Fine grinding of plant material, as commonly done for forage quality evaluation, may mask effects of variation in plant morphological traits on plant digestibility. Moreover, late-maturity genotypes are more digestible than early-maturity genotypes when harvested on the same date, but the reason for this difference in digestibility is not known. This study was conducted to determine if fineness of grind, leaf size, or maturity group affects rate or extent of digestion of orchardgrass (Dactylis glomerata L.). Plants from two maturity groups (anthesis differed by 8 d) were selected for short, long, narrow, or wide leaf blades. Plants were harvested from a field planting on common dates in spring (near anthesis) and summer (after 5 wk of regrowth), and from 5 wk of regrowth in a greenhouse. Stem + sheaths from spring growth, and leaves from summer and greenhouse regrowth were ground to pass an 8- or 1-mm screen. Samples were incubated in rumen fluid, and digestion kinetics determined. Generally, there were no leaf size × fineness of grind interactions, and digestibility of the plant genotypes ranked the same regardless of fineness of grind and leaf size. Variation in digestibility remained the same regardless of grind size. The indigestible residue was 12% greater in leaves and 49% greater in stem + sheaths of early than of late maturing clones. There was no significant difference in digestion rates of leaves from the early and late groups (0.076 and 0.079 h−1, respectively), but stem + sheaths of the early group digested at 0.068 h−1, and those of the late group digested at 0.087 h−1. Therefore, maturity group had a greater effect on digestion kinetics of stem + sheaths than of leaves.
Joint contribution of Field Crops Res. Unit and U.S. Dairy Forage Res. Ctr. of the USDA-ARS, and Iowa State Univ. Journal Paper no. J-14333 of the Iowa Agric. and Home Economics Exp. Stn., Ames. Project no. 2709.