CHAPTER 16 Updates to the Nutrient Requirements of the Horse: NRC 2007 Guidelines

The National Research Council (NRC), which was established in 1916 as the working arm of the National Academy of Sciences, produces many reports, including several related to the nutrient requirements of domestic animals. The NRC published the first edition of the Nutrient Requirements of Horses in 1961. Subsequent editions were released in 1966, 1973, 1978, and 1989. A committee to produce a sixth revision of the document was appointed in March 2004, and the report was released in 2007. Projects of the NRC are produced from the work of volunteer committees and are funded by nongovernmental sources. Much of the funding for this project was obtained from the livestock feed industry and equine breed organizations. Funds are used to support committee meetings, NRC staffing, and the technical aspects of publishing the document. Committees are appointed to reflect diversity in professional expertise and practical experience. The final report is compiled on the basis of published and unpublished research and consensus-driven discussion. Before publication, each report is submitted to rigorous independent review and revised appropriately. The sixth revised edition of the “Nutrient Requirements of Horses” was reviewed by equine nutritionists, feed industry professionals, and veterinarians from the United States, United Kingdom, and Germany.

The committee was given the charge to update the recommendations in the 1989 publication, review recent research on the nutrition and feeding of horses, and develop a user-friendly computer model to predict nutrient requirements for horses of different body weights and physiologic classes. Therefore, the 2007 edition of Nutrient Requirements of Horses covers a much broader range of topics than previous editions. The 2007 NRC includes more information on feeding management, feeding behavior, and feedstuffs than previous editions. A glossary of feed-related terms is included, as is a chapter on feed analysis, with a diagram that illustrates the chemical and analytical partitioning of carbohydrates in horse feeds. The new revision also includes discussion of several unique aspects of equine nutrition and feeding management, such as developmental orthopedic disease, equine polysaccharide storage myopathy, and recurrent airway obstruction. Information about feeding donkeys and other equids is also included in the 2007 NRC. As in previous editions, the document contains tables with estimates of the daily nutrient requirements of horses of different mature body weights and physiologic classes. In addition, a Web-based computer program is available that will calculate the nutrient requirements of horses on the basis of body weight and physiologic state. The computer program will also perform a ration analysis using feed compositions included in the feed database or feed compositions supplied by the user.

NONBREEDING ADULT HORSES

Nonbreeding adult horses are divided into two groups in the 2007 NRC: adult no work (also referred to as maintenance horses) and adult working (horses that have regular imposed exercise). The term horse is broadly applied to all members of Equus caballus, including ponies and draft horses. The 2007 NRC recognized that nonworking adult horses vary greatly in their voluntary activity and that there will be variation in the amounts of some nutrients needed to maintain body weight and condition. For example, it seems likely that a 12-year-old Quarter Horse gelding would be less active in its paddock or stall, and thus require fewer nutrients, than a similarly sized 4-year-old Thoroughbred kept in the same conditions. As a result, three levels of requirements were developed for adult nonworking horses: minimum, average, and elevated. The minimum category is designed for adult, nonworking horses that engage in limited voluntary activity, because of either confinement or temperament. Because of their sedentary lifestyle, these horses need fewer calories to maintain body weight and condition than horses in the average and elevated categories. Many horses in the minimum category would have been traditionally described as easy keepers. The elevated category was developed for horses often considered to be hard keepers, either because of increased voluntary activity or other factors.

The daily digestible energy (DE), crude protein (CP), calcium (Ca), and phosphorus (P) requirements of adult nonworking horses with mature body weights of 250, 550, and 850 kg (550, 1210, and 1870 lbs) are summarized (Table 16-1). Included are the 2007 and 1989 NRC recommendations. For the average 550-kg light horse, the 2007 and 1989 recommendations are similar. However, for the 250-kg pony and 850-kg draft horse, there are differences worth noting. In the 2007 NRC, DE and CP intakes for adult nonworking horses are calculated on a body-weight basis. However, in previous editions, methods were used that resulted in higher intakes per unit of body weight for smaller animals and lower intakes per unit of body weight for larger animals. The previous approach was founded on concepts relating surface area to body mass, which may apply across very broad ranges of animals. However, when that approach was applied to horses, it resulted in higher DE and CP requirements per unit of body weight for a pony than for a Thoroughbred. The new recommendations for DE and CP intake of ponies at the minimum and average levels are lower than the 1989 recommendations. Practical experience suggests that these lower values may be more appropriate for ponies because they are generally considered to be easy keepers compared with many light horses. However, if the minimum and average intakes prove to be too low, the elevated level is similar to the 1989 recommendation. The 2007 recommendations for DE and CP intakes of draft horses with average voluntary activity and an average temperament are higher than the 1989 recommendations (again, because of the different method used to calculate the requirements). In many cases, draft horses are considered easy keepers. Therefore, the minimum level of daily nutrient intakes may be most appropriate for nonworking draft horses.

Table 16-1 Nutrient Requirements of Nonworking, Nonbreeding Adult Horses

The three levels of nutrient intake for idle horses were developed to allow better management of individual variation among horses. If a horse is receiving nutrient intakes at the average level and unwanted weight gain results, then the minimum level may be applied. Similarly, if a horse does not maintain weight when fed at the average level, then the elevated daily intakes can be used. The range of DE intakes in Table 16-1 might not encompass all horses in all situations. The 2007 NRC guidelines also address the effects of cold and hot weather on daily nutrient requirements, so daily intakes can be adjusted to reflect environmental challenges.

The daily nutrient requirements of mature, idle horses for nutrients not included in Table 16-1 are similar to those proposed in 1989 guidelines, with the exception of iodine. In the original printing of the 1989 NRC guidelines, the iodine requirement was set at 0.1 mg/kg dietary dry matter (approx 1.1 mg/day for a 550-kg horse). A later printing of the same publication contained a range of values that would result in daily iodine intakes between 1.1 and 6.6 mg/d for a 550-kg horse. The 2007 NRC guidelines set the iodine requirement of a 550-kg, nonworking, nonbreeding adult horse at 3.85 mg/day (0.007 mg/kg body weight).