On October 28, 2004, the FDA approved the use of sodium monensin (Rumensin®) for improving efficiency of milk production in dairy cattle. This action allowed US dairy farmers to join the ranks of farmers around the world who are using ionophores to improve the efficiency of milk production by selective modification of rumen function. However, in order get the maximum value out of its application, it is important that the role of this tool be carefully examined alongside some of the other management tools that have found a place in dairy production systems.
Alltech’s Yea-Sacc®1026 yeast culture, based on Saccharomyces cerevisiae strain 1026, and other yeast culture products have been commonly used to improve the performance of dairy cattle in modern production systems. Since both monensin and yeast cultures have the ability to beneficially modify microbial activities in the rumen, it is important to understand the potential beneficial effects and pitfalls of using combinations of these products in dairy management programs.
The overall objective of this paper is to compare some of the basic effects of yeast cultures and the ionophore, monensin, on various aspects of nitrogen and energy metabolism, production efficiency, and health in lactating dairy cattle. Specific attention will be given to evidence that suggests that these two supplements can be used to synergistically enhance dairy performance.
Comparative effects of yeast culture and ionophores on microbial activities in the rumen
It is generally believed that monensin exerts many of its effects by altering the microbial populations in the rumen (Dinius et al., 1976; Richardson, et al., 1976; Van Nevel and Demeyer, 1977; Schelling, 1984; Russell and Strobel, 1989). Monensin and similar ionophores are noted for their antimicrobial activities against certain groups of bacteria, fungi, and protozoa (Chen and Wolin, 1979; Dennis et al., 1981; Russell and Strobel, 1989).
Inhibition of growth is believed to be accomplished through disruption of membrane-associated energy metabolism that essentially selectively limits the production of the useful high-energy compounds needed for the growth of these organisms (Russell and Strobel, 1989). Since Gram(+) bacteria have less complex membrane structures than Gram(-) bacteria, they tend to be more sensitive to the antimicrobial effects of ionophores.
This selective antimicrobial activity can explain the shifts in microbial populations associated with monensin supplementation. Gram(+) organisms that are associated with starch digestion and lactate, acetate, hydrogen, and methane production tend to be more susceptible to the effects of ionophores and are suppressed in the rumen. As a result, there is a proliferation of Gram(-) organisms that produce succinate and the glucogenic precursor, propionate.
Since products of monensin-sensitive organisms like hydrogen and methane represent energy losses, and lactate is commonly associated with the early stages of ruminal dysfunction, the shift in microbial populations away from the monensin-sensitive organisms not only results in a more efficient fermentation, but also in a more stable ruminal fermentation process that is less susceptible to dysfunction. These ruminal changes are all related to the ability of monensin to selectively inhibit specific groups of microorganisms in the rumen.
In contrast to the selective antimicrobial activities associated with the ionophores, yeast cultures have been shown to have selective stimulatory effects on certain groups of bacteria in the rumen (Dawson, 2000). Many investigators have confirmed the observation that yeast culture supplements can stimulate microbial activities and increase the concentrations of bacteria in the rumen (Wiedmeier et al.,1987; Newman and Dawson, 1987; Harrison et al., 1988; Dawson et al., 1990; Edwards, 1991; Newbold and Wallace, 1992; Kim et al., 1992; El Hassan et al.,1993; Girard et al.,1993).
Information obtained from both in vivo and in vitro studies with mixed populations of ruminal bacteria suggests that stimulation of specific groups of bacteria is critical to the overall beneficial effects of Yea-Sacc® yeast culture in the diets of ruminants. Many of the bacteria that are easily stimulated are also critical to the process of fiber digestion in the rumens of animals fed fibrous substrates, or to lactic acid utilization in the rumens of animals receiving grain-based, high-energy diets (Girard et al., 1993; Girard and Dawson, 1994).
The major differences in the mechanisms that induce changes in the rumen microbial populations suggest that Yea-Sacc® and monensin supplements may have complementary effects that can be used to further enhance ruminal fermentations. In comparing their specific in vitro effects on some of the major groups of rumen bacteria (Table 1), it is not difficult to hypothesize the potential for synergistic activities. With the possible exception of the effects on some of the Gram(+) cellulose-degraders (Ruminococcus spp.), it appears that Yea-Sacc® supplementation is very compatible with the selective effects elicited by monensin.
Table 1. Effects of monensin and yeast culture (Yea-Sacc®) on the growth of some representative groups of ruminal bacteriaa.
aGrowth was inhibited (-), strongly inhibited (—), not altered (0), stimulated (+) or strongly simulated (++).
bAdapted from Nagaraja et al., 1997.
cAdapted from Girard and Dawson, 1994; Martin and Nisbet, 1992.
Yea-Sacc® tends to stimulate the bacteria that are not inhibited by monensin, and monensin has little inhibitory effect on many of the bacteria that are stimulated by Yea-Sacc®. Both supplements have the potential to affect bacteria that are involved in the accumulation and/or elimination of lactic acid in the rumen. Yea-Sacc® tends to stimulate bacteria that actively convert lactic acid to propionate (Girard et al., 1993), while monensin inhibits bacteria that produce lactic acid or rapidly degrade readily degradable carbohydrates and favor lactic acid production (Dennis et al., 1981).
Comparative effects of yeast culture and ionophores on carbohydrate and energy metabolism
The ability of ionophores like monensin to alter microbial fermentations in the rumen has been well documented and is central to their abilities to beneficially change animal production (Schelling, 1984). The characteristic shift in fermentations that result in the formation of propionate at the expense of acetate and methane formation results in fewer energy losses and a much more energy-efficient fermentation process. This is reflected in an improved energy balance, with more productive energy being available to the animal.
Induced alterations in ruminal fermentations and particularly in propionate formation have been used to explain many of the benefits associated with ionophore supplementation. This ability of ionophores to improve energy balance has been well documented (Bergan and Bates, 1984; Schelling, 1984; Nagaraja et al.,1997), and induced changes in fermentation can be used in turn to improve milk yield, improve efficiency of milk production, and enhance immune responses (Ipharraguerre and Clark, 2003).
However, these changes in fermentation may often be at the expense of some of the beneficial digestive activities in the rumen. The antimicrobial effects of the ionophores may inhibit the activities of some of the key fiber-digesting bacteria in the rumen and may prevent optimal digestion. While the overall view in a review by Ipharraguerre and Clark (2003) suggests there is little effect of monensin on feed intake, most investigators agree that decreases in intake during the adaptation period are likely, and that this can be related to reduced digestibility of feed in the rumen (Lemenager et al., 1978; Schelling, 1984).
It has not been possible to consistently associate alteration in volatile fatty acid (VFA) production with yeast culture supplementation in the rumen. However, several studies have suggested that yeast cultures enhance the relative production of propionate and decrease the acetate:propionate ratio in the rumen receiving silage or forage-based rations (Harrison et al., 1988, Martin et al., 1992; Dawson et al., 1990; Williams and Newbold, 1990). As with monensin, such changes are indicative of improved fermentation efficiencies and decreased methane production, and suggest that yeast culture supplementation could provide additional energy for animal production (Williams and Newbold, 1990).
In the case of Yea-Sacc®, any increase in the production of propionate appears to be the result of increased conversion of lactic acid to propionate rather than through selection of bacteria that produce propionate precursors like succinic acid. Other studies have demonstrated increases or nonsignificant increases in the ruminal acetate:propionate ratio when straw or high concentrate rations were fed to ruminants (Wiedmeier et al., 1987; Williams and Newbold, 1990; Edwards, 1991). This work suggests that many of the fermentation responses to yeast culture supplements are diet dependent.
In contrast to monensin, Yea-Sacc® has been shown to have a stimulatory effect on many groups of fiberdigesting bacteria that can be seen in enhanced rate and/ or the extent of fiber digestion in the rumen. Some of these enhanced digestive activities may be directly related to stimulation of microbial growth and activity (Girard and Dawson, 1994).
While many studies have suggested that yeast culture supplementation may have little effect on the overall extent of digestion, many suggest that yeast cultures influence the initial digestion rates of fibrous substrates in the rumen (Wiedmeier et al., 1987; Arambel et al., 1987; Chademana and Offer, 1990; Williams et al., 1991; Wohlt et al., 1991; Erasmus et al., 1992; Wong et al., 1992; Olson et al., 1994; Dawson and Hopkins, 1991).
These studies indicate that yeast culture supplementation may have a significant effect on the time course of digestive processes in the rumen. With Yea-Sacc®, such changes in digestive function could increase the availability of nutrients in the rumen and are believed to have a significant impact on intake (Williams and Newbold, 1990).
In high-producing dairy cattle, the ability of monensin to influence lactic acid metabolism and the problems associated with depressed ruminal pH and acidosis have been well documented (Nagaraja et al., 1997). This is believed to be associated with the ability of monensin to inhibit the growth of the organisms associated with lactic acid production and the disruption of the fermentation processes that lead to lactate formation.
Many of the rapidly fermenting lactic acid producers are sensitive to the antimicrobial effects of ionophores (Dennis et al., 1981). The selective inhibition of the growth of lactic acid-producing bacteria is important in controlling the rapid degradation of readily fermented carbohydrates and the resulting formation of lactic acid.
One important activity of Yea-Sacc® in high concentrate rations is its ability to decrease ruminal lactic acid concentrations and moderate ruminal pH. Williams et al. (1991) demonstrated lower ruminal lactic acid concentrations in the rumens of steers fed a hay plus barley diet when Yea-Sacc® was used. This decrease in lactic acid concentrations was associated with greater ruminal pH and lower concentrations of oligosaccharides in the rumen.
These studies suggest that Yea-Sacc® can help control lactic acid concentrations in the rumen and in the prevention of the ruminal dysfunctions associated with the use of high-concentrate rations. However, this control is accomplished by a different mechanism than that associated with monensin.
The ability of Yea-Sacc® to control ruminal lactic acid concentrations appears to be related to stimulation of the activities of lactic acid-utilizing bacteria. Nisbet and Martin (1991) have demonstrated that extracts from yeast cultures can stimulate the utilization of lactic acid by the ruminal bacterium, Selenomonas ruminantium.
Girard and Dawson (1994) demonstrated the ability of Yea-Sacc® to stimulate the growth of other species of lactic acid-utilizing bacteria isolated from the rumen and have confirmed the ability of this supplement to enhance the rate of lactic acid utilization by ruminal bacteria after a lactic acid challenge (Girard et al., 1993).
This increased activity of specific bacteria was related to enhanced ability to transport lactic acid into the cell and is consistent with other studies that have demonstrated that the potential for the ruminal population to utilize lactic acid is enhanced by Yea- Sacc® supplementation (Girard et al., 1993).
The fact that yeast culture and ionophore supplementations enhance propionate production by such different mechanisms suggests that these management strategies may be complementary (Figure 1). Where ionophores work to inhibit organisms and pathways that lead to lactate production, yeast culture supplementation enhances lactic acid utilization and its conversion to propionate. This has been confirmed in detailed studies of lactate conversion in rumen fluid from heifers that have been adapted to diets constraining Yea-Sacc® (Figure 2).
In these studies, 13C labeled lactate was added to in vitro cultures established from animals fed the yeast supplement and cultures established from unsupplemented animals. The rates of propionate and lactate production and rates of lactic acid utilization were examined. The rates of propionate production from lactate were greater in ruminal cultures established from yeast-supplemented diets, and both Yea-Sacc® and monensin depressed the rates of lactate production.
However, data from these studies indicate that the greatest effects of monensin on lactic acid metabolism are observed in animals receiving Yea-Sacc®. This work clearly suggested a synergistic interaction between monensin and Yea-Sacc®. These in vitro studies have been supported by more practical studies. Edwards (1991) has demonstrated greater concentrations of lactic acid-utilizing bacteria in the rumen of cattle receiving a high-energy finishing ration with Yea-Sacc® supplementation.
In this study, the use of Yea-Sacc® complemented the effects of the ionophore, which added to control of lactic acid production in the rumen. The control of lactic acid metabolism in animals receiving high-energy diets is critical in preventing the development of acidotic conditions in the rumen and suggests that strategies using both monensin and Yea- Sacc® can be useful in controlling acidosis in high-producing dairy cattle.
Figure 1. Complementary regulation of ruminal fermentation in animals receiving high-energy diets containing monensin and Yea-Sacc®.
Figure 2. Effects of monensin on propionate production, lactate production and lactate reduction rates in in vitro cultures established with rumen fluid from animals fed Yea-Sacc® or control diets and challenged with ground corn and lactic acid (unpublished information from Girard and Dawson, 1995).
Comparative effects of yeast culture and ionophores on nitrogen metabolism
Monensin has been shown to have amino acid or proteinsparing effects and has been associated with higher circulating urea levels, lower ruminal ammonia levels, decreased fecal nitrogen output and decreased apparent nitrogen digestibility (Ruiz et al., 2001). Since many of the proteolytic bacteria in the rumen are resistant to the antimicrobial effects of monensin (Chen and Wolin, 1979), the major effects of monensin appear to be the result of direct effects on bacteria that degrade peptides and deaminate amino acids, (Wallace et al., 1981; Whetstone et al., 1981; Wallace and Newbold, 1993).
Russell’s research group has demonstrated that many of the most active ammonia-producing bacteria in the rumen are susceptible to monensin (Russell et al., 1988; Chen and Russell, 1989). Again, the effects of monensin on ruminal nitrogen metabolism can be explained by the selective antimicrobial activities of the ionophores.
It is believed that the protein-sparing effects of monensin can help overcome the low efficiency of nitrogen utilization in ruminants by preventing wasteful ruminal degradation of valuable amino acids from the diet.
A number of studies have demonstrated the beneficial effects of Yea-Sacc® supplementation on protein and nitrogen metabolism in ruminant animals. In view of the known stimulatory activities of yeast on ruminal microorganisms, these effects are not unexpected. One of the common observations associated with the use of Yea-Sacc® in ruminants and rumen-simulating fermenters has been the reduction of ammonia concentrations (Edwards, 1991; Dawson and Newman, 1988: Newbold and Wallace, 1992; Williams and Newbold, 1990).
In animal trials, ammonia levels have been reported to decrease by 20 to 34%, while 7 to 12% decreases have been reported in rumen-simulating cultures. Reduced ammonia levels have not been associated with decreased protein degradation or deamination (Williams and Newbold, 1990) and appear to be related to increased ammonia utilization by microorganisms in the digestive tract. The uptake and assimilation of ammonia into protein by the yeaststimulated microbial population explain these decreases in ammonia concentrations in the rumen and would be expected if more microbial mass were produced in the rumen.
However, such decreases have not been consistently observed in in vitro systems where there is other evidence of stimulated microbial activities (Martin and Nisbet, 1992). Such deviations may be related to differences in the nitrogen availability in feeds or in low baseline ammonia levels in the rumen and may be influenced by the nitrogen recycling mechanisms in the animals.
Several studies have evaluated the direct effects of Yea-Sacc® on protein synthesis in the rumen. Studies by Erasmus (1992) indicate that Yea-Sacc® supplementation can significantly increase the flow of microbial protein from the rumen and favorably alter the relative concentrations of amino acids in the microbial protein leaving the rumen.
Investigators in this group suggest that yeast may have selective stimulatory effects on specific rumen bacteria that result in a shift in the microbial population and would be reflected in increased protein synthesis and altered amino acid profiles. Such observations are supported by work in other laboratories, which indicates that certain strains of ruminal bacteria can be selectively stimulated by Yea- Sacc®, while other strains of bacteria are unaffected (Girard and Dawson, 1994).
Williams and Newbold (1990) demonstrated that daily absorption of nonammonia nitrogen in the intestine was increased by 23% when Yea-Sacc® is provided in the diet. These investigators suggested that this represents an increase in the flow of useful microbial protein to the small intestine.
In another investigation, Edwards (1991) demonstrated increased allantoin nitrogen in the urine and decreased urea nitrogen concentrations in plasma, which also support a mechanism for enhancing the conversion of ammonia nitrogen into microbial cell mass in the digestive tract. While there is considerable variation in the observed effects of yeast culture supplementation on microbial nitrogen metabolism in the rumen, evidence clearly indicates that the ability of yeast to stimulate microbial activities may also alter microbial protein synthesis and ammonia utilization under some dietary conditions.
The combined effects of monensin, which improves protein efficiency in the rumen, and yeast cultures, which stimulate the production of high quality protein, are consistent with a potential synergistic effect on ruminal nitrogen metabolism. Direct evidence for these beneficial interactions is currently lacking. However, there is little reason to believe that antagonisms will be observed.
Comparative effects of Yea-Sacc® and ionophores on milk production
Monensin has been clearly shown to be a valuable tool for increasing milk yields. In six studies with cows receiving doses of monensin between 80 and 450 mg of monensin per day reviewed by Ipharraguerre and Clark (2003), milk production was increased by an average of 1.5 kg per day. This increased production was associated with an average decrease in intake of about 0.5 kg, a decrease in milk fat content (2.3%) and a slight decrease in milk protein content (0.5%).
However, many investigators feel that it is not possible to consistently show a significant change in the fat or protein content of milk when monensin supplements are used (Lean et al., 1993). Most data suggest that dairy cows that are at risk of entering a negative energy balance or are at risk of metabolic disorders due to stress or dietary challenge are most likely to benefit from the use of monensin or similar ionophore supplements (Ipharraguerre and Clark, 2003). As a result, the potential benefits of ionophores must be evaluated in relation to basic herd management and nutrition programs.
Similarly, Yea-Sacc® has routinely been used as a nutritional tool for increasing milk yields in dairy cattle.
The average milk yield increase in 11 trials reviewed by Dawson (2000) was 1.8 kg per day. While intake values are not available for many of these trials, it appears that the increase in milk production is in most cases associated with increases in dry matter intake. No consistent change in milk composition has been reported. Again, the magnitude of the production responses to yeast culture is dependent on the types of management and nutritional strategies being used.
Production responses to Yea-Sacc® are likely to be greatest in situations where nutrition and management practices stress the animal and ruminal fermentations are likely to become dysfunctional.
Studies that evaluate the overall synergistic effects of Yea-Sacc® and monensin on milk production are currently lacking. This is largely due to the fact that monensin has only recently been approved for use with lactating dairy cattle in the United States.
However, basic studies that have looked at the performance of beef cattle have clearly established the value of using both monensin and Yea-Sacc® in high-energy diets (Table 2). In these studies, both Yea-Sacc® and monensin increased the efficiency of production (efficiency of feed utilization), but the synergistic effects of combining the supplements appears to be related to the ability of Yea-Sacc® to improve the depressed feed intake associated with animals that are being fed monensin. It was also interesting to note that the use of monensin in these studies did not decrease the concentrations of viable yeast in the rumen, indicating that the antimicrobial activities of monensin had no detectable effects on the ability of yeast to remain active in this environment.
Other production studies examining the effects of Yea- Sacc® in beef cattle have clearly shown that it was effective at improving performance even when used in monensin-supplemented feeds (Birkelo and Berg, 1994).
Table 2. Interactive effects of monensin and Yea-Sacc® on the performance and ruminal fermentation of cattle fed high-energy, corn-based rations1.
1Adapted from McLeod et al., 1990
aSignificant Yea-Sacc® effect (P<0.10)
bSignificant monensin effect (P<0.10)
cSignificant monensin x Yea-Sacc® interaction (P<0.10)
Comparative effects of yeast culture and ionophores on animal health
Monensin has been routinely used to reduce the incidence and severity of rumen acidosis (Nagaraja et al., 1997) and has been long known for its ability to decrease the severity of bloat problems (Bartley et al., 1983). The use of monensin to control acidosis can be expected to indirectly address acidosis-induced lameness (Heuer et al., 2001) and generally improve feed intake problems associated with ruminal dysfunctions.
Monensin has also been used to protect cattle from acute bovine pulmonary edema and emphysema by inhibiting the formation of 3-methylindole from tryptophan in cattle that have been introduced to lush green pastures (Potchoiba et al., 1992). The improved energy balance associated with monensin supplementation may have a number of additional beneficial health effects. These include the ability to reduce the risk of abomasal displacement (Duffield, 1997) and the incidence of subclinical ketosis (Sauer et al., 1989). Most investigators agree that many of the health benefits, other than those associated with bloat or the prevention of toxin formation, are probably the result of the improved energy status associated with altered ruminal fermentations.
Yea-Sacc® supplementation tends to stabilize the ruminal microbial population and can be used to indirectly address dysfunctional problems that influence animal health by altering ruminal fermentations (Dawson, 2000). The ability of Yea-Sacc® to prevent the accumulation of lactic acid and decreased ruminal pH have been well documented (Williams et al., 1991).
Like monensin, yeast culture-induced control of ruminal pH in animals fed high-energy diets can be easily related to enhanced immune function, decreased health and lameness problems, and may be expected to result in fewer ketotic episodes. In this respect, these ruminal effects seen with Yea-Sacc® are certainly consistent with and complementary to the beneficial effects of monensin on animal health. Yeast culture may also have competitive exclusion properties that would prevent the establishment of pathogenic bacteria in the digestive tract and modulate immune function (Jouany, 1999).
There currently is little research that has directly examined the potential synergistic activities of monensin and yeast culture supplementation on the health of dairy cattle. However, the basic synergistic beneficial effects of these two supplementation strategies on ruminal fermentations and ruminal dysfunction make it easy to hypothesize synergistic effects on many aspects of ruminal and animal health. Further work is needed to investigate the potential for these interactions.
Summary and conclusions
In many countries, the use of Yea-Sacc® as an alternative to ionophores is being considered. This is due to the increased regulatory restrictions that are directly influencing antimicrobial use in livestock diets and the perception that yeast culture supplements offer a natural alternative to antimicrobial use.
However, it is quite clear that due to differences in their modes of action, strategies that use combinations of ionophores and live yeast culture supplements may in fact be complementary and could further enhance ruminant performance. While there is considerable need for additional information about the practical implications of using both Yea-Sacc® and ionophores in dairy management, a strong case can be made for using combinations of these agents for controlling acidosis in dairy cattle fed high-energy diets.
The health implications and the potential for improving energy balance in dairy cattle during critical periods in the lactation cycle also make combinations worthy of consideration. Similarly, the ability of these agents to increase the efficiency of nitrogen metabolism and to stimulate the production of high quality microbial protein in the rumen makes such combination strategies attractive.
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