Herbage digestibility, which to a large extent governs the animal's intake, is not easily predicted for grazing animals, and can vary due to different grazing managements. The objective of this study was to build a model for a range of defoliation managements, which takes account of vegetative grass tiller structure, rather than dates of cutting or grazing, as done usually. An experiment was conducted in 1997, near Toulouse, to examine the digestibility of cocksfoot whole lamina and lamina segments (15 cm) for three defoliation regimes. These regimes mimicked severe and lenient intermittent defoliations plus a frequent defoliation treatment over a six-week period. Data were collected for (i) lamina mass and digestibility for the youngest expanded lamina, (ii) lamina appearance rate, lamina and sheath lengths.

Laminae digestibility, measured over time, varied greatly with the defoliation regime both for the youngest full expanded lamina and all laminae on a tiller. Digestibility of the youngest fully expanded lamina decreased from one leaf insertion level to the next, but this decrease was lowest when the defoliation regime was frequent. There was also a consistent decrease in the digestibility of a lamina from its tip to its base. Sheath and lamina lengths varied with treatment in a different way to lamina digestibility. During frequent defoliation, sheath and lamina lengths both increased slightly while they showed substantial increases in the two other treatments. Lamina length increased from one insertion level to the next, and resulted in the addition of less digestible lamina segments rather than reducing overall lamina digestibility. Sheath length, through its influence on leaf appearance rate and lamina length, could be used to predict the digestibility of laminae over time according to leaf insertion level and the difference in the digestibility observed from the tip to the base of a particular lamina. Sheath length could therefore provide a basis for predicting the time course of grass digestibility for different combinations of defoliation frequency and height.