I'm also no composting expert [member=210868]Markus99[/member] but I looked into collecting-up flail cuttings a while ago - mainly as a mulch, but with some composting.
(I'm told that my grandfather would never mulch with green material in his extensive veg' plot, but sorry Grandad, there are varying views on that.)
Unfortunately, simple 3PL or towed grass-rakes are neither widely available nor cheap (for what they are). I seem to recall there are available flail mowers with collectors, but way-beyond my budget (can't offer examples this far on from my searches). Of course, in the very old days they simply used manual hay-rakes to gather any cut grass/hay into bundles/rows ! I do this if I require a small "crop" for mulching and fork it into my rear tipper-box and/or cubic metre bags for mulch distribution. However, not so long ago I saw a TV prog' with a young lady working on her parents' small farm using a manual hay rake across a whole field of several acres !!