A Model of Lamina Digestibility of Orchardgrass as Influenced by Nitrogen and Defoliation.

The morphogenesis of grass-sward regrowth drives the relationship between sward management effects and herbage digestibility. Our objective was to create a model of herbage digestibility for a range of N fertilizer levels and defoliation practices on the basis of changes in vegetative grass tiller structure rather than on dates of cutting or grazing. An experiment was conducted for two spring and two summer regrowths to examine the digestibility of orchardgrass (Dactylis glomerata L.) whole laminae (Lw) and youngest fully expanded laminae (Ly) for two N rates, and two defoliation patterns differing in the initial cutting date. Data were collected for lamina digestibility, dates at which a new leaf and its ligule were visible, and lamina and sheath lengths at least three times during each regrowth. There was a significant effect of N and cutting date on Ly and Lw digestibility, both variables being correlated significantly. At the ligule stage, Ly digestibility decreased between two successive leaves on a tiller, but this decrease was least when the N rate was low. For each regrowth, a single significant relationship was found between Ly digestibility and the growth duration of the lamina outside the sheath. As lamina growth duration depends both on sheath length and herbage N status, Ly digestibility was expressed according to these two easily measurable sward states. The model used to predict the green lamina digestibility at the whole canopy level could also explain the faster decline of herbage digestibility when the N rate was higher or when daily temperatures increased. In both cases, sheath length increased faster from one insertion level to the next, leading to a longer lamina growth duration.