Manipulation of the pattern of fermentation by the use of feed additives (for example ionophores and microbial products) with the aim to improve animal performance has been achieved in many species of farm animal.
However, the level of improvement can be limited by a lack of fundamental knowledge concerned with the mode of action of feed additives and limitations over the application of technology (Whittemore, 2000). In 1982, Alltech Inc. introduced Yea-Sacc1026, a natural feed additive containing metabolically active Saccharomyces cerevisiae strain 1026. The product was researched extensively in the 1980s and 1990s by universities and research institutes interested in ruminant and monogastric (including poultry) farm animal species (Lyons, 1997). However the level of research concerned with the supplementation of equine diets with yeast culture is less extensive.
The opportunities to manipulate the fermentation of dietary components in the gut of the horse are extensive but currently limited partly by a lack of knowledge of the processes involved in the gastrointestinal tract of the animal and partly by the nature of the industry (Spring, 2000). The attitude of the general public towards the horse varies among cultures and nations. Some regard the animal as a companion, some see the horse as a commercial animal (breeding, sport and leisure, pregnant mare urine production and meat production) while others consider the animal as important in agriculture (Kennedy and Hill, 2000). It is therefore not surprising that detailed studies concerning the impact of feed supplements on the nutrition of the horse are lacking. This paper discusses the effects of supplementation of equine diets with Saccharomyces cerevisiae (Yea-Sacc1026) on the processes of utilisation of feed nutrients.
Yeast cultures: modes of action and application in animal nutrition
Yeast cultures have been used strategically for many years as supplements to ruminant, monogastric and avian feeds (Williams and Newbold, 1990; Newbold et al., 1996; Wallace, 1996; Power, 1997; Dawson, 2000). The aim of supplementation with yeast culture is to transfer viable, but not necessarily reproductively active populations of yeast to the site of digestion thus stimulating the microbial population already present (Wiedmeier et al., 1987; Harrison et al., 1988; Nisbet and Martin, 1991; Dawson, 1992; El Hassan et al., 1993). However, the interactions between the metabolically active yeast population and the native gut microflora are complex and not fully elucidated. Many studies have been performed in ruminants, pigs and poultry; however their findings have to be examined in light of differences in the digestive physiology of the equid.
Various theories have been put forward to explain the stimulatory effects of yeast cultures. However, these theories should not be taken in isolation but as an integrated system. Nisbet and Martin (1991) and Rossi et al. (1995) proposed that dicarboxylic acids (e.g malic acid) in combination with metabolically active yeast cells could enhance the growth of lactate utilising bacteria (e.g. Selenomonas ruminantium and Megasphaera elsdenii) thus moderating ruminal pH. This mechanism may have been validated in light of findings of Williams et al. (1991) suggesting lactic acidosis in the rumen of animal feed high grain diets could be controlled by yeast culture supplementation.
Chaudeyras et al. (1996) also suggested that other metabolites (e.g. amino acids, B-vitamins) produced by yeast cultures could be implicated in the stabilisation of the physio-chemical conditions within the rumen leading to higher rates of growth of ruminal bacteria and anaerobic fungi. Stimulation of growth of anaerobic fungi may be important in the degradation of plant cell walls, especially molecules with a high degree of ester cross-linkage. Cross feeding mechanisms (bacterial and fungal) and an increase in substrate availability could lead to increased numbers of ruminal bacteria. There is considerable variation in the supply of carbon and nitrogen from the diet; and the impact on the total microbial population may be great irrespective of supplementation with yeast culture. It is therefore possible that inconsistent or variable responses to yeast culture may be a result of an imbalance in the supply of nutrients from the basal diet (Garcia et al., 2000; Roa et al., 1997; El Hassan et al.,1993).
The removal of trace concentrations of oxygen from the rumen environment as a result of supplementation of the diet with yeast is a more controversial mechanism of enhancing microbial growth (Wallace, 1996). Strict anaerobes present in gastrointestinal fluids will be sensitive to the presence of trace levels of oxygen. However, facultative heterotrophs present in the rumen have the potential to remove oxygen from the rumen environment efficiently and therefore the impact of oxygen scavenging by metabolically active yeast is difficult to ascertain. Nevertheless, yeast cultures which have retained an active respiratory metabolism do have the potential to remove oxygen from the rumen environment. Further, if respiratory activity is lost from the yeast, there is a reduction in the rate of growth of cellulolytic bacteria (Wallace, 1996).
Dawson and Girard (1997) presented a model system to show the possible metabolic role of stimulatory peptides released by yeast on the growth of ruminal bacteria (Figure 1). The key stage of the process of stimulation of ruminal bacteria occurs during the stationary phase of the growth cycle.
Low molecular weight peptides (400–650 Da) released stimulate microbial protein synthesis and decrease the time to initiate growth of ruminal bacteria.
The decreases in time to initiate growth in conjunction with an increase in the rate of transition to the exponential phase of growth leads to an increase in maximum cell yield (Girard, 1996; Girard and Dawson, 1994; 1995). These increases in growth of various categories of ruminal bacteria have been summarised by Dawson and Girard (1997). Yeast supplementation of diets fed to ruminants has increased the numbers of colony forming units of cellulolytic bacteria (Dawson et al., 1990; Newbold et al., 1996), proteolytic bacteria, total anaerobes (Wiedmeier et al.,1987; El Hassan et al., 1993) and lactic acid-utilising bacteria (Girard et al.,1993). Furthermore, Chaudeyras and co-workers (1995a and 1995b) demonstrated enhanced activity of bacteria that convert molecular hydrogen to acetate in the rumen. This observation may have significant implications in the digestive physiology of the horse.
Figure 1. Model system of yeast culture mode of action on microbial growth and ruminal fermentation (after Dawson, 2000).
Our knowledge of the digestive response to yeast culture in the horse is poor in comparison to ruminant farm livestock. For the purposes of this paper, the processes involved in the utilisation of nutrients by the horse are outlined in three broad sections, energy, protein and mineral nutrition. Some basic information will be presented on the similarities and differences between the processes of digestion and utilisation of nutrients by horses and ruminants in an attempt to develop and test some of the hypotheses and modes of action suggested in this section.
VIABLE YEAST CULTURE AND ENERGY NUTRITION
There are various systems used to predict and fulfil the energy requirements of the horse. These systems have been reviewed recently by Cuddeford (2000). It is however necessary to identify certain issues concerned with the utilisation of dietary energy that differ between horses and ruminants and may enhance the understanding of the responses to supplementation of equine diets with yeast cultures. The process of ingestion of feed over a long duration of time (‘trickle-feeding’) reflects the anatomy and physiology of the digestive tract (Frape, 1998). Equids are hindgut fermenters and are more efficient than ruminants in capture of energy substrates, especially when diets of very low or very high nutritive value are offered (Russell and Gahr, 1999).
Nonstructural carbohydrates are utilised by the host and structural carbohydrates are fermented by hindgut microflora. The utilisation of nonstructural carbohydrates in the foregut of the horse is highly efficient, contributing glucose and lactate to the energy substrate pool (Cuddeford, 2000; Van Soest, 1995). The extent of fermentation of nonstructural and structural carbohydrate in the foregut in the horse is not known in detail (though assumed extremely limited); but fermentation increases nutrient flow in the small intestine of the animal. The foregut may also be a site that is responsive to probiosis.
The environmental conditions in the hindgut of the horse are less affected by the rate of arrival of nutrients than is the rumen; and hence the variations in volatile fatty acid (VFA) production and pH are lower than those observed in ruminants (Van Soest, 1995). However, if readily fermentable nutrients reach the hindgut, caecal pH declines rapidly as the buffering capacity of digesta is relatively low (Cuddeford, 2000). In these cases the risk of colic, equine rhabdomyolysis syndrome, azoturia and laminitis (fructan and endotoxin release) is increased particularly in race horses, endurance and eventing animals. The hindgut microflora is not rich in methanogens; and therefore methanogenesis is partially replaced by acetogenesis, thus leading to a higher efficiency of capture of carbon from nonstructural carbohydrates.
Horses also tend to regulate the rate of digesta flow in the gut in response to physical structure of diet and hence nutrient release. It is therefore important that the animal does not overload the foregut (regulated via rate of dry matter intake) thus leading to an enhancement in the rate of digesta flow and excess escape of digestible nutrients to the hindgut (Van Soest, 1995).
Enhanced nutrient digestion by the horse as a result of feeding yeast culture supplements is reported frequently in the literature. Increases in nutrient digestibility have been ascribed to changes in the composition of the microbial community in the caeco-colon complex. However, the caecocolon complex may not be the only site affected by supplementation with yeast culture. For example there may be an effect in the fundic region of the true stomach. Furthermore, the effect of diet composition (level of microbially available carbon) and the physiological state of the animal must be considered (Hintz et al., 1971; Willard et al., 1977; Glade, 1991).
Undoubtedly one of the potential benefits from an enhanced supply of energy yielding substrates from the caeco-colon complex as a result of yeast culture supplementation is the increase in efficiency of nutrient utilisation by the performance horse, especially animals involved in endurance, racing and eventing. Glade and Campbell-Taylor (1990) demonstrated that supplementation of the diet with yeast culture leads to increases in the fitness of the young adult horse during exercise programmes, for instance lower heart rates and lower increases in the concentration of lactate in the plasma.
Furthermore, Kolterman et al. (1993) and Miller-Graber et al. (1994) demonstrated a more rapid transition from utilisation of muscle glycogen to fat oxidation after supplementation, thus enhancing the utilisation of longterm energy supplies and potentially explaining the reduction in plasma lactate concentrations. This increase in supply of energy substrate to the animal is beneficial to horses under extended periods of heavy exercise, eg. threeday eventing (Harris, 1997). The key to the enhancement in fitness seems to be the period of adaptation. If the period of adaptation to the diet and the exercise regime is too short, the response to yeast culture is lower and therefore inconclusive (Biels et al., 1990).
Typical increases in dry matter (DM) digestibility of diets supplemented with yeast culture have been 15 to 50 g/kg (Glade and Sist, 1988; Hall et al., 1990; Glade, 1991; Hill and Gutsell, 1997; Medina et al., 2000) suggesting an increase of between 0.5 and 0.8 MJ DE/kg DM (Table 1). In recent work at Writtle, no significant increase in the digestibility of DM was noted in horses supplemented (10 g/day) with yeast culture. However the forage to concentrate ratio was extreme, being 0.95:0.05. The latter results reflect a lack of stimulation of cellulolytic bacteria and fungi in the caeco-colon complex that could be explained by an imbalance in the supply of microbially available carbon. Palmgren-Karlsson et al. (2000) demonstrated that associative effects in digestion of nutrients in the total tract of the horse occur when the balance of readily fermentable carbohydrate (starch) and cell wall is optimal (approximately 0.6:0.4 forage:concentrate).
In the case of extreme diets containing high levels of structural fibre, the response to supplementation with yeast can be variable. However, it is not known how the passage of metabolites from the ileum and foregut could influence the performance of the microbial community in the caecum and colon (Meyer et al., 1982; Radicke et al., 1991; Tisserand, 1992). In vitro studies by McClean et al. (1997) demonstrated a reduction in time for the production of 50% of total yield of gas from various feeds (e.g. hay, sugar beet pulp and molassed sugar beet pulp) when relatively high levels of yeast culture were added to caecal fluid. Reducing the level of supplementation with yeast by half tended to increase the initial rate of degradation of the concentrate types, but not the hay. These responses are related to differences in the chemistry of the cell wall and the level of easily degradable substrate (Hyslop et al., 1998).
Table 1. Typical effect of supplementation of yeast culture on apparent digestibility (g/kg) of dry matter, lignocellulose and total plant cell wall content in horses*.
(*after Pagan, 1989)
Many studies investigating the supplementation of equine diets with yeast cultures have shown an increase in digestibility of neutral and acid detergent fibre (NDF and ADF) (Godbee, 1983; Glade and Biesik, 1986; Glade and Sist, 1988; Glade, 1991; Hill and Gutsell, 1997; Medina et al., 2000). Typical increases in digestibility of NDF ranged from 20 to 50 g/kg whereas increases in the digestibility of ADF were marginally higher, ranging from 25 to 80 g/ kg. These increases in digestibility of the cell wall reflect the dynamic change in microflora within the hindgut and are potentially dependent on the flow of substrate within the caeco-colon complex and the balance of C:N within the cell wall fraction per se. The pH of the caecum is very important in the development and maintenance of cellulolytic activity; and rapid changes in pH can lead to the proliferation of microbial populations with a lower efficiency of utilisation of dietary fibre (Vermorel and Martin-Rosset, 1997).
As a result, negative associative effects in digestion of the cell wall in hay diets supplemented with starch have been noted by several authors (Hintz et al., 1971; Thompson et al., 1984; Palmgren Karlsson et al., 2000). It is not known if yeast stimulate the microbial population such that enhanced utilisation of hemicellulose occurs after supplementation. However, increased acetate production has been observed in our most recent studies after supplementation with yeast culture, suggesting an enhancement of the utilisation of cell wall carbohydrates. This observation may reflect an increase in acetogenesis.
The utilisation of D-xylose, 2-hydroxybenzoic acid and 4-hydroxybenzoic acid by the faecal microbial population isolated from horses fed a high forage diet supplemented with or without Yea-Sacc yeast culture was examined using a modified Biolog system (Biolog Inc., Hayward, California, USA) in the most recent series of trials. No significant differences in the rates of utilisation of D-xylose and 2-hydroxybenzoic acid were observed between supplemented animals and controls. The rate of utilisation of 4-hydroxybenzoic acid was however significantly greater in faecal cultures from animals supplemented with Yea-Sacc (Figure 2). These observations suggest the architecture of the cell wall and the composition of hemicellulose may be important in developing an understanding of the mode of action of yeast culture in the hindgut of the horse. The observation that an artificial analogue of a 4-hydroxy-substituted phenol was utilised at a greater rate after supplementation tends to support the whole animal studies of Hill and Gutsell (1997). To date, no substantial increases in the digestion of lignin or ligninlike components of the feed have been observed when horses have been supplemented with yeast cultures (Medina et al., 2000).
Figure 2. Utilisation of 4-hydroxybenzoic acid by faecal cultures of horses supplemented with Yea-Sacc yeast culture.
The lack of any perceived increases in apparent digestibility of the cell wall does not preclude an alteration of the cell wall leading to an increase in substrate availability within the hindgut. By assessing difference spectra derived from near infrared spectroscopy of feed–faeces pairing (Coleman and Murray, 1993), differences in spectral characteristics between control animals and those supplemented with yeast culture were observed at 2114, 2180 and 2216 nm (wavelengths associated with C-H and O-H rotations).
These differences may be ascribed to an increase in the utilisation of the cell wall (Coleman and Murray, 1993). While the impact of such differences is not understood on a whole animal basis, it could be important at a microbial level. Furthermore, using faecal inocula and Biolog systems, supplementation of the diet with yeast culture increased the rate of utilisation of cyclodextrin and cellubiose in vitro. Again, the impact of these changes is not understood on a whole animal basis; but the results do suggest an improved supply of measurements of diet digestibility.
The influence of processing on the microbial degradation of feeds has been demonstrated (McClean et al., 1998). Feeding diets containing inappropriately processed feeds can lead to a significant reduction in pH of the digesta in the caeco-colon complex and alter the proportions of VFAs, thus reducing efficiency of energy transfer and increasing the incidence of metabolic and physiological dysfunction. Once a feed particle has passed the cardiac sphincter, it is not re-comminuted and therefore it is presented to the digestive tract in that form. Further processing can occur as a result of solubilisation and enzyme degradation.
Ellis and Hill (1999) and Ellis et al. (2000) examined the outflow of particles from the equine tract in diets of relatively high forage to concentrate ratio (greater than 70:30). These studies suggested the mean particle size of faeces voided ranged from 1.6 to 1.9 mm. Supplementation of the diet with yeast culture did not lead to a reduction in mean particle size voided in the faeces, nor did it alter the pattern of intake, rate or efficiency of chewing of concentrate diets offered.
Even though the addition of yeast culture slightly reduced the rate of eating, it was not appreciable and did not alter the structure of the feeding bout.
This is in contrast to previous findings in horses fed distiller’s by-products where the odour and possibly the taste of the by-product disrupted the pattern of eating, leading to a significantly greater incidence of rejection of the feed (Hill et al., 2000).
ROLE IN PROTEIN NUTRITION
The digestibility of crude protein has been shown to be elevated in horses supplemented with yeast culture by several authors (eg. Glade and Sist, 1991; Glade and Biesik, 1986). The typical increases in the digestibility of crude protein are between 50 and 130 g/kg. These increases are important in the nutrition of the young horse, including race horses in training and during periods of active growth when the requirement for amino acids is high (Bennett-Wimbush et al., 1991; Glade, 1991; Glade and Campbell- Taylor, 1990). However, the actual mechanism and transformations of protein in the total tract and therefore the biochemical and microbial processes responsible for the apparent increase in protein digestion are not fully elucidated.
In ruminants, bacterial N flow from the rumen and increases in proteolytic bacteria (Yoon and Stern, 1996) as a result of increased microbial activity have been shown to occur when diets are supplemented with yeast cultures; however such data may not be appropriate or usefully applied in equine nutrition. The key issues in protein nutrition of horses are the reliance of the animal on absorption of amino acids before the large intestine, i.e to the point of ileal outflow (Macheboeuf et al., 1996; Potter et al., 1992), the lack of transmucosal flux of lysine, histidine and arginine across colonic tissue (Bochroder et al., 1994; Freeman and Donawick, 1991; Freeman et al., 1989) and efficient renal conservation of urea (Martin et al., 1996).
Therefore the site of activity of yeast culture in the digestive tract of the horse is important to ascertain. If the site of action cannot be elucidated, the results of total tract studies may not be important for practical equine nutrition.
The most recent studies at Writtle College have shown an increase in the digestibility of crude protein in diets containing very low levels of concentrate (0.05 of DM) after supplementation with yeast culture (Table 2). These studies also provided detailed information on the microbial transformation of the feed protein in diets predominantly composed of forage.
Table 2. Apparent digestibility of protein, excretion of true protein, concentration of ammonia N in faecal OM and faecal pH in horses given an extreme hay:concentrate diet (0.95:0.05) supplemented with Yea-Sacc (10 g/day).
Concentrations of true protein excreted in faecal organic matter were elevated significantly; and the pattern of excretion was such that the elevation was maintained for at least 12 hrs after feeding the yeast culture. The concentration of ammonia N in the faecal organic matter elevated significantly 4 hrs after supplementation of the diet with yeast. These observations contrast with ruminant models that suggested the concentration of ammonia N in rumen fluid is reduced substantially as a result of an increase in microbial utilisation of protein and energy substrate (Erasmus et al., 1992). Little variation was observed in the rate of utilisation of various amino acid substrates incubated with faecal inocula from horses offered yeast culture compared with controls. These findings suggest that the response to yeast culture may be two-fold, an increase in hindgut microbial biomass and an effect, presumably in the pre-caecal digestive tract, that facilitated an increase in protein utilisation.
Near infrared spectroscopy was used to examine feed-faeces difference spectra of animals offered diets supplemented with yeast culture compared with those not supplemented. Spectra difference peaks at 2064 nm with contributing shoulders at 2024 and 2160nm representing N-H bending with Amide II and Amide III combinations have been shown to occur as a result of supplementation of the diet with yeast. These observations correlate with areas of the near infrared spectra related to enhanced protein digestion (Coleman and Murray, 1993).
Supplementation of the diet with yeast culture has been shown to reduce rapid fluctuations in pH in the caecum and the rumen (Moore and Newman, 1994). The authors suggested that the pH of caecal fluid remains unaltered or at a higher level after supplementation while the concentrations of acetate and propionate tended to be higher (but not significantly). These observations are important, as control of hindgut pH can lead to an environment more suited to cell wall digestion and less likely to lead to colic, laminitis and founder. In our recent work it was observed that an increase in faecal pH after 4 hrs occurred in diets containing very high levels of forage (95:5) supplemented with Yea-Sacc compared to the control diet. This increase in pH can be ascribed to two factors, an increase in the utilisation of lactate (Figure 3) and an increase in the concentration of ammonia N in faecal organic matter.
The increase in utilisation of lactate is not unexpected in light of the findings of Williams et al. (1991), however the increased concentration of ammonia N is in contrast to the observations of Erasmus et al. (1992). The reason for the increase in ammonia N is not clear, but may be related to an increase in proteolytic bacteria in the hindgut (Yoon and Stern, 1996). These findings are significant in the context of overall utilisation of nitrogen substrates by the caecal microflora. If an increase in apparent fixation of dietary nitrogen reflects the increased production of energy substrates utilised as a result of enhanced microbial action on the forage cell wall, the wastage of nitrogen in the form of ammonia must relate to proteolysis of dietary protein. It is unlikely that enhanced microbial turnover will be noted with bacterial senescence while utilisation of energy substrates were enhanced.
Figure 3. Production of lactate by faecal cultures of horses supplemented with Yea- Sacc yeast culture. Hill and Gutsell (1997)
APPARENT ABSORPTION OF CALCIUM AND PHOSPHORUS
The increases in apparent absorption of phosphorus (P) and calcium (Ca) in response to yeast culture are important in the nutrition of the growing foal and the lactating mare (Pagan and Jackson, 1990; Pagan, 1989). Supplementation of the diets of many farm livestock species with yeast culture has been linked to an apparent increase in phytase activity. The major site of phosphorus absorption in the horse is the large intestine (Schryver et al., 1978) and therefore the pattern of absorption is dependent partially on the microbial fermentation of the feed and the level of digestible cell wall (Pagan, 1989; Pagan et al., 1998). In diets in which the supply of microbial available carbon is adequate to ensure elevated phytase activity, typical increases in apparent absorption of P ranged from 80 to 170 g/kg in previous studies with Yea-Sacc (Pagan, 1989; Pagan et al., 1998; Hill and Gutsell, 1997; Glade, 1991; Table 3). However in diets containing very high forage to concentrate ratios (e.g. 90:10 or greater), yeast supplementation does not enhance apparent phosphorus absorption, reflecting the lack of digestible cell wall.
Table 3. Apparent absorption of phosphorus (g/kg) from hay-concentrate diets (70:30) supplemented with yeast culture and offered to mature horses.
Hill and Gutsell (1997)
The link between absorption of calcium and the concentration of phosphorus in the diet of horses has been established (Cymbaluk and Christenson, 1989; Schryver et al., 1978). Increases in the apparent absorption of calcium of about 20 to 50 g/kg after supplementation of the diet with yeast culture have been observed (Pagan, 1989; Hill and Gutsell, 1997). The actual mechanism of how an increase in the apparent absorption of calcium is facilitated has not been elucidated. The primary site of absorption of calcium is the duodenum. Does the process of fermentation (or partial fermentation) in the fundic region of the stomach have an influence on the apparent absorption of calcium, and therefore do yeast cultures influence the pattern of fermentation in this region?
A proposed model system can be defined for the mode of action of yeast culture in the digestive tract of the horse (Figure 4). The tentative model suggests microbial action on the diet in the fundic region of the stomach or in the upper duodenum to partially explain increases in the apparent absorption of calcium and phosphorus and maybe other nutrients. In the hindgut, differences from the existing ruminant models of Dawson (2000) can be highlighted by increased ammonia excretion even though lactate utilisation is enhanced. The use of yeast cultures in the supplementation of the horse at all stages of growth and productive function is beneficial with many researchers demonstrating enhanced digestibility of fibre, protein and dry matter.
Figure 4. Model system examining the mode of action of yeast culture in the equine digestive tract.
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Authors: JULIAN HILL1, SUSAN V. TRACEY1, MYFANNWY WILLIS1, LOUISE JONES1 AND ANDREA D. ELLIS2
1 Faculty of Applied Science and Technology, Writtle College, Chelmsford, UK.
2 Research Institute for Animal Husbandry, AD Lelystad, The Netherlands.