Studies of cattle and sheep eating leaf and stem fractions of grasses. 2. Factors controlling the retention of feed in the reticulo-rumen

Abstract

Pangola grass (Digitauia decumbens) and Rhodes grass (Chlouis gayana) cut as 6 and 12 week regrowths were separated into leaf and stem fractions and fed ad libitum to four cattle and eight sheep fitted with ruminal cannulae. Cattle and sheep ate more of the leaf than of the stem fraction, and this was associated with the shorter time that neutral detergent fibre (NDF) was retained in the rumeno-reticulum (rumen). To study this difference, measurements were made of the retention time of lignin, passage rate of undigested NDF and water from the rumen, ruminal water volume, potential digestibility of NDF and rate of digestion of NDF by means of the nylon bag technique and in vivo techniques. Possible deficiencies of protein and minerals were minimized from this study by feeding casein and a mineral supplement. Lignin was used to measure the time that indigestible feed fractions remained in the rumen. The retention time of the NDF was closely correlated with the time lignin was retained in the rumen (r = 0.93, P < 0.01). This indicated that the longer retention time and lower intake found with the stem fraction was associated with the slower rate of passage of NDF through the reticulo-omasal orifice. The longer retention time of NDF by cattle compared with sheep could also be explained in this way. The passage rate constant for water leaving the rumen was slightly higher for cattle and sheep fed on leaf fractions and lower for cattle compared to sheep on all diets. The flow of water from the rumen through the reticulo-omasal orifice was 25 and 22% (P < 0.05) more for cattle and sheep respectively when they consumed leaf compared to when they consumed stem. The concentration of NDF in water passing through the reticulo-omasal orifice was similar for leaf and stem fractions (26 and 27 gl-1. There was a higher frequency of ventral ruminal sac contractions in sheep fed on leaf compared with those eating stem. The potential digestibility of the NDF was higher for leaf than stem fractions (0.658 v. 0.600, P < 0.05). Rates of digestion of leaf and stem samples in nylon bags suspended in the rumen of cattle were similar. Samples that had been masticated had a higher digestion rate constant than chopped grass (0.0216 v. 0.0159, P < 0.05). Digestion rate constants calculated from a rumen model showed large differences both between leaf and stem (0.0488 c. 0.0305, P < 0.05) and between cattle and sheep (0.0307 v. 0.0486, P < 0.05). It was suggested that the higher digestion rate constant calculated from a rumen model compared with the nylon bag method was due to additional mastication during rumination. It was concluded that the higher voluntary intake and shorter retention time in the rumen of the leaf than of the stem fraction of grasses was associated with an apparent higher rate of digestion of NDF in vivo, the higher rate of passage of the NDF from the rumen and the higher potential digestibility of the leaf. Sheep were found to retain NDF in the rumen for a shorter time than cattle, and this difference also was associated with a higher rate of digestion of the NDF and more rapid rate of passage of the NDF.