Despite the fact that horses have evolved as foragers, with a diet high in fiber and water, many horse owners feed their pets high-calorie pellets which are low in fiber. Researchers at Pomona Animal Hospital have published findings in the December issue of the Physiology & Behavior journal that demonstrate how horse behavior changes with diet. In the paper, titled "Horses’ Behavior and Welfare Negatively Impacted by Low-Fiber Diets", the authors outline how horses that are deprived of a natural foraging diet are at risk of developing gastric ulcers and negative behavior such as destruction of their stalls.
The study assessed eight mares fed two different diets, either unlimited orchard grass hay or a complete pelleted feed for three weeks. The horses were also trained to press a panel to obtain a food reward. The study found that horses fed hay spent 61.5% of their time eating, while horses fed pellets spent only 10% of their time eating, and that horses fed pellets spent 58% of their time standing while horses fed hay spent 37% of their time standing. In addition, horses fed pellets spent 11.5% of the their time sifting through the bedding of their stall looking for food whereas horses fed hay only spent 1.2% of their time practicing this "searching behavior".
The authors say that these findings show that horses being fed a pelleted diet became motivated to look for hay, and recommend feeding horses at least half a bale of first cutting hay per day, or offering unlimited access to hay. Horses exclusively fed a pelleted diet also appear to have too much "free time", and may develop destructive habits such as damaging wooden stalls and doors if they are not adequately worked.