| Summary: | This study aimed at examining the effects of rumen inoculum of steers receiving different combinations of ionophore and probiotics in their diets on in vitro gas production of corn silage. The fitting of gas production was performed with five mathematical models and its kinetics was evaluated. Four crossbred steers (403.0 ± 75.5 kg body weight) with ruminal cannula were assigned to a 4 × 4 Latin square design. The additives used were Monensin sodium (Rumensin<sup>®</sup> 100, 3 g/day), <i>Bacillus toyonensis</i> (Micro-Cell Platinum<sup>®</sup> 10<sup>9</sup>, 1 g/day) and <i>Saccharomyces cerevisiae boulardii</i> (ProTernative<sup>®</sup>20, 0.5 g/day). Additives were arranged into the following treatments, supplied daily into total mixed diet: (1) Monensin; (2) Monensin + <i>B. toyonensis</i>; (3) Monensin + <i>S. boulardii</i>; and (4) <i>B. toyonensis + S. boulardii</i>. The gas production data were fitted into the models of Gompertz, Groot, Ørskov, Brody, Richards, and Dual-pool Logistic. A perfect agreement between observed and predicted values in curves of accumulated in vitro gas production was observed in the Groot and Richards models, with higher coefficient of determination (R<sup>2</sup> = 0.770 and 0.771, respectively), concordance correlation coefficient (CCC = 0.871 and 0.870, respectively), and root mean square error of prediction (RMSEP = 1.14 and 1.15, respectively). Evaluating the feed additives throughout the Groot model, the <i>B. toyonensis + S. boulardii</i> treatment presented higher <i>V<sub>F</sub></i> (12.08 mL/100 mg of DM; <i>p</i> = 0.0022) than Monensin and Monensin + <i>S. boulardii</i> (9.16 and 9.22 mL/100 mg of DM, respectively). In addition, the fractional rate of gas production (<i>k</i>) was higher (<i>p</i> = 0.0193) in <i>B. toyonensis + S. boulardii</i> than in Monensin, not presenting a statistical difference (<i>p</i> > 0.05) from the other two treatments. Additionally, with the time of beginning to gas production, the lag time (λ), was greater (<i>p</i> < 0.001) with Monensin and Monensin + <i>B. toyonensis</i> than with Monensin <i>+ S. boulardii</i> and <i>B. toyonensis + S. boulardii</i>. The combination of Monensin and probiotics (<i>B. toyonensis + S. boulardii</i>) resulted in better kinetics of degradation of corn silage, being that the Groot and Richards models had the best fit for estimates of the in vitro gas production data of corn silage tested with different feed additive combinations.
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