Bedding for horses should absorb urine and excess water from the feces to keep the horse dry and comfortable. (Table 1,2)
Coir is by far the most absorbent material, absorbing 3.3 L per kilogram of material or 327% of its weight.
A 21 lb (9.6 Kg) bag of coir can be expected to absorb 32 Liters of urine
Lets put data into perspective: (Table 3)
Straw bale of 31.7 Lbs (14.4 kg) will absorb approximately 36 liters of urine.
Many horses will eat a proportion of their straw so extra straw may need to be added to the stall
A pound of wood shaving can only absorb close to 2 L of urine so you need to add more than one bale a day of wood shavings of 15 Lbs (6.8 kg) ~2 Cu.Ft to absorb the 20 L of urine that a horse approximately do per day.
You will need a minimum of 20 lbs of saw dust each day to absorb the urine from one horse.
Extraordinary Absorption Capacity
"Evaluating Performance of Several Horse Beddings"
Wheat straw, pine shavings, peat moss, and coir (product make from coconut hulls)
"Coir was by far the most absorbent material, absorbing 3.3 L per kilogram of material or 327% of its weight."
By Sarah Molnar, University of Guelph, Ontario and Dr. Bob Wright, Ontario Ministry of Agriculture Food and Rural Affairs, Fergus, Ontario, Canada