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ABSTRACT:The objective of this study was to identify and characterize yeast strains isolated from the ruminal ecosystem that are capable of enhancing fermentation in bovines that consume high-fibre diets recommended by livestock feed guidelines in Cuba. The yeasts were isolated from the rumen of Holstein cows that had been fed a biofermented product. Isolated colonies were purified, identified, and characterized using biochemical and molecular methods, and their effects on ruminal fermentation were compared by measuring in vitro gas production. Thirteen new strains enhancing gas production with potential use as additives in ruminal fermentation were identified and named Levica. These strains grew successfully in detection medium for non-Saccharomyces wild yeasts and had long survival periods in the rumen. PFGE analysis found four karyotypes and homology of D1/D2 domain of gene 26S rDNA sequence was similar to that of I. orientalis, R. mucilaginosa, P. guilliermondii, and C. tropicalis. Phylogenetic analysis classified the strains into clades A and B. Clade A was further divided into groups AI, AII, BI, and BII. The AI cluster contained Levica (L)23, L24, L29, L33, and formed a monophyletic group with I. orientalis, while group AII contained L18 and formed a monophyletic group with R. muciloginosa. The BI cluster contained L13, L15, L17, L27, L28, and L32, all derived from P. guilliermondii. Cluster BII was composed only of L25 located in a separate subclade, forming a monophyletic group with C. tropicalis. The most useful strain for preparing microbial feed products to improve ruminal fermentation was L25 because it showed an increase in gas production.

Solid-state fermentation can be used to produce feeds for ruminants, which can provide an enriched population of yeasts to improve ruminal fermentation. Fermentation of apple bagasse was performed to obtain a yeast-rich product, with the objective of isolating, identifying, and characterizing yeast strains and testing their capability to enhance in vitro ruminal fermentation of fibrous feeds. Yeasts were isolated from apple bagasse fermented under in vitro conditions, using rumen liquor obtained from cannulated cows and alfalfa as a fibrous substrate. A total of 16 new yeast strains were isolated and identified by biochemical and molecular methods. The strains were designated Levazot, followed by the isolate number. Their fermentative capacity was assessed using an in vitro gas production method. Strain Levazot 15 (Candida norvegensis) showed the greatest increase in gas production (p < 0.05) compared with the yeast-free control and positively affected in vitro ruminal fermentation parameters of alfalfa and oat straw. Based on these results, it was concluded that the Levazot 15 yeast strain could be potentially used as an additive for ruminants consuming high-fiber diets. However, further studies of effects of these additives on rumen digestion, metabolism, and productive performance of ruminants are required.

This study evaluated the effect of Candida norvegensis (C. norvegensis) viable yeast culture on in vitro ruminal fermentation of oat straw. Ruminal fluid was mixed with buffer solution (1:2) and anaerobically incubated with or without yeast at 39°C for 0, 4, 8, 16, and 24 h. A fully randomized design was used. There was a decrease in lactic acid (quadratic, p = 0.01), pH, (quadratic, p = 0.02), and yeasts counts (linear, p<0.01) across fermentation times. However, in vitro dry matter disappearance (IVDMD) and ammonia-N increased across fermentation times (quadratic; p<0.01 and p<0.02, respectively). Addition of yeast cells caused a decrease in pH values compared over all fermentation times (p<0.01), and lactic acid decreased at 12 h (p = 0.05). Meanwhile, yeast counts increased (p = 0.01) at 12 h. C. norvegensis increased ammonia-N at 4, 8, 12, and 24 h (p<0.01), and IVDMD of oat straw increased at 8, 12, and 24 h (p<0.01) of fermentation. Yeast cells increased acetate (p<0.01), propionate (p<0.03), and butyrate (p<0.03) at 8 h, while valeriate and isovaleriate increased at 8, 12, and 24 h (p<0.01). The yeast did not affect cellulolytic bacteria (p = 0.05), but cellulolytic fungi increased at 4 and 8 h (p<0.01), whereas production of methane decreased (p<0.01) at 8 h. It is concluded that addition of C. norvegensis to in vitro oat straw fermentation increased ruminal fermentation parameters as well as microbial growth with reduction of methane production. Additionally, yeast inoculum also improved IVDMD.

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