In situ particle size reduction as affected by forage species and grinding mesh size

Two ruminally cannulated heifers were used to evaluate effects of three forage species (gamagrass, orchardgrass, and red clover) ground through two mesh sizes (2 and 5 mm) in a 3 x 2 factorial arrangement on particle size reduction in situ. Residues in polyester bags incubated for 3, 6, 12, 24, 36, 48, 60, and 72 h were analyzed for mean particle size. Initially, gamagrass had a 10.2% greater (P < .05) proportion of DM as particles larger than 300 microns that did orchardgrass or red clover, whereas forages ground through a 5-mm mesh (5-mm forage) had a 7.5% greater (P < .05) proportion of DM as 300 microns or larger particles than forages ground through a 2-mm mesh (2-mm forage). Mean particle size decreased with incubation time, resulting in reductions of 44, 42, 53, and 49% of gamagrass, orchardgrass, and 2- and 5-mm forages, respectively, after 60 h. Mean particle size of red clover was reduced 67% after 36 h of incubation. Particle size reduction was initiated earlier (12 vs 36 h) and occurred at a 150% faster (P < .01) rate for red clover than for gamagrass or orchardgrass. Although there was no difference (P > .10) between grinding mesh size at the time at which particle size reduction was initiated (24 h), particle reduction occurred at a 136% faster (P < .05) rate for forages ground through a 5-mm screen than for forages ground through a 2-mm screen. A greater (P < .10) reduction (595 microns by 36 h) in mean particle size occurred in red clover than in gamagrass or orchardgrass (average 376 microns by 60 h) during ruminal incubation. In addition, 5-mm forages exhibited a greater (P < .10) reduction in mean particle size (527 microns) than did 2-mm forages (372 microns) after 60 h of incubation. Substantial particle size reduction occurred in situ due to microbial digestion. The faster particle size reduction found in red clover may contribute to the greater observed intake for legumes.