Digestion kinetics of leaf, stem and inflorescence from five species of mature grasses

The objective of this study was to measure the digestion kinetics for mature (green and non-senescent) components of five grass species using in sacco and in vitro incubations to define rates of degradation and nutrient release. Mature perennial ryegrass, tall fescue, Yorkshire fog, phalaris and paspalum were hand separated into leaf blade, stem and inflorescence (flower head) for incubations. Concentrations of neutral detergent fibre (NDF) in fractions were in the ranges 492-677 (leaf), 626-718 (stem) and 501-677 (inflorescence) g kg(-1) dry matter (DM). Crude protein concentrations in the DM of the respective fractions were 70-236 (leaf blade), 35-77 (stem) and 75-120 (inflorescence) g kg(-1) DM. Soluble DM (% of the total) determined after mincing accounted for 31-54% of leaf, 26-56% of stem and 20-49% of inflorescence and fractional degradation of the insoluble DM was very slow, in the ranges 0.04-0.11 (leaf), 0.03-0.05 (stem) and 0.03-0.08 (inflorescence) h(-1). After 24h of in vitro incubation, plant nitrogen content became limiting for fermentation in most instances, especially with tall fescue and paspalum. Volatile fatty acid (VFA) production appeared to be similar for leaf, stem and flower fractions, but the proportion of plant DM released as VFA after 48h was only 7-12%, with a higher value (19%) for tall fescue. Nitrogen concentration in forage DM was not directly related to VFA yield in vitro. In conclusion, the in sacco kinetics suggested slow colonisation of all components, but especially stem, which will limit the rate of nutrient production. Effective degradability was highest for leaf but rates in vivo will depend on the speed and extent of particle size reduction by chewing during eating and ruminating. (c) 2005 Society of Chemical Industry.