The effects of forage preservation method and proportion of concentrate on digestion of cell wall carbohydrates and rumen digesta pool size in cattle

Abstract Six bulls with ruminal and duodenal cannulae were used in a 6 × 6 Latin square design with a 2 × 3 factorial arrangement of treatments. Barn‐dried grass (G) and direct‐cut silage (S) from the same sward were fed together with 250 (L), 500 (M) and 750 (H) g kg‐1 total dry matter (DM) of a barley‐based concentrate (barley 875 and rape‐seed meal 125 g kg‐1) at the level of 80 g DM kg‐1 live weight0·75. Rumen and total digestibility of cell wall constituents were measured by a double marker and total collection method. Rumen pool sizes of dietary constituents were estimated by emptying the rumen. Particle‐associated enzyme activities were measured from rumen particulate material and feed particles incubated in nylon bags in the rumen. Neutral detergent fibre (NDF) and hemicellulose digestibility were higher (P<0·05) for G diets than 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 increasing level of concentrate. The proportion of total cell wall digestion in the rumen was unaffected by the forage preservation method and the proportion of concentrate. NDF, and especially cellulose digestibility, declined quadratically with increasing level of concentrate. Dietary effects on particle‐associated carboxymethylcellulase and xylanase activity were consistent with those observed in cell wall digestion. There were no differences between the forages in rumen pool size of total ingesta or any dietary constituent. Rumen pool size of total ingesta 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 minimum on M level of concentrate. The differences in rumen NDF pool size were mainly in the digestible fraction. Calculation of digestion kinetic parameters showed that both the rate of passage and especially the rate of digestion were markedly depressed in animals fed on the highest level of concentrate. The results suggest that a reduced fate of digestion of NDF with high concentrate diets can be partially compensated for by an increased NDF retention time in the rumen of animals fed at a restricted level of feeding.