Effects of grass maturity and legume substitution on large particle size reduction and small particle flow from the rumen of cattle.

Five ruminally and abomasally cannulated heifers (average weight 365 kg) were fed 6 kg/d of early (EOG) or late (LOG) maturity orchardgrass (OG) hay with or without replacement of EOG or LOG by 1.5 kg/d red clover hay (RC) in a 5 x 5 Latin square design. A fifth treatment, of LOG supplemented with 100 g/d casein (LOG + C), was used to evaluate the effect of supplemental protein. In situ fiber digestion, in vivo nutrient digestion and rate of passage from the rumen of large (retained on a 1.68-mm screen; labeled with La) and small (passed a 1.68-mm screen; labeled with Sm) EOG and LOG particles were determined in heifers fed these diets. Early maturity OG hay had 20% and 62%, respectively, greater (P less than .05) rates, and 61% and 73%, respectively, greater (P less than .05) extents of in situ NDF and ADF disappearance than LOG. Red clover substitution for OG increased (P less than .05) in situ extent of NDF disappearance by 6%. Early maturity OG large and small particles had 40% and 36%, respectively, greater (P less than .05) rates of passage than LOG. Large and small particle ruminal pool sizes were increased (P less than .05) by 115% and 33%, respectively, with LOG. Red clover substitution for OG increased (P less than .10) large and small particle output (kg/d) from the rumen by 15% and large particle rate of passage by 20%. Grass maturity may be affecting intake through rate of large particle size reduction and passage and small particle passage, although legume supplementation of OG may influence intake by increasing rate of large particle size reduction and passage.