THE EFFECTS OF FORAGE PRESERVATION METHOD AND PROPORTION OF CONCENTRATE ON DIGESTION OF CELL-WALL CARBOHYDRATES AND RUMEN DIGESTA POOL SIZEIN CATTLE

Six bulls with ruminal and duodenal cannulae were used in a 6 x 6 Lati n square design with a 2 x 3 factorial arrangement of treatments. Barn -dried grass (G) and direct-cut silage (S) from the same sward were fe d together with 250 (L), 500 (M) and 750 (H) g kg-1 total dry matter ( DM) of a barley-based concentrate (barley 875 and rapeseed meal 125 g kg-1) at the level. of 80 g DM kg-1 live weight 0.75. Rumen and total digestibility of cell wall constituents were measured by a double mark er and total collection method. Rumen pool sizes of dietary constituen ts were estimated by emptying the rumen. Particle-associated enzyme ac tivities were measured from rumen particulate material and feed partic les incubated in nylon bags in the rumen. Neutral detergent fibre (NDF ) and hemicellulose digestibility were higher (P < 0.05) for G diets t han for S diets while no differences were observed between the forages in acid detergent fibre (ADF) and cellulose digestibility. Both rumen and total digestibility of cell wall constituents decreased with incr easing level of concentrate. The proportion of total cell wall digesti on in the rumen was unaffected by the forage preservation method and t he proportion of concentrate. NDF, and especially cellulose digestibil ity, declined quadratically with increasing level of concentrate. Diet ary effects on particle-associated carboxymethyl cellulase and xylanas e activity were consistent with those observed in cell wall digestion. There were no differences between the forages in rumen pool size of t otal ingesta or any dietary constituent. Rumen pool size of total inge sta decreased with increasing level of concentrate. On the other hand, DM content of ingesta increased with the level of concentrate, while the pool size of DM, NDF and ADF declined quadratically reaching a min imum on M level of concentrate. The differences in rumen NDF pool size were mainly in the digestible fraction. Calculation of digestion kine tic parameters showed that both the rate of passage and especially the rate of digestion were markedly depressed in animals fed on the highe st level of concentrate. The results suggest that a reduced rate of di gestion of NDF with high concentrate diets can be partially compensate d for by an increased NDF retention time in the rumen of animals fed a t a restricted level of feeding.