Modelling grass digestibility on the basis of morphological and physiological plant characteristics

Grass digestibility is determined by the rate of plant development, mass of plant organs (leaf blades, leaf sheaths and stem internodes) and composition of organs. The development of an integrating model for grass digestibility necessitates the quantification of developmental characteristics of plants and their organs and the effects of environmental factors and management practices. The main objective of this study was a thorough analysis of changes in composition and digestibility of plant organs of two grass species ( Lolium perenne and L. multiflorum ). These characteristics were quantified in glasshouse experiments on vegetative and reproductive plants.Detailed analyses were made of the specific cell wall ( s cw ) and cell contents ( s cc ) mass of the various organs (mg OM cm -2), which determine cell wall content (CWC), and of cell wall digestibility (CWD). During growth, s cw and s cc of all plant organs increased. After full organ expansion had been reached, s cw remained unchanged, while s cc declined, resulting in an increase of CWC. CWD of plant organs declined during ageing. The increase in the proportion of indigestible cell wall per leaf appearance interval could be described by a negative exponential curve, with a fractional rate that was the same for all plant organs, temperatures and populations. The decline in CWD of whole shoots was a linear function of shoot development stage, i.e. number of appeared leaves. Cutting had only a marginal effect on the digestibility of whole shoots.The implementation of these trends in an object-oriented simulation model resulted in acceptable estimations of growth, development and digestibility of grass under contrasting environmental conditions in the field. Sensitivity analyses with the model demonstrated that the morphological plant characteristics leaf blade length and width, and leaf appearance rate have no systematic effect on composition and digestibility. The model using object-oriented principles offers large opportunities for simulation of collections of individual plants and plant organs in complex and varying environments.The cumulative gas production technique is a potential addition to in vitro measurement of digestibility. It was demonstrated that a multiphasic equation is required for mathematical description of gas production curves. Curves from incubations with strongly contrasting types and concentrations of substrate, medium and inoculum could be fitted precisely with one flexible, multiphasic sigmoidal equation. Parameters in the equation were related to biological phenomena. These findings contribute to improved interpretation of in vitro fermentation studies and fine-tuning of the cumulative gas production technique.