Analysis of the temperature effect on the components of plant digestibility in two populations of perennial ryegrass Jeroen CJ Groot,* Egbert A Lantinga, Jan H Neuteboom and Bauke Deinum CT De Wit Graduate School for Production Ecology and Resource Conservation (PE&RC), Wageningen University, Haarweg 333, NL-6709 RZ Wageningen, The Netherlands Abstract: For the development of mechanistic models of herbage digestibility, quantitative insight into the effects of age, temperature and cultivar on digestibility characteristics of individual plant parts is needed. Towards that goal, glasshouse experiments were conducted at day/night temperatures of 13/8, 18/13 and 23/18 °C with vegetative and reproductive crops of two populations of perennial ryegrass (Lolium perenne L) selected for differences in leaf blade digestibility. Cell wall content (CWC) and true cell wall and organic matter digestibility (CWD and OMD) of vegetative and reproductive tillers were related to dimensions, mass, CWC and digestibility of separate plant parts. Compared with the vegetative tillers, the reproductive tillers had higher rates of leaf appearance, organic matter growth and CWD decline. Strikingly, for both tiller types, no direct effect of temperature on whole tiller CWD was observed, since temperature effects could be eliminated completely by relating CWD to development stage (DVS) expressed as number of leaves appearing on the main tiller. Temperature effects on CWD were restricted to its influence on tiller development rate only. The decline of CWD of individual plant parts with DVS in the reproductive tillers could be described with a negative exponential curve, which reached an asymptote that was higher for leaf blades (755 g kg 1 ) than for leaf sheaths (491 g kg 1 ) and stem internodes (230 g kg 1 ). However, all plant parts in both tiller types had the same fractional CWD decline rate of 0.395 per leaf appearance interval, independent of plant part insertion level, population or temperature. Differences between temperature treatments in OMD were caused by the higher CWC of plant parts at higher temperature, due to a stronger decline of the specific organic matter mass than of the specific cell wall mass of plant parts at increasing temperature. Differences in whole tiller OMD between populations were observed only for vegetative tillers and were also caused by differences in CWC. It is concluded that temperature increase accelerated both the tiller development rate and the rate of decline of CWD during aging to the same extent, whereas plant parts responded similarly in the fractional CWD decline pattern as a function of DVS. These trends offer unique possibilities for modelling grass digestibility under contrasting temperature regimes. # 2003 Society of Chemical Industry Keywords: perennial ryegrass; digestibility; aging; temperature; vegetative; reproductive; cell wall; development stage INTRODUCTION Temperature and reproductive development are the most important factors determining composition and digestibility of grass. 1 Temperature affects digestibility through changes in three plant characteristics: (i) the rate of development, (ii) the ratio between cell wall and cell contents and (iii) the rate of cell wall aging, which results in a decline of cell wall digestibility. The contribution of these three plant processes to the generally observed temperature effect has not been quantified before and will be analysed in this paper. The analysis is based on glasshouse experiments with vegetative and reproductive tillers of two popula- tions of perennial ryegrass differing in leaf digestibility. The digestibility differences could have been caused by contrasts in cell wall digestibility, but differences in development rates could also play a role. This paper is a new contribution to an explanatory mechanistic analysis of changes in grass digestibility. In earlier papers the relationships between plant development and digestibility and the digestibility decline in consecutively formed individual leaves on the main shoot of Italian ryegrass were studied. 2,3 An important parameter related to plant development that will also be used here is the leaf appearance rate. 2 The aging of cell walls in leaves (decline of digestibility) starts even before leaf appearance, during the elonga- tion of the leaf in the sheath tube. 2–4 Therefore in (Received 11 February 2002; revised version received 4 September 2002) * Correspondence to: Jeroen CJ Groot, Group of Biological Farming Systems, Wageningen University, Marijkeweg 22, NL-6709 PG Wageningen, The Netherlands E-mail: jeroen.groot@wur.nl # 2003 Society of Chemical Industry. J Sci Food Agric 0022–5142/2003/$30.00 320 Journal of the Science of Food and Agriculture J Sci Food Agric 83:320–329 (online: 2003) DOI: 10.1002/jsfa.1315