Vertical distribution of biomass, chemical composition and pepsin - cellulase digestibility in a perennial ryegrass sward: interaction with month of year, regrowth age and time of day

The vertical distribution of biomass, chemical composition (crude protein ( CP); neutral detergent fibre (NDF); acid detergent fibre (ADF); acid deterg ent lignin (ADL); total soluble carbohydrates (TSC)) and organic matter dig estibility estimated from pepsin-cellulase (PCOMD) of a perennial ryegrass sward was studied under strip-grazing management. Measurements were carried out during three grazing cycles (May, June and October) for three regrowth ages in each month (21, 28 and 35 days) and at two times in the day for ea ch of these ages (08.00 and 19.00 h). Each grass sample was cut in four lay ers (0-5, 5-10, 10-15 and >15 cm), then freeze-dried, ground and analysed. Very large vertical gradients were observed in the different chemical const ituents, with the following mean variations from the upper to the lower lay er of the sward: +80 g DM/kg fresh grass, -100 g CP/kg OM, -30 g TSC/kg OM, +250 g NDF/kg OM, +22 g ADL/kg OM and -25 units PCOMD (%). These variation s in chemical composition linked to height in the sward were often of great er magnitude than the variations measured on the whole plant for each month , regrowth age or time of day. For a given chemical constituent, the rankin g between layers was unaffected by season, regrowth age or time of day, exc ept in the case of TSC. However, the differences in contents of certain con stituents between layers were sometimes seen to vary strongly with the mont h (bulk density, CP, TSC), regrowth age (CP) or time of day (TSC). The vert ical distribution of NDE ADL and PCOMD showed relatively little variation w ith month, regrowth age or time of day within the vegetative stage studied. Equations were established which relate chemical composition to height in t he sward. From these results, it is possible to simulate the influence of s ward defoliation depth on the chemical composition of the diet selected by grazing ruminants. (C) 2000 Elsevier Science B.V. All rights reserved.