Effects of Dietary Restriction in Horses

Horses evolved as grazers, programmed to eat more or less continuously on forage of varying quality without access to large meals of carbohydrate-rich grains. Modern management strategies have strayed from this pattern, and many horses are overweight or obese because they are often allowed full access to high-quality forage as well as regular supplementation with grain-based feed products. Obesity has been linked to equine metabolic syndrome, a condition in which horses may develop excessive fat stores, cresty necks, insulin resistance, and a higher risk for laminitis.

Two recent studies have looked at the effects of dietary restriction on equines. One study conducted at the University of Liverpool in the U.K. was designed to investigate changes in insulin sensitivity in horses with equine metabolic syndrome following a period of dietary restriction. A second goal was to see if improvement in insulin sensitivity would be greater in horses receiving short-chain fructo-oligosaccharides (sc-FOS). Horses in the study were fed grass hay at 1.25% of body weight. The hay was soaked in cold water before feeding to remove some nonstructural carbohydrates (38% reduction), digestible energy (almost 7%), and water-soluble minerals. The horses were also given a vitamin-mineral supplement with or without the addition of sc-FOS at a rate of 10 g/100 kg body weight.

After six weeks of dietary restriction, horses lost an average of 6.8% of body weight. They also had lower body condition scores and smaller belly circumferences. Results of a combined glucose insulin test showed improvement in insulin sensitivity, with the degree of improvement related to the degree of resistance recorded at the start of the study and each horse’s extent of weight loss and reduction in body condition. However, the improvement in insulin sensitivity was independent of whether or not the horse received sc-FOS. Based on this study, a strict dietary program combined with a specifically designed vitamin and mineral supplement can be recommended to produce improvements in body weight, body condition score, and insulin sensitivity in horses with equine metabolic syndrome.

A study conducted at the University of Gottingen in Germany evaluated the effect on health and welfare of potential feed shortage on Shetland ponies. Ten Shetland mares were managed in semi-natural outdoor group housing for seven months. Five of the ponies had restricted feed for four of the seven months. Restricted ponies lost an average of 18.4% of their body mass, and their body condition scores also decreased by an average of 2.2 points on a scale where 0 indicates emaciation and 5 indicates obesity. There was an increase in total bilirubin in restricted ponies, with these levels out of the reference range during the trial. Nonesterified fatty acids (NEFA) also rose in restricted ponies compared to control ponies, indicating the mobilization of stored fat. Levels of total protein and beta-hydroxybutyrate were also lower in restricted ponies at the end of the trial. Blood parameters of ponies in the control group were within reference ranges. Results of this study suggest that year-round outdoor management with additional feed supply is an adequate housing system for robust breeds like Shetland ponies.

Owners of overweight horses and those with equine metabolic syndrome should consult with an equine nutritionist before putting horses on a restricted feeding program. Serious health problems can result if horses are severely restricted in energy and forage intake levels.