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Rumen Acidosis

Signs, causes, risk factors and solutions

Ruminal acidosis is a bovine metabolic disease that affects feedlot as well as dairy cattle. Acidosis in cattle is usually associated with the ingestion of large amounts of highly fermentable, carbohydrate-rich feeds, which result in the excessive production and accumulation of acids in the rumen.  

Ruminal acidosis can be present in different forms, reaching from peracute life-threatening forms to chronic illness, which is difficult to detect (Oetzel, 2003).  

Two main forms of rumen acidosis are: 

  1. Acute ruminal acidosis 
  2. Subacute ruminal acidosis (SARA) 

The difference between acute and subacute forms are that during acute ruminal acidosis the pH depression is more severe (Oetzel et al., 1999), and clinical signs more prominent (Kleen et al., 2003). Acute rumen acidosis is common in feedlots, whereas SARA is more common on dairy farms (Krause and Otzel, 2006). Contrary to acute acidosis, in SARA the pH depression is apparently due to the total accumulation of volatile fatty acids alone and is not due to lactic acid accumulation (Krause and Otzel, 2006). 

Definition of subacute ruminal acidosis 

Subacute ruminal acidosis is the most important nutritional disease in dairy cattle since it can negatively impact the dairy industry by decreasing dry matter intake, milk production, profitability, and increasing culling rate and death loss (McCann et al., 2016).

The current definition of SARA is based on a time period during which the rumen pH is below a certain threshold. Although, there is no general agreement on the pH threshold for SARA, the two main definitions indicate 5.24 hours below 5.8 (Zebeli et al., 2008) and 3 hours below 5.6 (Plaizier at al. 2008).  

    Diagnostic technique 

    Among the scientific community, the most common diagnostic techniques are based on rumen pH determination. The most widespread methods are listed below:

    • Indwelling pH Data Logger Method:  this is currently the best methods to record real time pH fluctuations. Nevertheless, different rumen areas have different pH and uncontrolled sensor movements can generate unreliable data.
    • Rumenocentesis: consists of percutaneous needle aspiration of rumen fluid from caudoventral rumen. The disadvantage associated with this method is that it is quite invasive, and can result in abscesses at the site of puncture (Aceto et al., 2000). 
    • Oral – Stomach Tube Technique: it is not considered a reliable technique because pH may vary depending on intra-ruminal localization, time of sampling in relation to feeding and saliva contamination (Enemark et al., 2002).  
    • Rumen Cannula Method

    Other promising non-invasive diagnostic techniques include:

    • Fecal Lipopolysacharide (LPS): feeding high-grain diets to induce subacute ruminal acidosis (SARA) in dairy cows has been associated with the increase in the fecal concentration of lipopolysaccharides (LPS; = endotoxins) originating from gram-negative bacteria (Li et al., 2012).
    • Blood Gas Analysis: to detect acid-base imbalance in the blood (Giansella et al., 2010)
    • Milk fatty acid pattern: which may help identifying cows with different susceptibility to a SARA challenge within a herd (Jing et al., 2018)

    Causes of subacute ruminal acidosis (SARA) 

    SARA occurs when ruminal buffering is not adequate to contrast the volatile fatty acids (VFAs) production. This may be due to different reasons:

    1. Excessive carbohydrate feeding at the expenses of dietary fiber. Long fiber particles (> 4 mm) stimulate chewing activity, which drives saliva production. Saliva, with a pH value of approximately 8.2 and a high sodium bicarbonate level, has a buffering effect in the rumen.
    2. Excessive long forage particles (sorting). Physical structure of fiber also matters. Even if diet fiber levels are adequate but particles are too long or unpalatable, sorting for concentrates occurs soon after feed delivery, causing the cow to consume a diet that is low in in physically effective fiber (Oetzel, 2007).
    3. Failure to adapt to rapid diet changes. The classical example is the passage from the dry diet rich in roughage, to the early lactation diet rich in concentrates. Rumen bacterial population on one side and rumen papillae on the other need time to be ready to digest large amount of carbohydrates and consequently to absorb large amount of VFAs.

    Risk factors for SARA 

    Cows are more at risk to develop SARA in the following circumstances: 

    1. Early lactation due to the instability of the bacterial population (Devries et al., 2009) and to the diminished size and absorptive capacity of rumen papillae following feeding of lower energy diets during dry period (Stone, 2004). 
    2. Primiparous cows, for the same reasons described above and additionally because the animals were never exposed to a lactation diet before (Enemark et al. 2004) (Krause and Otzel 2006).   
    3. Cows subjected to heat stress. Increased respiratory rate during heat stress lowers blood bicarbonate concentrations thus decreasing ruminal buffer capacity of the animal. Moreover, in summer, atypical meal patterns in response to heat avoidance can decrease meal frequency and increase acidotic bouts.
    4. Errors in ration calculation and management: wrong dry matter calculation, errors in TMR mixing, feeding time-schedule and feed bunk space per cow (Kleen et al. 2003).

    Prevalence 

    Prevalence of SARA increases as cows consume more total dry matter and as cows consume diets containing higher proportion of grain. However, in intensive dairy farming, the problem is technically unavoidable. Group feeding and strong variability among individual cows in terms of rumen microbiome, are the main reasons of failure (Figure 1) 

    Figure 1. Ruminal pH measured 5 days after calving in two cows (best and worst-case acidosis cows) fed the same lactation diet (Penner, Beauchemin and Mutsvangwa, unpublished data).
    Figure 1. Ruminal pH measured 5 days after calving in two cows (best and worst-case acidosis cows) fed the same lactation diet (Penner, Beauchemin and Mutsvangwa, unpublished data).
    PrevalenceCountryReference
    19% early lactation
    26% mid-lactation
    US (15 farms)Garrett et al. 1997
    20.1% early and peak lactationUS (14 farms)Oetzel et al. 1999
    13.8% overallNetherlandsKleen et al. 2009
    11% grazing cowsIrelandO'Grady et al. 2008
    33% early lactationItalyMorgante et al. 2007
    11% early lactation
    18% mid-lactation
    Germany/NetherlandsKleen et al. 2004
    20% overallGermanyKleen et al. 2013
    14% overallPolandStefanska et al. 2016

    Table 1. SARA prevalence in dairy herds reported by different authors.

    SARA prevalence ranges between 11% and 33% in early lactation (Kleen et al. 2004; Morgante et al. 2007) and between 18% and 26% in mid-lactation (Kleen et al. 2004; Garret et al. 1997). Table 1 summarizes the prevalence of SARA in dairy herds reported by different authors.

    Clinical signs of SARA 

    The diagnosis of SARA is difficult under farm conditions as clinical signs are commonly subtle and delayed (Humer et al., 2018). The clinical findings that may direct the veterinarian’s attention to the possible occurrence of SARA have recently been summarized by Oetzel (2017) and include, for example, a poor body condition score and frequent cases of infections. The recommendation is to look at the presence of multiple signs, as the ones listed below, since there is no specific and unique indicator for SARA.  

    Some symptoms of SARA: 

    1. Liver abscesses may occur as the result of a cascade of events starting with rumenitis and rumen parakeratosis. Once the rumen epithelium is inflamed, bacteria can leak into portal circulation and causing abscesses. A more specific finding indicative of SARA is liver abscesses at slaughter that may reach prevalences of >30% in cull cows (Rezac et al., 2014). Limit: post mortem information often lost. 
    2. Variable intake and/or milk production. In cows affected by SARA, a fluctuating feeding pattern has been described as the most consistent symptom. During mid-lactation, variable feed intake may be indicated by the observation of variable milk production; however, during early lactation, this will likely go unnoticed, because of mobilisation of body reserves (Humer et al., 2018). 
    3. Milk fat depression. The interpretation of low milk fat is quite difficult, as the normal milk fat percentage depends largely on breed, DIM and season. Further, herd means may obscure outlier cows with very low or high milk fat content. Therefore it might be useful to interpret milk fat content as a proportion of cow with very low (<2.5% for Holstein cows) test results; whereby these cows should not represent more than approximately 10% of the herd (Oetzel, 2007). In addition, it is important to keep in mind that also other factors can lead to low milk fat concentration like feeding excessive amount of plant lipids rich in polyunsaturated fatty acids. 
    4. Alterations in feces and diarrhea. SARA affects the consistency and particle size of feces; however, those alterations are usually transient. Typical feces appearance is bright yellowish with a sweet–sour smell (Kleen et al., 2003). Furthermore, feces may appear foamy with gas bubbles and whole cereal grains as well as higher amounts of undigested fiber might be present. The size of fecal particles may be enlarged being around 1–2 cm instead of the more normal size of less than 0.5 cm (Hall, 2002). 
    5. High incidence of lameness. During SARA, vasoactive molecules like histamine, LPS and lactic acid, are released into the blood stream. These molecules play an important role in the laminitis etiology, weakening the hoof tissue and predisposing the animals to lameness. Reference values are impossible to set since environmental factors play a big role in such kind of disease.    

    Consequences of SARA in dairy cattle 

    SARA has long-term devastating health and economic consequences in dairy cattle. The most visible and direct effect of the acidotic stress can be observed in the work published by Khafipour et al. 2009, where a SARA challenge was induced in lactating cows. In this experiment about 20% of a 50:50 forage to concentrate (F:C) ratio TMR, was replaced by a pellet containing 50% barley and 50% wheat for one week, resulting in an F:C of 40:60. 

    The SARA challenge induced in this experiment reduced the DMI (15%), milk yield (3.3 kg/d), and milk fat (0.12% point).  

    Few attempts have been done in order to calculate the potential economic impact of SARA. One of the most cited, is a case study conducted on a 500-cow dairy in central New York (Stone, 1999). Stone calculated a cost of $400 to $475 lost income per cow per year due to SARA. This rough estimation was simply calculated multiplying a reduced milk yield by 2.7 kg/day, milk fat by 0.3% point and milk protein by 0.12% points for the entire lactation.  

    Daily averages of DMI and milk yield in dairy cows fed a basal TMR during control or TMR with wheat-barley pellets during subacute ruminal acidosis (SARA) treatment. Error bars indicate standard error of difference between treatments (SED); within each day, * = P < 0.05.
    Figure 2. Daily averages of DMI and milk yield in dairy cows fed a basal TMR during control or TMR with wheat-barley pellets during subacute ruminal acidosis (SARA) treatment. Error bars indicate standard error of difference between treatments (SED); within each day, * = P < 0.05.

    The performance loss can be explained at physiological level, by the systemic inflammation status occurring in cows subjected to an acidotic challenge. An unbalance between carbohydrate and physically effective fiber causes a shift towards Gram-negative bacteria resulting in the release of cell-free lipopolysaccharide (LPS) in the rumen. LPS can then translocate trough gut and to a lesser extent trough rumen epithelium and reach systemic circulation triggering a strong inflammation response (Zebeli and Metzler-Zebeli, 2012).  

    However, any estimation on performance loss would be inevitably inaccurate for two main reasons: 

    1. The loss in performance depends on the severity of the stress induced as described by Li et al., 2012.
    2. By impairing cow health, SARA has also been associated with other diseases like displaced abomasum, fatty liver, liver abscesses, laminitis and downer cow syndrome, thus increasing risk of culling and veterinary treatments (Abdela 2016).

    How to minimize the risk of SARA 

    Given the lag time following the onset of SARA, there is no specific treatment. This reinforces the importance of prevention.  

    Besides offering the cows a balanced diet in terms of amount and degradability of carbohydrate and quantity and size of fiber, feeding management is of vital importance to minimize the risk of SARA.

    A list of practical management indications have been listed in the review of Humer et al. 2018, and includes: 

    1. Check the particle size distribution of the diet. An appropriate distribution of small and large feed particles will results in a less selective feeding behaviour and in a more stable rumen pH (Table 2. Adapted from Heinrichs and Kononoff, 2002).
    2. Provide sufficient space at the feeding lane (at least 60 cm/cow), avoiding feed competition and the consumption of large meals.
    3. Do not overmix the TMR (max 3 to 5 min after last ingredient is added). Overmixing decreases the structure of the ration. 
    4. Add water to a dry TMR until a DM content of 55% is reached, to minimize sorting. 
    5. Deliver feed more often and increase the number of feed push ups to encourage small and frequent meals. 

    Table 2. Recommendations for TMR particle size distribution when the TMR is composed of ground concentrates (TMR 1), with pelleted concentrates (TMR 2) or the diet is offered as partial mixed ration (PMR) (partly adapted from Heinrichs and Kononoff, 2002)

    Particle fractionScreen sizeTMR 1 (%)TMR 2 (%)PMR (%)
     Large particles>19 mm3-83-815-25
     Medium particles8-19 mm30-4035-4535-65
     Fine particles1.18-8 mm30-4040-5015-25
     Very fine particles<1.18 mm<20<10<8

    How to mitigate the effects of SARA 

    Supplementation of feed additives represents another commonly used approach to mitigate the consequences of SARA. Commonly used feed additives include:  

    • Yeast supplements
    • Essential oils or phytogenics
    • Toxin binders 

    Levabon® Rumen E yeast supplement

    One of the most used additive class in this sense are yeasts products which can be provided as live yeasts, dead yeasts, or yeast culture products. 

    Levabon® Rumen E is an autolyzed yeast, with a prebiotic mode of action, which has shown beneficial effects in cows exposed to an acidotic challenge. In a study, done in cooperation with the University of Veterinary Medicine, Vienna, rumen-cannulated cows were fed a pure forage diet and switched to a 65% concentrate diet on DM basis, to induce an acidotic stress. Supplementing the concentrate rich diet with Levabon® increased duration of eating, total chewing and DMI compared with the control diet (Kröger et al. 2017).

    Moreover Levabon® caused pronounced effects on the concentration of biogenic amines, like histamine, during the first acidotic challenge, showing adecrease by 31% compared with control. 

    Its beneficial effect is also reflected at microbiome level. In the same study Levabon® decreased gram-negative bacteria thanks to the binding activity of certain yeast components like mannan oligosaccharides, β-glucans, chitin, peptides, AA, and nucleotides. The same components also acted as substrate for cellulolytic bacteria, promoting growth of Ruminicoccus and Clostridium spp. in the challenged rumen and contributing to the maintenance of a physiological ruminal pH (Neubauer et al., 2018).     

    Mycofix® toxin deactivator  

    Mycofix® binding complex demonstrated a high affinity towards lipopolysaccharides (LPS) in vitro. LPS binding has been tested both in presence of high aflatoxin concentrations, to exclude competition between Afla and LPS for the same binding sites, and in rumen fluid using Rumen Simulation Technique (RuSiTec). (Image available if needed)

    On this basis, in vivo studies have been conducted to test Mycofix® in cows subjected to acidotic stress. In a study done by Prof. Zebeli and his team, non-lactating cows were challenged with intermittent high concentrate diets to induce SARA. As a result of this nutritional stress, levels of liver enzymes such as AST and GLDH, considered markers for hepatocyte integrity of dairy cows (Bobe et al. 2004), increased compared to levels detected in control cows (Figure 3). 

    The liver plays a major role in the inflammation response caused by LPS release and subsequent translocation in the blood circulation. This mechanism is widely described by Zebeli and Metzler-Zebeli, 2012. The beneficial effect of the clay mineral CM (Mycofix®) supplementation on improved liver health, can be explained by a lower toxic load in the rumen and systemic circulation because of its known capability to absorb LPS (Humer et al., 2019).   

    Figure 3. AST and GLDH concentration measured in cows fed control diet and clay mineral additive CM. (Adapted from Humer et al., 2019)
    Figure 3. AST and GLDH concentration measured in cows fed control diet and clay mineral additive CM. (Adapted from Humer et al., 2019)

    Another interesting effect observed was the significant reduction of certain biogenic amines whose production is increased during high-grain feeding. Among all biogenic amines, histamine, which is also known to play a role in the pathogenesis of laminitis, was significantly reduced by 28% in cows supplemented with Mycofix® (Humer et al., 2019).  

    Finally, to estimate the global effect of Mycofix® on all the measured parameters, a multivariate analysis was conducted to identify characteristic trends or grouping among cows fed a pure forage diet (Baseline), the SARA diet (65% concentrate; = control) or the diet with Mycofix®. The outcome was that SARA samples clustered separately from the baseline samples, whereby the cows receiving Mycofix® clustered closer to the baseline than control cows (Humer et al., 2019) (Fig. 4).

    A partial least-squares discriminant analysis (PLS-DA) of the blood metabolites that were affected by the feed additive.
    Figure 4. A partial least-squares discriminant analysis (PLS-DA) of the blood metabolites that were affected by the feed additive. The two-dimensional score plot distinguishes the metabolic profiles of cows fed either a pure forage diet (Baseline; red) or a 65% concentrate diet (subacute ruminal acidosis, SARA) without feed additive (control, CON; blue), or a clay mineral-based product (CM; green).

    The beneficial effect of Mycofix® on cows affected by SARA is reflected also at microbiome level. In another paper originating from the same experiment, Mycofix® showed a certain potential to reduce bacteria that are associated with low pH, such as Lactobacillus, and favor high abundant genera such as Campylobacter, Butyrivibrio, and lower abundant Gram-positive commensal bacteria. Along with what has been previously shown in vitro, Mycofix® showed a decreasing effect on possible harmful bacteria, especially Gram-negative genera, such as Treponema, Fusobacteria, and Succiniclasticum. These groups include LPS producing species and potential host pathogens (Neubauer et al. 2019).

    Digestarom® phytogenic feed additive 

    Digestarom® contains a blend of spices, herbs and essential oils that showed potential to modulate reticular pH in different ways. The product was tested in an extensive study done in cooperation with the University of Veterinary Medicine in Vienna, where non-lactating cows were fed an intermittent high concentrate diet to induce SARA.

    One of the main finding was that supplemetation of Digestarom® improved reticular pH dynamics when the lowest reticular pH readings were observed in control cows subjected to acidotic stress. In particular Digestarom® increased the time spent ruminating and total chewing (Fig.5) (Kröger et al. 2017). 

    Figure 5. Duration of reticular pH <6.0 in dairy cows fed either a control diet (CON), or a diet supplemented with phytogenic compounds (PHY) per day of experiment. The solid line indicates the SARA threshold of a reticular pH <6.0 for longer than 314 min/d. Treatments with different letters (a,b) differ significantly within the same day (P < 0.05) (Adapted from Kröger et al. 2017).
    Figure 5. Duration of reticular pH <6.0 in dairy cows fed either a control diet (CON), or a diet supplemented with phytogenic compounds (PHY) per day of experiment. The solid line indicates the SARA threshold of a reticular pH <6.0 for longer than 314 min/d. Treatments with different letters (a,b) differ significantly within the same day (P < 0.05) (Adapted from Kröger et al. 2017).

    However, the positive effects of Digestarom® in shortening the time of reticular pH <6.0, cannot be solely attributed to changes in ruminating behavior. In fact, during the second concentrate challenge (CONC 2), there was a positive effect on pH without any influence in chewing variables (Kröger et al. 2017).  

    The explanation could come from the modulating effect that Digestarom® showed towards ruminal bacteria community with potential effects on reducing the ruminal degradation of starch-rich feeds. Almost all of the bacteria that were decreased by Digestarom® are known starch utilizers, including the genera Shuttleworthia, Olsenella, Bacteroides, Bifidobacterium, Roseburia, and Syntrophococcus (Calsamiglia et al., 2007; Patra, 2011), whereas no fiber fermenting taxa were reduced (Neubauer et al. 2018).  

    In a high-concentrate diet, the increase in starch provided would normally supply amylolytic bacteria with sufficient substrate to promote growth.  The decrease of amylolytic bacteria seen with supplementation of Digestarom® in this experiment, supports the mode of action stated by Calsamiglia et al. (2007) and Cobellis et al. (2016).  

    A reduction in starch utilizers would potentially delay the onset of SCFA fermentation, reduce SCFA (short-chain fatty acids) accumulation, and thereby prevent rapid and extended duration of low pH after feeding (Neubauer et al. 2018). 

    Moreover, with a decrease in starch utilizers, cellulolytic bacteria might have a chance to evolve due to less competition (Patra and Yu, 2015). This is supported by the higher reticulo-rumen pH previously reported for Digestarom® (Kröger et al., 2017). 

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