The effect of replacement of wheat bran by graded levels of molasses on digestion, degradation and digestion kinetics of fibre and rumen digesta pool size in crossbred ( Bos taurus × Bos indicus) steers fed native grass hay

Abstract

This 4 × 4 Latin square experiment evaluated the effect of replacement of wheat bran by graded levels of sugar cane molasses on digestibility, degradation of fibre, digestion kinetics of fibre and rumen pool sizes. Four ruminally fistulated crossbred ( Bos taurus × Bos indicus) steers were given a basal diet of grass hay ad libitum and 4.0 kg dry matter (DM) of wheat bran day −1 (Diet M0). The other three diets consisted of replacement of wheat bran with 1.0 (Diet ML), 2.0 (Diet MM) or 3.0 (Diet MH) kg DM of molasses. Urea was used to make the three diets (ML, MM and MH) isonitrogenous with the basal diet (M0). Intake and digestibility of neutral detergent fibre (NDF) decreased from 4.65 to 3.31 and 0.65 to 0.55 (linear effect, P < 0.01) with increasing levels of molasses. Similarly, NDF digestion expressed as a proportion of potentially digestible NDF decreased linearly ( P < 0.01) when the level of molasses increased. The disappearance of hay DM from nylon bags decreased (significant linear effect after 48 h incubation period and onwards, P < 0.05) with increasing wheat bran substitution by molasses. The disappearance of NDF of hay from nylon bags decreased significantly only at 48 h (linear effect, P < 0.01) and at 96 h (quadratic effect, P < 0.05) when the amount of molasses increased. Although there were differences between diets in rumen pool sizes of DM, NDF and in digestion kinetics of NDF, these were not, in general, explained by polynomial orthogonal contrasts. Increasing the level of molasses caused a clear linear effect ( P < 0.01) only for rate of digestion ( k d) of NDF and true rate of digestion ( k dt) of NDF. There was also a quadratic effect ( P < 0.01) for rate of passage ( k p) of indigestible NDF, showing that diets ML and MM had greater rate of passage compared with the other two diets when the level of molasses was increased.