The objective of the present study was to examine the potential utilization of the yeast strain Pichia guilliermondii (Levica 27) on the ruminal fermentation of corn stover. Two in vitro treatments were evaluated: (A) filtered rumen liquid + powdered corn stover + yeast-free culture medium and (B) filtered rumen liquid + powdered corn stover + culture medium inoculated with yeast. Samples were collected at 0, 6, 12, and 24 h and examined for effects on culture pH, ammonia nitrogen (NH₃-N), the fiber digestibility percentages of neutral detergent fiber and acid detergent fiber, the digestibility of dry matter, and the concentration of volatile fatty acids (VFAs). The results showed that the addition of Levica 27 had no effect on the ruminal pH, NH₃-N or the digestibility indicators (p > 0.05). However, at 12 h, the inclusion of Levica 27 increased (p < 0.05) the molar concentrations of total VFAs and acetic and propionic acids in ruminal fluid and decreased (p < 0.05) the C2:C3 ratio. It is concluded that Levica 27 seems to have a stimulating effect on ruminal fermentation over a range within 6 and 12 h, a result that should be taken into account when in vivo studies are performed with this yeast strain.

The potential of hops (Humulus lupulus L.) and β-acid extract were evaluated for improving the quality of maralfalfa grass (Cenchrus purpureus) silage (with added sorghum grain, sorghum straw, and urea) during aerobic exposure and their residual effects on in vitro ruminal fermentation characteristics. Silage samples and ground hops pellets (Galena and Chinook varieties) as well as β-acid mixtures were incubated at 37 °C for 24 h and then maintained under aerobic exposure for 12 h. The sample pH, counts of filamentous fungi, yeasts, and total coliforms, and volatile fatty acid (VFA) concentrations were determined. Subsequently, in vitro ruminal fermentation was conducted to determine total gas production and concentrations of hydrogen, methane, carbon dioxide, and VFAs. The β-acid treatment controlled yeast populations, but an increase (p < 0.05) in pH values was observed for the Galena and Chinook treatments compared to the Control. However, pH did not differ significantly (p > 0.05) between the Control and the β-acid treatment. Butyric acid concentrations in the silage were lower (p < 0.05) compared to the Control, except in the silage treatment with Galena. In the in vitro ruminal fermentation, the β-acid treatment showed higher butyric acid levels than the Chinook and Galena, but these differences were not significant (p > 0.05). There were no differences (p > 0.05) in methane between the treatments. An increase (p < 0.05) in propionic acid concentration was observed in the in vitro ruminal fermentation with β-acids. It was concluded that β-acids could help reduce silage deterioration during the aerobic phase, reducing the butyric acid and yeast populations, and their residual effect could improve ruminal fermentation, increasing propionate and acetate concentrations.

This study aimed to evaluate the effect of three yeast strains, Pichia guilliermondii (Levica 27), Candida norvegensis (Levazoot 15), and a commercial product (Levucell® SC 10) containing Saccharomyces cerevisiae, on in vitro rumen fermentation of two substrates: corn stover and a total mixed ration (TMR). Samples were taken 24 hours post-fermentation to measure pH, ammonia nitrogen (NH3-N), gas production, and volatile fatty acid (VFA) concentrations. The addition of yeasts did not affect gas production at 24 hours for either diet. For pH, differences between yeast strains were noted when TMR was used. The lowest NH3-N concentration (6.1 mM) was recorded with corn stover, showing similar results across strains. Conversely, NH3-N levels with TMR ranged from 47 mM to 66 mM. The inclusion of all three yeast strains increased the molar concentrations of total VFA, as well as acetic (C2), propionic (C3), and butyric (C4) acids in the rumen fluid. The C2:C3 ratio remained unchanged with corn stover but decreased with the inclusion of Levica 27 and Levazoot 15 in the TMR. With the commercial product Levucell® SC 10, this ratio was comparable to the control treatment. In conclusion, Levica 27 and Levazoot 15 demonstrated a stimulatory effect on in vitro rumen fermentation for the evaluated diets, evidenced by increased VFA concentrations.