Resistance of leaf and stem fractions of tropical forage to chewing and passage in cattle.

The voluntary intake (VI) of separated leaf and stem fractions of a grass and legume (Panicum maximum and Lablab purpureus respectively) was determined using Hereford steers fistulated at the rumen and oesophagus. VI of leaf fractions was higher than that of the stem fraction (8.23 v. 3.67 kg/d, P less than 0.001) while that for the legume diets was higher than for the grass diets (6.65 v. 5.22 kg/d, P less than 0.05). The total number of eating chews per day was higher on the leaf than stem fraction (1.6 x 10(4) v. 9.8 x 10(3), P less than 0.05). The mean number of rumination chews (2.4 x 10(4)) was similar (P greater than 0.05) for all four diets. The mean resistance of large particles (LP, i.e. retained on a 1.18 mm sieve during wet sieving) to breakdown (chews per g LP breakdown) during eating was lower for leaf than stem fractions (8.4 v. 23.7) and lower for the grass than legume diets (10.5 v. 21.6). The mean resistance to breakdown of LP by rumination (chews per g LP breakdown) was lower in leaf than in stem fractions (8.2 v. 13.2, P less than 0.01) and higher in grass than in legume (12.5 v. 9.0, P less than 0.05). The resistance of LP to breakdown during rumination was higher than during eating for the grass diets, but was lower for the legume. Fractional passage rates (FPR) of small particles (i.e. passing through a 1.18 mm sieve during wet sieving) from the reticulo-rumen were negatively related to dimensions of particles, with greater ease of outflow for legume than for grass particles of the same length or diameter. When corrected for content of cellulase-indigestible fibre, FPR of small particles of leaf was greater than for small stem particles. It was concluded that VI of tropical forages was associated with the resistance of LP to breakdown by chewing during both eating and rumination and that the patterns of escape of small particles from the reticulo-rumen were only partially explicable in terms of particle dimensions, and that other properties of the particles may be of importance.