Effect of Rumen Inoculum on Diet Utilisation and Ruminal Fermentation Parameters of Commonly Used By-Product Ingredients in the Rumen Simulation Technique (Rusitec)

There has been increased interest in the use of by-products (BP) for ruminant feeds, yet a deficit in information regarding nutrient content of these feeds exist. The objective of this study was to assess the effect of different rumen fluid inoculum on in vitro digestibility, including Dry Matter Digestibility (DMD); Organic Matter Digestibility (OMD); Crude Protein Digestibility (CPD); Neutral Detergent Fibre digestibility (NDFd); Acid Detergent Fibre digestibility (ADFd), and fermentation parameters of four commonly used BP feeds: Maize Distillers (MD); Wheat Distillers (WD); Palm Kernel (PK) and Soya Hulls (SH). Rumen inoculum was sourced from six rumen-fistulated continental heifers blocked on weight and age and fed one of two divergent diets vis. 50: 50 silage: concentrate (C, rumen inoculum) on a dry matter (DM) basis or grass only (G, rumen inoculum). In vitro incubation trials were carried out with the rumen simulation technique (RUSITEC). The dietary in vitro treatments consisted of 60% silage (18% DM, 13.37% CP, 51.5% NDF and 37.39% ADF) and 40% of the BP ingredient on a DM basis. These were incubated in one of two rumen inoculum (C or G), replicated three times and distributed evenly among three eight vessel RUSITEC units, resulting in a 2x4 factorial design. No interaction was detected (P > 0.05) between rumen inoculum and BP ingredient for any of the parameters examined. Rumen fluid inoculum had no effect on the digestibility of DM, OM, CP, NDF or ADF of concentrate or whole diet; pH; daily gas production; total VFA or molar concentrations of individual VFA proportions (P >0.05). However, ammonia production was higher (P <0.05) in the C compared to G derived inoculum. The DMD of the BP were: WD (70%); MD (60%); PK (53%) and SH (34%) (P <0.0001). By-product ingredient had no effect on gas production, pH or total VFA production. However, concentration of individual VFA varied with by-product ingredient. Palm kernel and SH exhibited the highest acetate: propionate (A: P) ratio of 2.41 and 2.34 respectively. Wheat distillers resulted in the lowest A: P ratio of 2.08. Daily ammonia production was affected by by-product ingredient, resulting in WD > MD > PK >SH (P <0.01). In conclusion, in the RUSITEC culture system the in vitro digestibility and fermentation profile of by-product feeds are consistent and unaffected by rumen inoculum when sourced from divergent diets within a single animal species.