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

Abstract

The vertical distribution of biomass, chemical composition (crude protein (CP); neutral detergent fibre (NDF); acid detergent fibre (ADF); acid detergent lignin (ADL); total soluble carbohydrates (TSC)) and organic matter digestibility 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 each of these ages (08.00 and 19.00 h). Each grass sample was cut in four layers (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 constituents, with the following mean variations from the upper to the lower layer 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 variations in chemical composition linked to height in the sward were often of greater magnitude than the variations measured on the whole plant for each month, regrowth age or time of day. For a given chemical constituent, the ranking between layers was unaffected by season, regrowth age or time of day, except in the case of TSC. However, the differences in contents of certain constituents between layers were sometimes seen to vary strongly with the month (bulk density, CP, TSC), regrowth age (CP) or time of day (TSC). The vertical distribution of NDF, ADL and PCOMD showed relatively little variation with month, regrowth age or time of day within the vegetative stage studied. Equations were established which relate chemical composition to height in the sward. From these results, it is possible to simulate the influence of sward defoliation depth on the chemical composition of the diet selected by grazing ruminants.