The effect of pelleting on the voluntary intake and digestibility of leaf and stem fractions of three grasses.

1. Leaf is eaten in greater quantities than stem of similar digestibility. To determine whether this difference is caused by physical or chemical factors, leaf and stem fractions from Digitaria decumbens, Chloris gayana and Setaria splendida were fed ad lib, to sheep in the chopped and pelleted forms. Pellets were made from leaf and stem which had been ground through a screen with 3 mm holes. All sheep received a protein and mineral supplement. 2. Voluntary intake of chopped leaf was 34 percent higher than that of the chopped stem fraction (40-3 and 30-0 g/kg body-weight 0-75 respectively, P smaller than 0.01) although dry matter digestibility ratios were similar (0-478 and 0-450 respectively, P greater than 0-05). The higher intake of leaf was associated with a larger surface area (13 400 and 5200 mm2/g for chopped leaf and stem respectively), lower bulk density (60 and 180 kg/m3 respectively) and lower neutral-detergent fibre (706 and 724 g/kg respectively), acid-detergent fibre (383 and 413 g/kg respectively) and lignin (42 and 59 g/kg respectively) contents. Chopped leaf was retained in the reticulo-rumen for a shorter time than the stem fraction (19.9 and 26.4 h respectively). 3. Grinding and pelleting increased the voluntary intake of the leaf fraction by 88 percent and the stem fraction by 60 percent. This increased voluntary intake caused by grinding and pelleting was not accompanied by any significant changes in the chemical composition of the diet. Grinding and pelleting reduced the time that the food was retained in the reticulo-rumen and this change appeared sufficient to account for the observed increases in voluntary intake. 4. It was concluded that the higher intake of the leaf fraction of grasses is caused by differences in retention time of food in the reticulo-rumen. These differences in retention time are caused by differences in physical properties and not chemical composition.