The first cut is the.... most productive?

Grass does loose a lot of its volume pretty quickly as it decomposes and too much in the bin will make it very wet. Mixed 50-50 with cardboard should be good. Earlier in the year it is good to use as a mulch if slugs are not too much of a problem as it smothers weeds and conserves moisture in the ground. I pile it on round raspberries and fruit trees.