Diet for healthy gut: The secret for overall health and productivity of dairy cattle

Introduction

Gut health is intrinsically linked with animal health, which in turn dictates cost efficient production. Animal health and productivity are intrinsically linked, as are gut health and animal health. In many diseases, diet is implicated as a contributing factor by having direct effects on host metabolism, immune responses, and microbiome composition, subsequently altering disease susceptibility (Plaizier et al. 2018). Gut health is defined as the balance of homeostasis between host and microbiome (normobiosis) as well as the resistance to external and endogenous disturbances (Lozupone et al. 2012). Dysbiosis is a state in which the microbiota produces harmful effects via changes in relative abundance, changes in metabolic activities or changes in diversity and distribution (Garcia et al. 2017). The strongest disturbances relate to diet, including differences in form, composition, dry matter intake (DMI), the application of antibiotics, or addition of transient bacteria through feed additives or water. Gut homeostasis is maintained when the diet meets the needs of the commensal microbiota, the dynamic barrier function and tolerant immune responses of the host (Khafipour et al. 2016; Steele et al. 2016). This dynamic network, when in balance, provides a symbiotic, and mutually beneficial relationship (Plaizier et al. 2018). This relationship is especially unique in foregut fermenters, where the microbial ecosystem utilizes nutrients unavailable to the host to produce short chain fatty acids (SCFA) and microbial protein, which the cow can then metabolize for milk production (Russell and Rychlik, 2001). In this regard, when feeding cows, we are actually feeding the microbes (Neubauer et al. 2018). Therefore, feeding management is not only crucial for the metabolic adaptation for high production dairy cows, but for the balance of the rumen microbial ecosystem as well. When the symbiotic relationship in the rumen is disturbed, rumen health becomes a systemic concern. Besides the luminal environment, the integrity and health status of the rumen epithelium and gastrointestinal mucosa is also a very important factor in gut health. In addition to their central metabolic roles, rumen epithelial cells are the first line of defense against hostile rumen conditions such as acidic pH, high osmotic pressure, and harmful microbial-derived metabolites (Chen et al. 2012).

Challenges to Maintaining Gut Health and Normobiosis in Dairy Cows

One major gut health challenge is the prioritization of gluconeogenesis for milk production. Especially in early lactation when metabolic demand is high and insufficient dietary precursors for glucose production are provided, the body will mobilize tissue resulting in ketosis and fatty liver disease (Zebeli et al. 2015). In an attempt to prevent metabolic disorders due to a negative energy balance, it is common to provide high energy, rapidly fermentable diets to increase the energy availability at a time when dry matter intake (DMI) is often limited (Zebeli et al. 2015). In practice, this means concentrates are fed, often at the expense of fibre-rich forages, which can cause major imbalances in the acid-base balance by reducing salivary buffering contribution to the rumen through reductions physically effective neutral detergent fibre (peNDF; Zebeli et al. 2012; Figure 1). These changes result in a cascade of changes within the microbial ecosystem that can have a negative impact on rumen and animal health. Also problematic is the increase in feed intake in high producing dairy cows around peak lactation. This increase in fermentable substrate results in an increase in rumen fermentation and a larger pool of VFAs for rumen buffering. Since chewing time is reduced in diets with high grain, low forage content, the salivary contribution to ruminal buffering is also decreased (Beauchemin, 2018). Again, this increase in VFA results in lowered pH and gastrointestinal dysbiosis (Enemark, 2008).

Another major challenge to maintaining gut health is the impact of diet-induced dysbiosis on secondary disease susceptibility. When disturbances occur, functional adaptations away from growth towards cellular pH stabilization by more sensitive members of the gastrointestinal microbial ecosystem may decrease relative abundances in the community (Russell and Dombrowski, 1980) and impair efficient nutrient utilization (Russell and Wilson, 1996). This, in turn, results in the increased flow of nutrients to the hindgut from the rumen causing further disturbances in gastrointestinal microbiota (Gressley et al. 2011). Increased nutrient flow increases hindgut fermentation, reducing digesta pH, resulting in diarrhea (Li et al. 2012). Alterations in pH, osmolality, and the microbial community structure throughout the gastrointestinal tract impact absorptive and barrier functions of the epithelia causing inflammation (Khiaosa-ard and Zebeli, 2014). Hindgut acidosis is therefore similar to SARA and can also disrupt nutrient utilization, impair the gastrointestinal microbiota, reduce the absorptive and barrier capacities of the epithelia, and trigger inflammatory responses (Plaizier et al. 2018).

Dietary disturbances can result in localized inflammation, tissue damage affecting barrier function, and the infiltration of pathogens into the peripheral blood circulation (Kleen et al. 2003; Plaizier et al. 2008; Figure 1). Increased stress on the gastrointestinal microbes, as a result of an accumulation of protons, results in microbial cell death for pH sensitive microbes, and the release of microbial-derived toxic compounds into the digestive milieu including lipopolysaccharides (LPS) and biogenic amines (Ametaj et al. 2010; Aschenbach et al. 2011; Humer et al. 2018a). As a result, lowered pH and the increase in LPS also causes disruptions to the mucosa barrier function of the gastrointestinal tract (Aschenbach and Gäbel, 2000), offering the opportunity for microbe-derived toxic compounds to translocate into the systemic circulation. This event could lead to systemic inflammation and repeated bouts of dysbiosis may reduce immune responsiveness due to exhaustion from repeated exposure to systemic endotoxins and possibly opportunistic gastrointestinal pathogens. A derailment of metabolic function and immune response can result in an increased incidence rate for other diseases including laminitis (Nocek, 1997), displaced abomasum (LeBlanc et al. 2005), and liver lipolysis (Ametaj et al. 2010).

Additionally, host variation is an additional challenge to managing gut health, as susceptibility to dysbiosis varies between animals, even within a herd with similar genetics and identical environmental factors (Khafipour et al. 2009; Penner et al. 2009; Petri et al. 2013). Variations in physiological parameters, including previous exposure to lactation diets, as well as social parameters impacting eating behavior are all sources of possible variation within a herd (Figure 1). Variations in individual microbiomes and host epithelial function are also believed to play a role in animal variation (Petri et al. 2019).

Figure 1. Challenges associated with maintaining gut health in high production dairy cows

Metabolic Adaptations and Underlying Mechanisms in Cows

The rumen microbial ecosystem is a vast community of bacteria, protozoa, archaea and fungi, working in a coordinated manner to optimize nutrient utilization, exemplifying a symbiotic or mutualistic relationship with the host (Garcia et al. 2017). The gastrointestinal microbiome of cattle is no different in its complexity or symbiotic relationship. However, the role of microorganisms in other segments of the gastrointestinal tract (GIT), such as the small and large intestine, have received comparatively little attention (Mao et al. 2015). Members of gut microbiota differ in their functionality and ability to utilize different groups of substrates (Henderson et al. 2015; Mao et al. 2015; Petri et al. 2017). Hence, a higher richness and diversity of microbiota is beneficial as it enhances the stability and often enables a more efficient use of nutrient resources (Russell and Rychlik, 2001; Ley et al. 2006). However, abiotic stresses including those induced by high grain feeding, can temporarily or permanently change the composition and functionality of the microbiota. These changes include reduced richness, evenness and diversity of microbiota, as well as a reduction in the abundance of many beneficial microbial taxa in both the reticulo-rumen (Khafipour et al. 2009; Mao et al. 2013, 2015; Petri et al. 2013) and in the hindgut (Mao et al. 2015; Plaizier et al. 2017). Specific changes in the ruminal abundances of lactic acid producing and lactic acid utilizing species, and reductions in the relative abundances of fibrolytic species are most commonly documented in association with highly fermentable diets (Petri et al. 2013; Plaizier et al. 2017). Similarly, in the hindgut, increased starch feeding may increase rumen bypass and as a result the composition and functionality of the hindgut microbiota are shifted. During a state of dysbiosis, the microbiota is prone to invasion and overgrowths of pathogens exploiting the niches that are left open after disturbances. Changes to the populations of pathogenic bacterium in the rumen and gastrointestinal tract such as Escherichia coli and Streptococcus bovis are often associated with the disruption of ecological balance within the ecosystem caused by highly fermentable diets (Khafipour et al. 2009; Petri et al. 2013). The overgrowth and invasion of Fusobacterium necrophorum resulting in liver abscesses is an example of a potential trigger of disease in cattle fed high grain diets resulting in damage to the rumen epithelium (Nagaraja and Chengappa, 1998). While it is clear that the changes in the microbial populations leading to an overgrowth of pathogenic microbes promotes disease development, the mechanisms involved are less clear.

Microbial Metabolites

Microbes, including the commensals, produce an array of products that include metabolites and parts of their cell membranes known as microorganism-associated molecular patterns (MAMP; Garcia et al. 2017). Most conventional studies investigating the effect of low pH and high grain diets on the metabolism of cows have focused on single metabolites (Zebeli et al. 2011) such as LPS (Nagaraja et al. 1978). However, metabolomics technology has become an important part of livestock science to better understand health and disease. Using metabolomics, Ametaj et al. (2010) and Saleem et al. (2012) performed comprehensive studies characterizing the bovine rumen fluid metabolome, reporting potentially toxic biomarkers in the rumen fluid of dairy cows that experienced ruminal dysbiosis after being fed barley-rich diets. Humer et al. (2018a) looked at the metabolomic profile of cows fed either an all forage or a high-concentrate diet and found negative relationships between rumen LPS and biogenic amines and blood metabolites including amino acids, phosphatidylcholines and sphingomyelin. This indicates an impact of microbial metabolites on host metabolism and function. Such microbial-derived toxic compounds have been long suggested as biomarkers of dysbiosis (Plaizier et al. 2012), but the mode of action is not completely understood. However, from studies in other mammals, it is clear that frequent perturbations of the normal microbial population in the GIT can lead to an inappropriate activation of the host immune system, promoting chronic inflammation and subsequent disease (Carney, 2016; Garcia et al. 2017). It has been suggested that endotoxins are part of immune regulation mechanisms that prevent excessive inflammation, when the costs may exceed the benefits. However, this tolerance means immune responses are either limited or absent, which may result in increased susceptibility to secondary infections. Gott et al. (2015) observed grain induced ruminal dysbiosis reduced acute phase proteins (serum amyloid A) in milk after intra-mammary challenge with E. coli LPS, which could theoretically make cows more susceptible to mastitis associated pathogens.

Host gene expression

The GIT is the largest interface between the animal and the environment, not only is it responsible for facilitating nutrient uptake by the host, but it also functions as a barrier preventing the uptake of harmful microbes and some harmful products (Farhadi et al. 2003; Garcia et al. 2017). The stratified squamous structure of the ruminal epithelium appears to have evolved to deal with the abrasive feed and large microbial population colonizing the rumen and is more adapted to lower ruminal pH (Garcia et al. 2017). The hindgut is lacking this stratified structure and instead has a protective mucous layer (Steele et al. 2016). Furthermore, the mechanisms employed by the epithelial structures in the rumen and mid or hindgut differ based on these dramatically different epithelial structures. Specifically, specialized cells (goblet, paneth and M cells) are not present in the foregut, and the rumen epithelium and lamina propria lack organized lymphoid tissues (Garcia et al. 2017). Some studies have associated ruminal epithelium damage with compromised rumen integrity in cattle fed high grain diets, which may enable the translocation of microbes and their products to the bloodstream (Devant et al. 2016; Garcia et al. 2017). Khafipour et al. (2009) postulated that the translocation of pathogenic organisms and their metabolites from the GIT into systemic circulation is possibly occurring from the hindgut as opposed to the rumen based on structural design. The lack of stratified squamous structure would possibly make the hindgut more sensitive to low pH associated with hindgut dysbiosis. Although this is still believed to be true, the mechanisms and mode of action for these events remains largely undocumented and requires further research.

Advances in technology over the last decade have enhanced our understanding of the microbiome’s role in regulating host gene expression. We now understand how activation of specific genes coding for key molecular compounds results in immune activation during dysbiosis, systemic inflammation and other metabolic disturbances of the host, as well as a lowered mucosal capability for absorption and metabolism of SCFA (Penner et al. 2009; 2011; Aschenbach et al. 2011). Numerous studies have looked at the response of pattern recognition receptors (PRR) in the rumen in response to dietary induced dysbiosis (Minuti et al. 2015; McCann et al. 2016; Petri et al. 2018; 2019) with varying success. Recognition of a mucosal associated molecular pattern (MAMP) by a toll-like receptor (TLR) complex leads to the activation of inflammatory mediators (Garcia et al. 2017). Several studies have reported the expression of TLR in ruminal tissue and the potential to response to ruminal LPS (Chen et al. 2012; Minuti et al. 2015). And though multiple studies have reported associations between ruminal LPS concentrations, and its translocation into circulation (Khafipour et al. 2009; Humer et al. 2018a), the mechanisms by which this occurs are not fully understood. Postulated mechanisms include passive diffusion through the damaged rumen epithelium or via the lower gut, due to disruptions in the intestinal tight junctions (Turner, 2009; Garcia et al. 2017). High production and or reduced absorption of ruminal VFA may also induce increments in osmotic pressure, disturb Na transport, and impair ruminal barrier function, further increasing the risk of microbial and microbial-derived compound translocation (Owens et al. 1998; Humer et al. 2018; Petri et al. 2019). Regardless of whether or not translocation occurs via rumen or lower gut, microbes and or microbe-derived compounds can rapidly enter the portal vein and be taken up in the liver, increasing the changes of systemic inflammation and secondary infections (Andersen et al. 1994).

Dietary Strategies to Manage Gut Health in Transition Cows

With the push for reduced usage of antibiotics in animal production, it is critical that a combination of alternative strategies be used to manage gut health. Gastrointestinal dysbiosis due to high grain feeding may be attenuated by a variety of strategies. The guiding principle is management of diet during the early and mid-lactation period as an important factor influencing gut health, subsequently milk production, and metabolic disorder incidence in high production cows. The possible approaches that can be taken are:

1) Feed management inclusive peNDF content, and adaptation periods

2) Supplementation with feed additives inclusive phytogenic compounds, probiotics/direct-fed microbials (DFMs), and yeast derived products

Feeding Management

The feeding management principles for mitigating dysbiosis in high production cows should aim to maintain acid-base regulation in the rumen so that the production of SCFA does not overwhelm saliva and epithelial buffering mechanisms (Aschenbach et al. 2011). The ingestion of large meals in a short time predisposes cows to rumen disorders because rumen pH decreases following meals in general and the rate of decrease in rumen pH is high when a meal is large (Krause and Oetzel, 2006). This is explained by the reduced salivary secretion when dairy cows ingest a large meal in a short time, resulting in a decrease in the buffering capacity of the rumen and a consequent depression in the rumen pH (Beauchemin et al. 2008). Therefore, to maximize the buffering capacity of the rumen and lowering the episodes of low rumen pH, dairy cows should be managed so as to consume their diet slowly and more frequently in small meals during the day. Although the feeding of total mixed rations (TMR) aims to ensure the adequate ingestion of peNDF and minimize the selective consumption of grains or fine particles, these sorting behaviours still occur (Armentano and Pereira, 1997; DeVries et al. 2008). Management of these behaviours can be achieved by distributing the TMR more frequently, providing sufficient eating space, avoiding stress and adequately mixing the feed. Diets low in peNDF and in excess of rumen degradable starch (RDS) should also be avoided as this will be converted to lactic acid and can drastically reduce rumen pH (Nagaraja and Titgemeyer, 2007). Though exact recommendations of particle size vary based on composition of the diet and feeding management, peNDF is best management with regular measurement of both TMR and the orts will provide the most accurate estimation of particle size distribution based on various dietary composition and the associated sorting behaviours (Humer et al. 2018b). In addition, by providing adequate fiber and particle length (Zebeli et al. 2012) and >30% NDF for adequate production of microbial protein, which is the primary source of amino acids for the cow (Humer et al. 2018b). Along with a consistency in the supply of feed, these feed management components are important in the prevention of ruminal dysbiosis, as they are most often adjusted in used in studies trying to invoke a dysbiotic state (Nagaraja and Titgemeyer, 2007). Monitoring rumination activity is another feed related management strategy as research suggests that cows with a greater risk of developing rumen disorders have a slower increase in rumination time after calving (Calamari et al. 2014).

Although nutritional management practices can reduce the incidence of dysbiosis, some animals are more susceptible to the effects of a high-grain diet than others (Brown et al. 2000; Bevans et al. 2005; Penner et al. 2009; Petri et al. 2013). Feeding strategies for high producing cows should target adaptation of the rumen epithelium and the microbiome to the large amounts RDS and decreases in forage content to maintain the balance between production and absorption of SCFA (Kleen et al. 2003; Zebeli et al. 2015). Humer et al. (2015) showed longer periods of time below pH 5.8 and 6.0 in primiparous cows compared to those in second or higher lactation and postulated that those cows possible had fewer papillae and different microbial community profiles compared to mature cows (Penner et al. 2007; Bramley et al. 2008). Therefore, this adaptation is particularly important and in primiparous cows which have a higher susceptibility to disbalance as a result of physiological naivety. Recent studies have shown that physiological adaption of the rumen epithelium to changes in diet requires at least 14 days (Petri et al. 2019) and would be beneficial up to 4 - 6 weeks (Bannink et al. 2012; Zebeli et al. 2015). The length of recovery of the microbiome and epithelial microbiota and host gene expression from a dysbiosis challenge shows a similar adaptation time (Wetzel et al. 2017; Petri et al. 2019). This would imply that for at risk animals, such as those that are naïve to high-grain diets, it is of special importance to optimize the above feeding and management parameters.

Supplementation with Feed Additives

In light of these feed management strategies, it can still be difficult to achieve a balance between feeding readily fermentable carbohydrates to meet the high energy levels required for productivity and to minimize adverse health risks that result from feeding starches and sugars. Recently there has been increasing research in the usage of feed additives for the stabilization and recovery of rumen homeostasis. Current control strategies are unlikely to manage ruminal dysbiosis in the entire herd, but in combination with feed additive strategies, may significantly decrease the risk of nutrition related diseases.

Sodium bicarbonate and magnesium oxide are commonly used rumen buffers (Staples and Lough, 1989; Golder, 2014). By affecting the dietary cation-anion difference, bicarbonates and buffers might prevent an overgrowth of acid tolerant, lactic acid bacteria by preventing pH depression. However, literature on the effects of feeding buffers on ruminal pH are inconsistent and generally are used only as a supportive measure due to their minimal effects on the overall acid-base balance (Krause and Oetzel, 2006).

Similar to rumen buffers, the supplementation with lactate-utilizing microorganisms has been investigated to reduce the impact of lactic acid production when high grain diets are fed. A number of products of single or mixed bacterial cultures are used in cattle, including strains from Bifidobacterium, Enterobacteria, Streptococcus, Prevotella, Bacillus, Lactobacillus, Megasphaera, and Propionibacteriumspp. have been used in the dairy industry in different stages of production (McAllister et al. 2011). However, responses are inconsistent and reflecting differences in inclusion level, diet, feeding management and other factors (Humer et al. 2018b). There is often an increase in milk production as a result of supplementing DFMs, increased health and performance in calves and reduced SARA (Krehbiel et al. 2003). Direct-fed microbials have been shown to decrease the time and duration spent at low pH, increase propionate concentrations, and maintain community diversity (Krehbiel et al. 2003). Although there has been a substantial amount of work on establishing the mechanism, the evidence is limited.

Despite some contradictory results, the supplementation of feed additives including yeast, enzymes, and phytogenic compounds show modest effects on the support and recovery of the rumen homeostasis in cattle fed high grain diets. Live yeasts have been shown to increase rumen pH, reduce lactic acid, increase fibre digestion and increase SCFA production (Chaucheyras-Durand et al. 2008). Yeast fermentation products have shown increased chewing time (Kröger et al. 2017), increased DMI, and stabilization of the rumen under SARA conditions (Neubauer et al. 2018). Similar results have been seen for phytogenic compounds showing effects of the rumen pH (Kröger et al. 2017) as well as butyrate concentrations, and the proportions of rumen microbes (Neubauer et al. 2019). While mode of action is generally not understood, it is postulated that supplementation may delay the onset of VFA accumulation. However, this and further research regarding the possible synergistic properties of these feed additives in preventing gastrointestinal dysbiosis and other related diseases is still needed.

Conclusions

Understanding the mechanisms resulting in animal variation in susceptibility to dysbiosis and conditions such as SARA remains a primary concern in the prevention of metabolic diseases and the improvement of animal gut health and production. The advancement of ‘omics technology is continually providing insights into the host-microbiome interactions, as well as the potential mode of action for feed supplementation products. However, the use of this knowledge for the identification of dysbiosis susceptibility or intervention methods is still a ways off. Therefore, a combination of measures including feed management, adaptation periods, and assessment of chewing and sorting behavior will be required assessing and improving gut health. In addition, under periods of metabolic adaptation, and during high grain feeding, feed supplements can be used to reduce dysbiosis and supporting gut homeostasis.

Published in the proceedings of the Animal Nutrition Conference of Canada 2020. For information on the event, past and future editions, check out https://animalnutritionconference.ca/.