Evaluation of natural additives on in vitro gas production kinetics and digestibility of Xaraés grass combined or not with dry fiber and solubles

This study aimed to assess the impacts of various additives on the kinetic parameters of gas production and in vitro dry matter digestibility (IVDMD) and in vitro neutral detergent fiber digestibility (IVNDFD) in substrates with high fiber content. The additives evaluated were: control (CON) - without additives; Monensin (MON) - 20 mg/kg DM; Flavomycin (FLAVO) - 4 mg/kg DM; Live yeasts (LY) - Saccharomyces cerevisiae - 0.5 g/kg DM; Yeast culture (YC1) - 1.3 g/kg DM; Yeast culture (YC2) - 1.3 g/kg DM; and Essential Oils (EO) - 3.5 g/kg DM. The substrates used were: forage Urochloa brizantha cv. Xaraés (11.5% of CP) alone, Dry Fiber with Solubles (DFS) alone, and a combination of the two (50:50). Ruminal fluid was obtained from two castrated F1 Nellore x Angus (BW = 400 ± 25 kg), fitted with a rumen cannula, grazing Urochloa brizantha cv. Marandu, and receiving mineral supplement without additives. Three consecutive incubations were conducted with gas production volume (GP) measured on times 3, 6, 9, 12, 18, 24, 36, and 48 hours using a semi-automatic reader. A total of 67 flasks per incubation (7 additives x 3 substrates x 3 replicates) and four additional flasks as blanks (rumen liquid + buffer solution) were used in each run. Data were analyzed in a 7 x 3 factorial arrangement (7 additives and 3 substrates). No interactions between additives and substrates (P > 0.05) were observed for variables analyzed. Asymptotic GP was higher (P < 0.05) for combination of the forage + DFS substrate, intermediate for DFS alone, and lower for forage alone. The FLAVO supplementation increased GP (P < 0.05) compared to other additives, while MON and FLAVO inclusion reduced (P < 0.05) the digestion rate. Lag time was higher (P < 0.05) when only forage was used as a substrate, with no differences between additives. The MON and FLAVO decreased (P < 0.05) IVDMD, IVNDFD, and NH3-N. The substrate DFS stimulated microbial biomass synthesis (P < 0.05), with no significant difference observed between additives. In conclusion, regardless of the substrate, FLAVO inclusion promotes greater in vitro gas production, whereas MON and FLAVO had detrimental effects on DM and NDF digestion.