Horses like to sleep in a soft bed just like their owner's do.
How do we solve the problem of having to cut down trees, or having to dispose of tons of soiled bedding?
My horses are not currently in stalls (Lucky to live with my horses on my property with a super large paddock), so I make a big "mattress" of about 1.5 feet of bedding. I noticed that I don't go through bags and bags of bedding like I did when they lived at a barn. I realized that if I routinely forked and turned some of the shavings, a few tosses whenever I thought about it, that I was effectively mulching the shavings, and as they degraded, they turned to a sweet smelling compost. I even preferred this mulch over a new bag of tannic acid smelling shavings (or whatever the shavings producers may be adding to retard mold on the shavings - are there such additives? need to do more homework for another article). The mattress of shavings lasted indefinitely.
Once the mulch is degraded, where it is too fine to withstand moisture and turns into muck when pee'd-on, THEN I add more clean shavings, or rice hulls, which when combined with shavings, seems to forestall the degrading of the bedding.
It gets trickier if your horse lives in a stall most of the time.
To my delight, while I haven't yet verified availability in the United States let alone Southern California, the Swedes have been recycling horse manure into bedding. Ok, I want to be partners with the genius who charges ranchers to pick up their manure, and then sell the stuff back to them as bedding.