Grazing comes naturally for horses, but there's more to it than just chewing pasture. Researchers know horses can adjust their rate of feed intake based on food source availability, but they recently set out to determine if horses can select for or against certain nutrients or nutrient compositions.

Sarah E. Redgate, PhD, now a senior equine lecturer at Nottingham Trent University in England, worked with colleagues from the University of Lincoln and the WALTHAM Equine Studies Group (both also in England) to evaluate if horses' voluntary feed intake, selection patterns, and behavior could be influenced by dietary composition.

The team employed seven geldings, ranging in age from 6 to 20 years, in good health and dental status. During the study, the horses resided in individual stalls with drylot turnout and received daily exercised in a mechanical horse walker. The researchers fed the horses three diets containing a chopped timothy hay base to ensure digestive health and natural feeding behavior; each diet contained similar levels of digestible energy but were individually rich in either protein, hydrolysable carbohydrates (carbs that can be broken down with water), or fat. In addition, study horses received free-choice haylage, soaked beet pulp, and chopped forage daily, and a vitamin/mineral supplement twice daily to meet their nutritional requirements.

Then the researchers collected data via a technique not previously used in horses. They initially allowed horses access to all three diets—each at 2-2½% of the horse's body weight—in a "self-selection" phase. Next, they fed the horses every diet alone for two three-day periods each during a "monadic" phase, which the researchers said was crucial to the study design as it ensured the horses learned to associate each diet with its individual nutritional characteristics. Finally, the horses underwent another self-selection phase with free-choice feed access. This allowed the researchers to gauge whether horses had changed their diet selection after consuming each one individually, the team said