Date of Award


Degree Name

MS in Agriculture - Animal Science


Animal Science


Mark S. Edwards


Estimating nutrient and energy requirements of exotic animals is a necessary component of nutrition management in zoos and other wildlife facilities. In the absence of species-specific data, domestic animal models are often referenced. Herbivorous hindgut fermenters, such as horses, zebra, and rhinoceros, rely on microbial fermentation in the cecum and colon to utilize dietary structural carbohydrates. The study objective was to measure the digestible energy of two (LOW, HIGH) complete pelleted diets by the horse as a model for nondomestic hindgut fermenters. Seven, individually housed, adult Quarter Horse (Equus caballus) geldings were assigned to one of two diets as 100% of intake in a randomized crossover design. Experimental diets both contained similar ingredients including soybean oil as an added source of supplemental fat (LOW 1.7%, HIGH 6.9%). Diets differed in predicted digestible energy (LOW 2.29 Mcal/kg, HIGH 2.85 Mcal/kg, DE), ether extract (LOW 4.00%, HIGH 7.41%, EE), and acid detergent fiber (LOW 33.7%, HIGH 26.2%, ADF). Daily feed quantities were offered at 33.3 kcal DE BWkg-1 equally distributed over three meals to maintain target BW. Daily feed intake was quantified. Horses had ab libitum access to water. Horses were transitioned from all forage to 100% test diet over 14 d, acclimated to the test feed for 19 d prior to 4 d acclimation and 6 d total fecal collection using hygiene collection harnesses (Equi-San Marketing Pty Ltd). Diet transition between periods occurred over 8 d. Total fecal output was quantified every 8 h, thoroughly mixed and 10% of measured mass output was subsampled for further analysis. Body weights (BW) recorded weekly did not change significantly throughout the trial (P = 0.420). Apparent digestibility of diet within horse and day was evaluated by a nested ANOVA (Minitab 16). The apparent digestibility of EE (P < 0.000), neutral detergent fiber (P = 0.008), and ADF (P = 0.002) differed between the two diets. Apparent digestibility of DM (P = 0.137), OM (P = 0.140), and GE (P = 0.418) were not different. Excess fat not digested and absorbed in the small intestine (by-pass fat) will enter the hindgut and may cause disruption of normal microbial activity. Additionally soybean oil, when consumed in quantities that allow by-pass to occur, has been shown to have a negative effect on fiber digestibility in hindgut fermenters. A negative effect on fiber digestibility in the higher fat diet could result in diets closer in DM, OM, and GE digestibility than initially predicted. The NRC (2007) recommends that no more than 0.7 g/kg BW/d of soybean oil be fed to the horse. The HIGH diet provided 0.91 g/kg BW/d soybean oil. Feeds that contain concentrations higher than recommended may not be appropriate as the sole dietary ingredient of hindgut fermenters. Further studies are needed to evaluate the use of soybean oil and to determine the threshold at which soybean oil will begin to suppress hindgut fiber digestion. In vivo measurements of digestibility in model species may provide useful benchmarks from which diets for nondomestic hindgut fermenters, as well as horses, may be formulated.