Andrea Ellis and coworkers researched the effectiveness of different hay nets in a study entitled: Effect of forage presentation on feed intake behaviour in stabled horses, published in the journal, Applied Animal Behavior Science. They found that hay nets with small hole sizes of 25-30mm were equally effective to prolong eating time (by five minutes) compared to a traditional hay net with large (75mm) holes.
Horses consuming hay from the small hole hay nets appeared to spend more time overnight eating, though all horses were able to consume their equivalent allotments of hay over a 24-hour period. Based on these findings, it can be recommended to feed horses that are being offered limited amounts of hay (due to dietary restriction) in hay nets that have smaller (25-30mm) holes.