Until this question is answered definitively, the best suggestion is to choose a hay cube product made from coarsely chopped hay rather than finely ground forages, and if possible, to combine feeding hay cubes with some natural pasture or long-stemmed hay.
Hay cubes are favored by many trainers of Thoroughbred and Standardbred racehorses because they tend to be highly digestible and less "bulky" in the gut than long-stemmed hay, writes Karen Briggs in the January edition of The Horse. But that very digestibility -- the ease and speed with which they are chewed and swallowed -- can be a potential problem. Horses that are fed exclusively on hay cubes rather than regular hay tend to find themselves with an unsatisfactory grazing urge that can result in chewed lumber, a lot of consumed bedding, or even in vices like cribbing or stall walking. Horses that bolt their feed also might be at increased risk of choke when fed hay cubes (although soaking the cubes in water for about 10 minutes before feeding tends to slow down these horses somewhat). There's also some question as to whether the short particles of chopped forage do as good a job of stimulating the hindgut as do the long stems of ordinary hay. Some researchers feel that the two- and three-inch particle length found in hay cubes is quite sufficient to keep the gut moving along as it should, but others say the jury is still out and that horses on a hay-cube diet might be at increased risk of colic.