Introduction

Ruminant animals are adapted to digest and metabolize mainly forage diets. However, growth rates and milk production are increased substantially when cows consume high-starch diets. One consequence of feeding excessive amount of concentrates with inadequate fibre to ruminants is Sub-Acute Ruminal Acidosis (SARA), also known as chronic or sub-clinical acidosis. It is a well-recognised digestive disorder that is an increasing problem in most dairy herds.

Nowadays, farmers push the boundary of milk and growth production but they need to focus on the animals’ health to have a good production. Results from field studies indicate a presence of SARA in 11-29.3% of the early lactation cows and in 18-26.4% of the mid lactation cows (Garrett et al, 1997; Kleen, 2004; Tajik et al, 2009). It is an important nutritional, economic and health problem, predominantly in high yielding cows as producers respond to the demands for increased milk production with higher grain, lower fibre diets that maximize energy intake.

Dairy herds experiencing SARA will have feed intake depression, reduced fibre digestion, milk fat depression and impaired cows’ health which often results in a high culling rate.

The challenge for dairy farmers and dairy nutritionists is to implement feeding management and husbandry practices that prevent or reduce the incidence of SARA.

 

What is SARA?

SARA is currently defined as a decrease in ruminal pH below a certain threshold for prolonged periods each day. Ruminal fluid pH is a measure of the acidity or alkalinity of ruminal contents, which has diurnal fluctuations. For optimum fermentation of the diet and fibre digestion, ruminal pH should be between 6.0 and 6.4 because the cellulolytic bacteria, which allow the fibre digestion, are inhibited when pH is less than 6.0 (L. Commun, 2011).

However, even in healthy cows, the ruminal pH will fluctuate below this level for short periods during the day due to the digestion of carbohydrates (e.g. Starch), from cereal grains. Grain diets are high in readily fermentable carbohydrates which mean they will rapidly be broken down and lead to volatile fatty acids (VFA) and lactic acid.

Absorption of VFA from the rumen occurs passively across the ruminal wall. This passive absorption is enhanced by finger-like papillae that project away from the rumen wall. These papillae provide a huge surface area for VFA absorption. Ruminal papillae

Absorption of VFA from the rumen occurs passively across the ruminal wall. This passive absorption is enhanced by finger-like papillae that project away from the rumen wall. These papillae provide a huge surface area for VFA absorption. Ruminal papillae increase in length when cattle fed higher grain diets; this presumably increases the surface area and absorptive capacity, which protects the animal from acid accumulation in the rumen. If the absorptive capacity of these cells is impaired (e.g., chronic rumenitis with fibrosis), then it becomes much more difficult for the animal to maintain a stable ruminal pH following a meal. Accumulation of VFA in the rumen is the principal cause of SARA.

Ruminants possess highly developed systems to maintain ruminal pH within a physiological range about 5.5 to 7.0. As ruminal pH begins to drop following a meal, ruminant’s first response is to stop eating. Depending on the amount of fibre content in the diet, animals will start ruminating and by consequence producing a large amount of buffers via their saliva which will increase the ruminal pH. Conversely, if the proportion of fibre in the diet is too low, the buffers in saliva will not prevent the decrease because of a lack of rumination, the ruminal pH will drop under a threshold during several hours the day and the animal will be considered in Sub-Acute Ruminal Acidosis.

Two situations are likely to represent the risk of SARA. First, the transition from the pregnant non-lactating state to the non-pregnant lactating state is the period during which the majority of metabolic diseases occur and where SARA is the most present. During this period, which ranges from 3 weeks before until 3 weeks after calving, the cow is switched from a high-fibre, low concentrate diet to a diet that is higher in concentrate feeds and lower in fiber. The ruminal absorption capacity for acids, due to a reduction in the length and density of rumen papillae, can decrease by 50% during the dry period. It will take several weeks for this capacity to be restored after high concentrate diets are reintroduced (Dirksen et al, 1985; Duffield et al, 1999). Secondly, further in lactation, inaccurate calculation of dry-matter-intake (DMI) leading to wrong roughage/concentrate ratio, an inadequate content of structure within the diet or mistakes in preparing of total mixed rations may produce SARA (Kleen et al, 2003).

 

How was it diagnosed?

SARA can be a difficult condition to diagnose because its appearance is subtle. The symptoms of SARA are not easy to see and often too late.

The most common clinical sign associated with this disease is reduced feed intake in order to reduce the acidity of the rumen. However, where cows are housed and fed in groups it becomes harder to see the individual feed intake and often it is unnoticed. Other signs are reduction in milk fat content, feed conversion efficiency, decreased digestion of fibre (Lean et al. 2000), laminitis causing lameness (Nocek 1997; Owens et al. 1998), liver abscessation (Owens et al. 1998), scouring (Nocek 1997), and a higher incidence of left and right displacements of the abomasum (Shaver 1997).

Nonetheless, these signs appear after an episode of Sub-Acute Ruminal Acidosis, sometimes several months later, which enhances the subtlety in the detection of the disease.

The only reliable and accurate diagnostic test for SARA is measuring ruminal fluid pH, which can be done by different techniques, with different ease to do and accuracies.

First technique was to use a cannulated cow and sample an amount of ruminal fluid and measure it with a portable pH meter. Then the development of an indwelling probe was study using a fistulated cow and they found reliable data from it. After some research, it has been found that there is a difference between in situ pH measures and cannula pH measures (Smith, 1941; Dado and Allen, 1993; Garett et al, 1995).

Furthermore, fistulated cows are only for research purpose and are too expensive to be on farm. Thereby, techniques able to be done on-farm have been developed.

While stomach tubing has been used on-farm to collect ruminal fluid samples for pH measurement, it has been quickly determined that the results are often inaccurate due to contamination with saliva (Duffield et al., 2002; Alzahal et al., 2007).

Rumenocentesis, sometimes referred to as percutaneous needle aspiration, involves inserting a needle (1.3 mm, 12.7 cm long) into the ventral rumen, and withdrawing a sample of ruminal fluid using a 10-mL syringe. Although reliable data have been measured from rumenocentesis, the time of ruminal fluid sampling relative to feeding is important to allow proper interpretation of results. Moreover, rumen pH varies significantly among sites within the rumen (Duffield et al, 2004; Kimura et al, 2012) and by consequence samplings have to be done at the same place each time. Rumenocentesis is an invasive technique for the animals and could be a problem of milk decrease, haematomas and abscess formation at the puncture site if it is not done properly (Kleen et al., 2004; Garrett et al, 1999).

 

New method of diagnosis

Recently, a new device has been developed, using a wireless indwelling probe, called a Bolus (Mottram et al, 2008). The bolus is constituted of a pH electrode, a data logger, a battery and an antenna to send the data. The bolus measures data every minute and averages every 15 minutes, which gives 96 data points per day. It gives reliable data for up to several months and it is advantageous because it can detect rapid fluctuations in variables that are often more difficult to acquire with spot samples due to diurnal variation (Keunen et al, 2002; Duffield et al, 2004). The indwelling pH measurement and data transmitting system is a very useful and proper tool for long term measurement of ruminal pH in cow (Gasteiner et al, 2012).

The bolus can be used in intact cows, it is swallowed by the cow and goes directly to the reticulum. As it sits in the reticulum, the term reticuloruminal pH is used. The pH measured is higher than in the ventral sac (Kumara et al, 2012) and need to be taken in consideration when interpreting the data.

 

Conclusion

The definition of Sub-Acute Ruminal Acidosis is general, there is no defined specific critical point to determine it. The threshold depends upon the method used to measure the ruminal pH, the time after feeding (for spot sampling), and the site of sampling. The time below this limit is also important due to the diurnal fluctuation of the rumen pH.

Technique used to measure rumen pH affects pH value and continuous recording of rumen pH is a useful method by its ease of use and its accuracy. It also shows the ruminal pH variation over the day and after feeding, which is really helpful by determine the presence of SARA in dairy cattle.

SARA is a subtle condition in most high-producing dairy herds, leading to unnecessary economic losses. Dairy cattle nutrition has advanced sufficiently over the years to avoid SARA and the new technology developed provides a convenient technique for detecting SARA. The data needs interpretation by a vet and/or nutritionist to aid in farm management.

 

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