EFFECT OF FORAGE PARTICLE LENGTH ON RUMEN DIGESTA LOAD, PACKING DENSITY AND VOLUNTARY FEED-INTAKE BY SHEEP

The effect of forage particle length (4 to 101 mm) on packing density of oesophageal extrusa was investigated following ingestion of chopped wheaten hay by sheep. Packing density of oesophageal extrusa from 4 mm hay was greater than that from hay 21 mm and longer; the difference was associated with smaller particle size and a lower modulus of fineness. There was a significant positive correlation between the intake rate of hay by individual sheep and size of extrusa particles for three of the five forage lengths. The effect of packing density of ingested forage on rumen digesta load and voluntary intake was examined subsequently by feeding sheep hay chopped to either 4 or 21 mm lengths. Reducing the particle length of hay from 21 to 4 mm resulted in small increases (P < 0.05) in the intake of dry matter (DM) [26.0 cf 27.8 g per kg rumen digesta-free liveweight (RFLW)] and organic matter (OM) (23.8 cf. 25.5 g per kg RFLW). Increases in intake were paralleled by increases (P < 0.01) in loads of both rumen DM (29.7 cf. 33.9 g per kg RFLW, for 4 and 21 mm hay respectively) and OM (25.2 cf. 29.1 g per kg RFLW). The higher digesta load was associated with higher concentrations (P < 0.05) of DM (133.6 cf. 122.5 g per kg total digesta) and OM (114.9 cf. 103.8 g per kg total digesta) in the rumen, but not with a difference in the measured packing density of particulate matter. The increase in the intake of 4 mm hay was totally attributable to the greater DM load of rumen digesta because the fractional outflow rate of particulate matter was lower for the 4 mm hay (0.55 cf. 0.63/day) and the apparent fractional rate of digestion was similar (0.42 cf. 0.43/day) in sheep fed the two lengths of forage.