Abstract: In recent years, the regulation of the rumen microbial system of ruminants has become a research hotspot. As a natural regulator of rumen fermentation, active dry yeast can not only improve the rumen environment and production performance, but also has no drug residue problems similar to antibiotics. It has been widely used in ruminant production. This article briefly describes the research progress on its mechanism of action and application effects.

Keywords: active dry yeast, rumen, microorganisms, regulation

For many years, many researchers have been committed to the study of the microbial system in the rumen of ruminants, aiming to improve the production efficiency of ruminant livestock by regulating the microbial system in the rumen. There are currently many methods used to regulate ruminal microflora, such as ionophores and antibiotics. However, the series of problems caused by their use and the requirements for safe feed have prompted researchers to devote themselves to the development of new non-antibiotic safe feed additives.
Active dry yeast is a live, naturally occurring yeast. As a regulator of rumen fermentation, it can not only improve the rumen environment, but also has no drug residue problems similar to antibiotics. It has been widely used in ruminant production. The definition of active dry yeast by the American Feed Control Office (AFCO) is: a yeast product that has been dried in some way and retains most of its fermentation ability, and does not contain any grains or other fillers. Each gram of active dry yeast product must contain no less than 15 billion viable yeast cells. Highly active dry yeast can play a role by regulating the rumen environment, especially by regulating bacteria in the rumen. It can increase feed intake, increase lactation, and reduce the harmful impact caused by environmental stress. This article gives a brief description of the machenisim of action and application effect of active dry yeast in ruminants.

1. Mechanism of action
A large number of studies have confirmed that the rumen of ruminants itself cannot directly digest and absorb feed, what really participates in the decomposition and digestion of food is the microbial flora in the rumen. The microorganisms in the rumen mainly include bacteria, protozoa, and fungi. The degradation of carbohydrates and nitrogen-containing substances is mainly completed by bacteria and protozoa, the degradation of fibrous carbohydrates is mainly completed by anaerobic fungi. Generally speaking, there are about 40 billion to 50 billion bacteria per milliliter of gastric juice, and the number of protozoa is also in the tens of millions. A healthy and vigorous rumen microbiota can provide 50% to 80% of the protein requirements and 80% of the energy requirements of ruminants. Therefore, it is particularly important to promote the stability and health of beneficial microbial flora in the rumen. Currently, the mechanisms by which active dry yeast stimulates the growth and activity of rumen microorganisms have been recognized to a certain extent, including the oxygen control theory, the nutritional theory and the small peptide nutritional trigger theory.
The oxygen control theory holds that although yeast is a facultative bacterium, it mainly functions as an aerobic bacterium. It consumes oxygen in the rumen to protect the anaerobic bacteria in the rumen from O2 damage and maintain the normal reproduction and function of beneficial bacteria. Anaerobic fungi account for 8% of the rumen microbial flora and play an important role in the physical degradation of fiber in the rumen (Orpin and Joblin. 1989). Newbold et al. (1996) reported that the two selected yeasts could stimulate the increase of total anaerobic bacteria and fiber-decomposing bacteria in the artificial rumen, and when doubled the dosage, the oxygen disappearance rate in the rumen fluid increased by about 50%, but after the yeast is mutated with ethidium bromide, it cannot stimulate the growth of total anaerobic bacteria and fiber-decomposing bacteria in the artificial rumen.
Nutritional theory believes that active dry yeast is rich in amino acids, B vitamins, organic acids, sugars, calcium, phosphorus and other nutrients, which just provide nutrients for the growth and metabolism of certain rumen microorganisms such as lactic acid utilizing bacteria and fiber decomposing bacteria, thereby increasing the intensity of fermentation (Henderson, 1980; Nisbet et al., 1990, 1991; Callaway et al., 1997).
The small peptide nutritional trigger theory believes that active yeast may contain a substance similar in structure to a small peptide, which has a great stimulating effect on the microorganisms in the rumen. Newbold et al. (1991) and EIHassan et al. (1993) found that after radiation inactivation of yeast, most of the stimulating activity in the artificial rumen was still maintained, but after inactivation by high temperature and high pressure, this activity was lost. It was speculated that saccharomyces cerevisiae stimulates the ability of bacteria to grow may depend on an as-yet-unknown thermally unstable factor. This small molecule compound of nitrogen can stimulate rumen microorganisms to enter the exponential growth phase, and its basic chemical properties are the same as those of biologically active small peptides (Girard, 1996). The synthetic small peptides containing amino acid also have similar stimulating effects and can also stimulate the growth of rumen cellulolytic bacteria and other bacteria (Dawson, 2000). This also confirms to a certain extent that metabolically active yeast cells stimulate the growth of rumen microorganisms by continuously providing a source of peptides.
In short, although the above three theories have been proposed, there may still be other unknown mechanisms, and whether these mechanisms act alone or in combination is still unknown and needs further research.

2. Impact on ruminants and application effects

          2.1 Impact on the rumen environment

Research on the effects of active dry yeast on rumen fermentation has shown mixed results. Harrison reported that adding yeast culture medium to the diet of Holstein cows reduced the ratio of acetic acid and acetic acid to propionic acid in the rumen, and increased the molar ratio of propionic acid and valeric acid. Willams (1990) found that when yeast preparations were added to the diet of dairy cows in the early and middle stages of lactation, the ratio of acetic acid to propionic acid decreased, the production of propionic acid increased, and the production of butyric acid decreased, while valeric acid and branched-chain volatile fatty acid were not affected. However, some studies have pointed out that adding live yeast has no effect on ruminal fermentation. The inconsistent results of these studies may be due to different dosages, concentrations, substrates, different dietary compositions, and different feeding methods.
Active dry yeast improves the stability of the ruminal environment. Feeding ruminants high-concentrate diets can easily cause a decrease in rumen pH and cause rumen dysfunction. By adding Aspergillus or yeast preparations, lactobacilli can be proliferated, lactic acid utilization can be improved, rumen pH can be increased. There are reports that active dry yeast is more effective when the rumen pH is between 4 and 5.
Active dry yeast improves nitrogen metabolism in the rumen. Cows fed yeast preparations had lower ruminal nitrogen concentrations and increased ruminal bacterial populations (Newbold et al., 1992). In addition, duodenal microbial nitrogen production is also increased (Erasmus et al., 1992). The increase in microbial protein production shows that adding yeast preparations can stimulate the growth of rumen microorganisms, thereby converting ammonia nitrogen into microbial protein more effectively.
Active dry yeast stimulates the increase in the number and activity of beneficial bacteria in the rumen, especially lactic acid-utilizing bacteria and fiber-decomposing bacteria. Found in recent research. Yeast preparation treatment can greatly increase the number of fiber-decomposing bacteria and lactobacilli in the rumen, but does not affect the number of proteolytic bacteria, amylolytic bacteria and protozoa. Dawson (1987) in vitro test results confirmed that adding yeast culture can increase rumen anaerobic bacteria by 5 times and fiber decomposing bacteria by 2 times; in vivo test results show that adding yeast culture can increase fiber decomposing bacteria in the rumen by 1.5 times. Dawson (1990) found that adding live yeast preparations to the diet of lactating cows increased the number of fiber-decomposing bacteria by 5 to 40 times compared with the control group. Other studies have shown that adding active dry yeast to the diet can increase the total number of beneficial microbial flora in the rumen by 30%, among which the number of bacteria involved in decomposing fiber substances and hay can increase by 60-70%.

           2.2 Impact on feed intake

Active dry yeast can be added to the diet to increase dry matter intake. Willimas et al. (1991) added yeast preparation to dairy cows fed a high-concentrate diet, and the dry matter intake increased by 1.2 kg/d. Wang Cong et al. (2005) found through a 70-day feeding experiment of Holstein dairy cows that yeast can significantly increase the dry matter intake and slow down weight loss. Chen Hongliang et al. (2007) also found that compared with the control group, adding active yeast and extracts could increase their dry matter feed intake. The direct reason may be related to the sweet and sour smell and pure fragrance of general live yeast and yeast extract. However, Williams et al. (1991) believed that adding yeast preparations to dairy cows fed high-concentrate diets can reduce the concentration of lactic acid in the rumen. The decrease in lactic acid concentration leads to an increase in rumen pH, which increases the degradation of structural carbohydrates in the rumen, thereby increasing feed intake.

          2.3 Effect on milk production and milk composition

Gunther et al. (1989) reported that adding 10 grams of active dry yeast culture to dairy cows' diets could increase their standard milk production from 28.4kg/d to 34.3kg/d. Komari et al. (1999) and Kang et al. (1998) found that adding yeast to dairy cow diets can significantly increase milk production, and increase milk fat and milk protein rates. Sun Manji et al. (2005) tested high-concentrate feed cows in early lactation stage and confirmed that adding yeast can significantly increase milk production, milk protein, milk fat rate, and total dry matter content in milk. Jiang Xiaoyi et al. (2007) conducted experiments on Chinese Holstein cows calving for 30 to 45 days and showed that different types of yeast basically have a certain positive impact on the milk production and milk composition of cows. Wang Cong et al. (2005) selected Holstein lactating cows with about 80 days of lactation and conducted a 70-day feeding test and found that yeast can significantly increase the daily standard milk production by 2.25kg/head and the milk fat rate by 0.3%.
The effect of active dry yeast on milk production and milk components is not only related to yeast strain, but also has a great relationship with the dietary forage/concentration ratio and lactation period of dairy cows (Harris et al., 1988; WJuiams et al., 1991; Zheng Xiaozhong, 1996; Yuan Manju, 2001). Judging from the existing research data, under high-concentrate diet conditions, adding active dry yeast to the diet of lactating cows can significantly increase the milk production in the early and middle stages of lactation, while in late lactation or under low-concentrate conditions, the impact on milk production is relatively small. Whatover, different dietary compositions have completely different effects on milk production and milk composition.
In addition, some scholars believe that adding active dry yeast to increase dietary dry matter intake is consistent with the increase in lactation, and the energy provided by the increased feed intake is considered to be used to increase milk production (Gomaz- Alarcon, 1990). This also explains to a certain extent the direct reason why active dry yeast increases milk production.

           2.4 Effect on heat stress

Many studies have found that active dry yeast can alleviate heat stress in lactating cows to a certain extent. Wallentlne (1992) reported that in the early lactation period, the rectal temperature of the treatment group with active dry yeast dropped significantly compared with the control group, while the rectal temperature in the late lactation period was not affected by the experimental treatment, and its effect was related to the environmental temperature. When the environment temperature is lower than 27°C, it has no effect on the rectal temperature, but when the ambient temperature is higher than 37.5°C, the rectal temperature decreases significantly. Under heat stress conditions, a large-scale trial of early lactation cows in California confirmed that adding yeast can significantly increase milk production and reduce the incidence of lameness, but there is no significant difference in milk composition (Zhang Yinglai et al., 2006). Chen Hongliang et al. (2007) conducted experiments during midsummer and found that compared with the control group, adding active dry yeast and extracts could increase their dry matter feed intake and significantly increase milk production, but the milk composition did not change much.

          2.5 Effect on the number of somatic cells in milk

Active dry yeast can reduce the number of somatic cells in milk to a certain extent, thereby regulating the overall health of the animal. Wang Cong et al. (2005) found that after adding live yeast, the number of somatic cells in milk significantly decreased. Song Lihua et al. (2006) found that yeast active substances can reduce the number of somatic cells in milk by 45.8%. Jiang Xiaoyi et al. (2007) also found that certain yeasts can significantly reduce the number of somatic cells, which is of great significance in reducing the incidence of mastitis in dairy cows.
In addition, studies have shown that active dry yeast can improve the appearance of stool. Sun Manji et al. (2005) showed that cows fed yeast had better fecal dryness and humidity during the experimental period.

3. Conclusion
In scientific research and actual production applications, there are many factors that affect the effectiveness of active dry yeast in ruminants, mainly including the following aspects: 
The influence of yeast strains. It can be seen from many research reports that different yeast strains may have an impact on the result.
The impact of the amount of addition, the amount of addition refers to the total number of viable yeast added, and the optimal addition amount of different strains may be different.
The impact of diet composition and energy concentration, the basic mechanism of diet affects active dry yeast is not very clear.
The relationship between the lactation period and the effect of active dry yeast is not clear.
The mechanism of active dry yeast in reducing the number of somatic cells in milk and resisting heat stress needs further study.