Yeast culture refers to a micro-ecological product formed by sufficient anaerobic fermentation of yeast on a specific medium under the control of specific process conditions. It is mainly composed of yeast extracellular metabolites, medium after fermentation and culture. A small amount of inactive yeast cells are formed. Metabolites are a general term for extracellular metabolites. Some of these substances are familiar, such as peptides, organic acids, oligosaccharides, amino acids, flavoring substances, and aromatic substances, and many others are not. Familiar but proven to be "unknown growth factors" and other substances that are beneficial to the growth of livestock. The decades of scientific research and production application practice have proved that extracellular metabolites contained in yeast culture can significantly improve the productivity of ruminants, optimize the nutritional value of feed, and improve the health of animals. At the same time, people have carried out a lot of research work on the mechanism of yeast culture in ruminants. Most scholars believe that yeast culture mainly uses the metabolites to increase the quantity and vitality of microorganisms in the rumen, thereby improving rumen fermentation and improving. The efficiency of digestion and utilization of feed ultimately plays a role in improving animal performance. This paper reviews the mechanism and application of yeast culture in ruminants from the rumen microflora, rumen fermentation function, nutrient intake and utilization, in order to have a new understanding of yeast culture.
1 Effect of yeast culture on rumen microflora
Many researchers believe that the role of yeast cultures is to alter rumen fermentation and microflora (Williams and Newbold, 1990; Dowson, 1992; Newbold, 1996; Wallace, 1996), increasing the number of bacteria in the rumen (Wallace and Newbold, 1992). ), improve bacterial protein synthesis, improve bacterial AA composition (Beharka and Nagaraja, 1991; Dawson and Hopkins, 1991; Erasmus et al, 1992). Although Doreau et al. (1998) believe that yeast culture does not affect the balance between bacteria. However, many scholars believe that yeast cultures can selectively stimulate specific microorganisms in the rumen, thereby changing the microflora. Among them, the most consistent report is to increase the number of anaerobic and fibrolytic bacteria (Wiedmeier et al, 1987; Harrison et al, 1988; Newbold and Wallace, 1992). Studies have also shown that yeast cultures can increase the concentration of lactic acid-utilizing bacteria (Edwards, 1990; Girard et al, 1993), proteolytic bacteria (Yoon and Stern, 1996), and hydrogen-producing acetogens (Chaucheyras et al, 1995b). In addition, yeast cultures can also increase the activity of fungi (Chaucheyras et al., 1995a) and protozoa (Plata, 1994) in the rumen. The increase of the concentration of beneficial microbial flora in the rumen and the increase of microbial activity are conducive to the digestion of crude fiber and other nutrients, as well as the metabolic intermediates that can cause rumen imbalance due to destruction and degradation. The ability of yeast cultures to stimulate the growth of specific flora is consistent with many reports of the effects of physiological metabolism in the rumen, as well as improving protein synthesis, improving rumen stability, and improving microbial protein synthesis.
1.1 fiber decomposition bacteria
Yeast cultures increase the concentration of rumen anaerobic microorganisms, particularly fibroinolytic bacteria. In vitro studies have shown that yeast cultures directly stimulate the production and reproduction of rumen anaerobic microorganisms, fibrolytic bacteria (Wiedmeier et al, 1987; Harrison et al, 1988; Newbold and Wallace, 1992). Wiedmeier (1987) reported that the total number of viable rumen bacteria and fibrolytic bacteria in non-laced cows fed yeast cultures also increased significantly. Harrison et al (1988) showed that yeast cultures (provided by Diamond V Mill) were provided to lactating cows 8 weeks postpartum, and the cellulytic bacteria count in the rumen fluid was from the control group (6.6 × 107). Increase to (12.0 × 107) MPN / ml. Dawson (1990) added yeast to a crude rumen-based simulated rumen medium and found that the number of fiber-decomposing bacteria was significantly increased. Further experiments showed that the number of rumen anaerobic microorganisms and fibrolytic bacteria in the sputum bulls supplemented with yeast culture was significantly increased. Wienmeier (1987) reported a significant increase in the total number of viable bacteria and the number of fibrolytic bacteria in non-lactating dairy cows fed yeast culture. Callaway et al. (1997) reported that the addition of yeast culture filtrate to a medium containing only cellobiose 6g/l stimulated the utilization of cellobiose by fibrinolytic strains F.succinogenes S85, R.flavefaciens FD1 and R.albus B199. . The total number of microbes in the rumen and the increase in the number of microorganisms contribute to the digestion of crude fiber and other nutrients, thereby promoting the production performance of ruminants.
1.2 Lactic acid utilization bacteria
The yeast culture contains a large amount of amino acids, glucose, B vitamins, organic acids (malic acid, formic acid, succinic acid, aspartic acid) and the like. These nutrients, such as lactic acid-utilizing bacteria (S. ruminatium and M. elsdenii), are also required for the growth and metabolism of certain rumen microorganisms. Girard (1993) reported that at high concentrate levels, the addition of yeast culture not only increased the concentration of lactic acid-utilizing bacteria, but also increased the rate of lactic acid utilization. Edwards (1991) reported that supplementing yeast cultures with high-energy fattening beef cattle increased the concentration of lactic acid-utilizing bacteria. Nisbet and Martin (1991) found that yeast culture stimulated the growth of S. ruminatium when the bacteria were cultured purely. Callaway and Martin (1997) reported that S. rubinatium HD4 was cultured in a medium containing 5 g/l of DL-lactic acid, Tryptioase and yeast culture, and 1%, 5% yeast culture filtrate was added to the medium to stimulate S. Ruminatium HD4 increased by 7% and 15%, respectively. In the medium supplemented with only 5g/l DL-lactic acid, 1%, 5% yeast culture filtrate stimulated S. rubinatium HD4, H18 and M.elsdenii B159, For the growth of T81, the yeast culture filtrate increased the acetic acid and total VFA production of S. rubinatium HD4, increasing the propionic acid and total VFA production of S. rubinatium H18.
The rumen mixed bacteria can quickly utilize organic acids (fumaric acid and malic acid) and aspartic acid. The yeast culture filtrate contains malic acid, which stimulates the growth of the lactic acid-utilizing bacteria S. Ruminatium. Linehan et al. (1978) found that S. Ruminantium HD4 requires L-aspartate (Asp), CO2, P-aminobenzoic acid and biotin in the growth of lactate medium, wherein aspartic acid can be L- Replace with malic acid or fumaric acid. Nisbet and Martin (1990) found that aspartic acid, fumaric acid, and malic acid all stimulated the use of lactic acid by S. ruminatium HD4, with the most obvious effect of malic acid stimulation. Nisbet et al. (1991) found that different levels of L-malic acid stimulated the use of lactic acid by S. ruminantium HD4, but the effect was greatest at 10 mM levels. Henderson (1980) found that S. ruminantium WPL 151/1 produced a significant increase in succinic acid production in the H2 environment containing glucose, suggesting that S. ruminantium WPL151/1 contains fumarate reductase because of malic acid and fumaric acid. It is the precursor of this bacterial randomizing pathway to synthesize succinic acid. It can be speculated that the ability of S. rubinantium HD4 to grow with malic acid or fumaric acid is inseparable from the presence of extracellular H2.
1.3 Other Microorganisms
Reports on the effects of yeast cultures on proteolytic bacteria are rare. Yoon and Stern (1996) found that yeast cultures stimulated the growth of proteolytic bacteria (3.09 vs 2.00 x 108/ml).
In the rumen, H2 is an intermediate produced by decomposing plant cell walls by cell-decomposing bacteria such as R. albus, R. flavefaciens, B. succinogenes, B. ruminocola, and anaerobic fungi. However, H2 does not accumulate in the rumen. This is due to the methanogens that are dominant in H2 utilization in the rumen to synthesize methane. Although acetic acid producing bacteria can also use H2 and produce acetic acid, the competitiveness of the acetic acid producing bacteria H2 in the rumen. Far less than methanogens, so most of the H2 is lost in the form of methane. Chaucheyras et al. (1995a) found that the effect of live yeast on the production of H2 microbes, acetogen-producing bacteria, and methanogenic bacteria in artificial rumen, found that the addition of live yeast can produce H2 (hydrogenotrophic) metabolism of acetogens. Acetic acid production increased by more than 5 times; in the control group without yeast and the mixed group of acetogenic and methanogenic bacteria, H2 was mainly used for the synthesis of methane, and once added to live yeast, the use of acetogenic bacteria H2 was stimulated. ability. This suggests that the addition of yeast can increase the competitiveness of the acetogen-producing H2 utilization while also reducing methane losses.
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