Analysis of the temperature effect on the components of plant digestibility in two populations of perennial ryegrass

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 23/18degreesC 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. (C) 2003 Society of Chemical Industry.