Comparative Starch, Fiber, and Energy Digestibility and Characterization of Undigested Starch Using Confocal Laser Scanning of Pulse and Cereal Grains in Growing-Finishing Pigs

Starch content is included in the equation to predict NE value of feedstuffs (NRC, 2012), but NE value may vary depending on apparent ileal digestibility (AID) vs. apparent total tract digestibility (ATTD) of starch. The botanical origin of starch determines starch structure and, thus, may affect the AID of starch. The objective was to determine digestibility of starch, fiber, and energy, and to characterize undigested starch of grains in growing pigs. Seven ileal-cannulated barrows (initial BW 30 kg) were fed 6 diets containing 96% of 1 of 6 feedstuffs (pulse: faba bean, field pea, chick pea; cereal: barley, wheat, corn) or a N-free diet in a 7×7 Latin square at 2.8 × maintenance DE to collect feces and digesta. Apparent hindgut fermentation (AHF) was calculated as ATTD−AID. Starch structure of feedstuffs and digesta was analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SCEM). The model included diet as fixed effect, and pig and period as random effects. Cereals contained more starch than pulses (57-68% vs. 41-45%), but pulses contained more total dietary fiber (TDF) than cereals (23-27% vs. 14-21%). The AID of starch was lowest (P<0.05) in faba bean (85.3%) followed by field pea and chick pea (87-90%), and cereals (93-96%). The ATTD of starch was almost complete (>99%) among grains; therefore, AHF of starch was greatest (P<0.05) for faba bean and field pea (12-15%). The AID and ATTD of TDF was greatest (P<0.05) in field pea and chick pea and lowest in corn. The AHF of TDF was greatest (P<0.05) for corn (42.0%) and lowest for wheat (23.8%). The AID of energy was greatest (P<0.05) in field pea and wheat; and AHF of energy was greater (P<0.05) in pulses than cereals (17-20% vs. 8-11%). The CLSM revealed that larger pulse starch granules were embedded in protein matrixes; hence, more starch granules remained intact in digesta. In SCEM images, the greater surface area of small corn starch granules may contribute to its 97% AID. Smooth and cracked surfaces of starch granules in digesta among sources indicated different stages of digestion. In conclusion, AID of starch is greater in cereals than pulses; however, ATTD of starch was complete among grains. Hindgut fermentation of starch and TDF of pulses will contribute to their energy value. Defining the digestible fraction of starch and fermentable fractions of starch and fiber may enhance accuracy of equations to predict NE value of feedstuffs.