In Vitro Digestion Kinetics of Temperate Perennial Forage Legume and Grass Stems

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Abstract
Digestion of plant cell walls (CW) in the rumen is determined part by digestion kinetics. This study was conducted to gain information regarding plant factors that affect in vitro kinetics of CW digestion. Analyses were conducted on the basal 150 mm o immature and mature stems of field‐grown cultivars of alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), red clover (Trifolium pratense L.), smooth bromegrass (Bromus inermis Leyss.), and orchardgrass (Dactylis glomerata L.). Stem samples were digested in rumen fluid, and data were fitted with a first‐order nonlinear model to estimate potentially digestible CW (PDCW), indigestible residue (IR), digestion rate of PDCW, and digestion lag. These parameters of digestion kinetics were related to CW constituents determined previously. Maximum size of IR was about two‐thirds of CW mass. Within legume and grass stems, IR was closely correlated with lignic noncentration (r = 0.84 and 0.94, respectively). The IR:lignin ratio, however, was 71% greater in grasses than in legumes. The IR concentration was positively related by stepwise regression to total nitrobenzene oxidation products (NBO) of core lignin, lignin concentration, p‐coumaric acid concentration, and arabinose:xylose ratio; and was negatively related to proportion rhamnose in CW neutral sugars. Digestion rate of PDCW was 50% faster in legume than in grass stems and 70% faster in immature than in mature stems. By using stepwise regression, digestion rate of PDCW was negatively related to NBO:lignin ratio, and was positively related to proportion mannose in CW neutral sugars and to the arabinose:xylose ratio. Results of this study suggest that selecting for low NBO:lignin ratio and low p‐courmaric acid concentration will reduce the inhibitory influence of lignin on digestive kinetics.
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