381S.L. Ralston et al. APPARENT DIGESTION OF HAY/GRAIN RATIONS IN AGED HORSES-REVISITED

Research on digestion in aged (> 20 yrs old) Quarter Horse and Thoroughbred horses in the mid-1980s revealed that the population studied at that time had reduced apparent digestion of protein, phosphorus and fiber relative to younger horses fed pelleted alfalfa (Ralston et al., 1989). The digestive profile of the aged horses was very similar to that reported for horses which had had 90 % resection of the large colon (Bertone et al., 1989a and 1989b). In two subsequent digestion trials on aged horses conducted in New Jersey in the 1990s, the reductions of digestibility, especially in protein, were not as apparent. In trial 1, conducted in 1990, eight light horse mares> 19 years old were used in a simple crossover design to compare digestibility of a commercial textured sweet feed (OM) and one formulated specifically for aged horses (ES). The grains were fed in amounts calculated to provide 50 % of the recommended caloric requirements (NRC, 1989) for maintenance, with the rest of the ration being provided as long stem hay. The mares were adapted to the rations for 2 weeks and then subjected to a standard 5-day digestion trial, during which intake was recorded and al