Intake, rumen fermentation and nutrient flow to the omasum in beef cattle fed grass silage fortified with sucrose and/or supplemented with concentrate

The objective was to determine the relative effects of a specific increase in grass silage sucrose concentration, or a specific supplement of a starch-based concentrate, on rumen fermentation and nutrient supply to the omasum in beef cattle. Four ruminally cannulated Holstein–Friesian steers were fed grass silage only (G), G plus 3 kg concentrates/day (GC), G plus 90 g sucrose/kg dry matter (DM) (GS) and G plus 90 g sucrose/kg DM plus 3 kg concentrates/day (GCS) in a 4 × 4 Latin Square design experiment. Omasal flow was estimated using Co-EDTA, Yb-acetate and indigestible neutral detergent fibre (INDF) as digesta flow markers and purine bases as microbial markers. Concentrate supplementation reduced (P < 0.01) silage and increased (P < 0.001) total DM intake whereas sucrose had no effect. There was a sucrose × concentrate interaction (P < 0.05) for rumen pH whereby addition of sucrose to grass silage alone decreased pH and to grass silage plus concentrate had no effect. Rumen ammonia N (P < 0.01), total volatile fatty acid (VFA) concentration (P < 0.05) and the molar proportions of valerate (P < 0.05) and butyrate (P < 0.001) increased with concentrate supplementation whereas, sucrose supplementation had no effect on rumen fermentation parameters. Organic matter (OM) intake, omasal OM flow, the quantities of OM apparently (OMAD) and truly digested (OMTD) in the rumen (P < 0.001) and total tract OM digestibility (P < 0.01) increased, and apparent and true ruminal OM digestibility decreased (P < 0.05) with concentrate supplementation. Supplementation with concentrate decreased (P < 0.05) ruminal neutral detergent fibre (aNDFom) digestibility and increased (P < 0.05) aNDFom omasal flow. There was a tendency for addition of sucrose to increase (P < 0.1) ruminal OMAD and OMTD, while there was no effect of sucrose addition on intake or digestion of aNDFom. Concentrate supplementation increased (P < 0.001) N intake, flows of N, non-ammonia N (NAN), microbial N (MN) (P < 0.05) and non-ammonia non-microbial N (NANMN) (P < 0.01) and apparent total tract digestibility of N (P < 0.01), whereas sucrose reduced (P < 0.05) N intake and apparent ruminal N digestibility. There was no effect of concentrate or sucrose on N use efficiency or efficiency of microbial protein synthesis. Concentrate supplementation increased (P < 0.001) plasma β-hydroxybutyrate levels. In comparison to supplementing unwilted, well preserved grass silage of moderate digestibility with 3 kg starch-based concentrate per day, the limited response to the rate of sucrose supplementation employed suggests that increasing the water-soluble carbohydrate (WSC) concentration of grass silage through agronomic and/or ensiling practices will have relatively little effect on intake, rumen digestion or efficiency of microbial N synthesis.