The voluntary intake, digestibility and retention time by cattle and sheep of stem and leaf fractions of a tropical legume (Lablab purpureus)

Abstract

Hay made from the tropical forage legume Lablab purpureus was chopped and separated into leaf and stem fractions by using a gravity separator. Each diet was fed to four cattle and six sheep fitted with rumen fistulae to measure the digestibilities of the dry matter (DM), organic matter (OM) and neutral detergent fibre (NDF), and the retention times of DM, OM and NDF in the rumeno-reticulum (rumen) and the rumen volume. Mean voluntary intake of the DM in the leaf fraction was 79 % higher by cattle and 61 % higher by sheep than the stem fraction, and this appeared to be associated with the shorter time it was retained in the rumen. Both leaf and stem DM were digested to a similar extent by cattle. Sheep digested the leaf fraction to the same extent as cattle. However, the DM digestibility of the stem eaten by sheep was 0,061 lower than that of the leaf fraction (P < 0.05). Cattle retained the DM, OM and NDF in all diets for longer periods in the rumen than did sheep. This difference led to a 48% higher voluntary intake (g(kg body d-1) by sheep. The proportion of large particles that disappeared within the rumen was lower (P < 0.01) for cattle (0.968) than for sheep (0.994). The large particles of the leaf and stem fraction were retained for the same mean time by sheep (5.7 and 5.6 h), which was shorter (P < 0.01) than the corresponding times for cattle (13.8 and 21.6 h). The rate of disappearance of large particles in the rumen appeared to be only a minor factor causing the longer retention time of stem fractions in the rumen. The concentrations of volatile fatty acids and ammonia in the rumen were higher for leaf than stem fractions with no differences between cattle and sheep. The proportion of acetic acid for cattle (0.75) was higher (P < 0.05) than that for sheep (0.69) while the corresponding propionic acid proportions were lower (P < 0.01) in cattle (0.14) compared with sheep (0.19). These differences were associated with a higher passage rate constant for water flowing from the rumen and a higher digestion rate constant in sheep. It was concluded that both cattle and sheep ate more of the leaf than the stem fraction of the legume Lablab purpureus and that the higher intake of leaf was associated with the shorter time that it was retained in the rumen and was not due to differences in digestibility. Cattle had a smaller relative voluntary intake of legume than sheep, a difference that was associated with the longer time that digesta was retained in the rumen.