plants weeded and trimmed.