Effect of Particle Size upon Digestible Cell Wall and Rate of In Vitro Digestion of Alfalfa and Orchardgrass Forages1

Alfalfa and orchardgrass forages chopped through a 13-mm screen were larger in particle size than were rumen ingesta samples from sheep fed the same forages. Samples of the chopped forages were ground through 1-, 4-, 8-, and 12-mm screens; particle sizes of the 8- and 12-mm ground forages were not significantly different from those of rumen ingesta samples or chopped forages. The 1- and 4-mm ground forages were significantly smaller in particle size than the 8- and 12-mm ground forages, rumen ingesta and chopped forage. Alfalfa contained less cell wall at the beginning of in vivo digestion, more cell wall residue at the end of digestion and less potentially digestible cell wall than did orchardgrass. The cell wall of alfalfa was 44% digestible, whereas that of orchardgrass was 61% digestible; particle size did not affect the amount of digestible fiber. Alfalfa ground through 1- and 4-mm screen openings had digestion rate constants that were about twice those of the 8- and 12-mm granulations (.10 less than P less than .20). Particle size did not affect rate constants of orchardgrass. These data suggest that in vitro kinetic studies based on examination of finely ground forages (1) may not reflect in vivo conditions, and (2) may depend upon the type of forage. Differences between legumes and grasses in (1) amount of digestible fiber and (2) digestion rate constants and the interaction of the above two factors with particle size and passage rate have important implications for digestibility of forage fiber.