Rumen monitoring with the eBolus

Monitoring the rumen has been of scientific interest ever since fistulation became common practice, but methods of rumen monitoring have always been plagued by inaccurate or unnatural and
expensive techniques.

Professor Toby Mottram a

Professor Toby Mottram

In 2001 Toby Mottram was asked by a major pharmaceutical company to develop a bolus to measure pH in the reticulum. Although the project was eventually cancelled the idea was too good to pass on and bolus R&D continued. In 2007 some boluses were shipped to friends in Brisbane who tested them and allowed publication of “A wireless telemetric method for monitoring clinical acidosis in dairy cows” in 2008. The biggest problem in pH monitoring is to maintain the accuracy of the sensor whose base line tends to shift.  The best way to achieve this, which was patented (GB0719740.3) by eCow in 2007, is to use a high quality pH glass bulb electrode and to maintain its orientation downwards using a weighted bolus. Currently the eBolus is produced regularly and shipped worldwide with customers as far reaching as California and Japan.

How to best monitor the rumen – eBolus

The eCow eBolus is a wireless telemetric device swallowed by the cow that provides continuous and accurate measurements of pH, temperature and reduction potential inside the reticulum. It records data at 1 minute intervals and saves averages every 15 minutes to provide 96 readings per day. This data is stored on the bolus and then can be retrieved every time the mobile phone handset is brought within range of the cow.

For more information see For researchers – the eBolus, Bolus Specification or our FAQs.

Bolus Picture

Advantages of the eBolus

  • Smallest bolus size available today – just 27mm diameter by 135mm long
  • Longest bolus life available – 5 months of data collection
  • Reliable, stable data – eCow uses the best quality sensors available
  • Direct measurement of rumen conditions to give more accurate picture
  • No need to change the feeding pattern of the animal – boluses can be inserted at any point in the daily feeding regime with a standard bolus gun
  • Auto power off when outside of the animal to preserve battery life
  • One end of the bolus is weighted to encourage the bolus to sink into reticulum and keep sensor in the rumen liquor
  • In reticulum pH is consistently 0.25 pH units above rumen – less fluctuation than rest of rumen
  • Metal cap protecting pH sensor from rocks and hardware swallowed by cow
  • Smooth surface as to not cause distress or harm to animal – no protruding edges
  • Small handset with separate antenna for easy use on farms – farmer can stand next to cow (familiar for cow) and advisor can stand back with handset (50cm cable, with longer ones available on request)
  • Visual readout of download status – no wasted time (approx 1 min to connect, 2 min to download)
  • Instant data analysis showing pH graph, lowest pH, days since last reading and battery life

pH Measurements

The pH of a substance is a measure of the acidity of an aqueous solution. More specifically, it is the negative logarithm of the (thermodynamic) activity of the solvated hydrogen ion – meaning pH is a dimensionless scale. Due to the acidity of a substance being correlated to the relative hydrogen ion concentration, and as the hydrogen ion is electrically charged, it is possible to measure pH electrically.


The main feature of the eBolus is its ability to accurately and continuously monitor pH (+/- 0.1 pH unit). When in constant use (i.e. above 31°C) the sensor readings can remain stable for up to 150 days (5 months), far longer than similar products. Although after this time the reference junction inside the bolus may tend to drift towards pH 7, as well as exhibiting a reduction in the pH range.

The effect of pH drift in eCow boluses is caused by rumen fluid slowly seeping into the glass bulb pH sensor, through the protective triple junction and contaminating the reference electrode. Inside the boluses, pH is measured by the differential potential (mV) between the hydronium ions affecting the measurement electrode and the Ag/AgCl reference electrode protected by the triple junction. As rumen liquor begins to leak into the sensor and contaminate the reference the potential difference in mV between the two electrodes narrows so that the pH value appears to be rising.

We do not apply any drift correction, although with the eBolus customers have the ability to re-calibrate the device to correct the pH reading and lengthen the life of the device.

Temperature Measurements – monitor drinking behaviour and heat stress

As well as the pH functionality, the eBolus continuously logs the temperature of its environment with an accuracy of +/- 0.1°C. Unlike the pH measurements, readings of the temperature of the rumen do not drift over time and will therefore only be limited by the battery life of the bolus.


Measurements of rumen temperature can be used to monitor drinking activity, as cold water entering the rumen correlates with a sharp but brief drop in the temperature reading. It is important to note that these events are not changes in the overall body temperature of the cow and should not be treated as such.

However, changes in the mean daily temperature of the cow’s rumen can be linked to an overall change in the body temperature of the cow. As it is consistently 1°C above the rest of the body, the temperature of the rumen can be relied upon to give an accurate indication of overall body temperature. Peaks in temperature are often observed when a cow is suffering from an infection such as mastitis. Infections such as this may also be observed as a lack of drinks taken and a drop in ruminal pH.

Reduction potential

If necessary, the ability to measure the reduction potential (values in mV) can be included with the eBolus. Reduction potential is a measure of the tendency of a particular chemical species to gain electrons and therefore be reduced.

Limited experimental redox data is available. As the reticulorumen is an anaerobic environment, the values for reduction potential are consistently negative and in the ventral sac values can vary significantly depending on its contents. The reticulum however is much more stable and fluctuations in reduction potential have been hypothesised to be mostly caused by drinking events since they line up with drops in the temperature data. We would therefore only recommend redox functionality for research customers with the ability to choose the location within the rumen that the bolus resides. Typical values for reduction potential lie within the range -150 to -300mV.Temperature and redox fluctuations match

Follow this link for a paper on measuring reduction potential with the eBolus.

Solubility of certain nutrients through reduction potential

The process of digestion involves many enzymatic reactions, often involving the oxidation of one compound and the reduction of another, hence oxidation-reduction reactions.  The environment in which an enzyme exists dictates its ability to perform oxidation-reduction reactions, often referred to as the oxidation-reduction state of the environment, or its reduction potential (Eh). With the rumen being an anaerobic environment, values for reduction potential exist as negative Eh values typically between -150 to -300 mV.  Through measuring the reduction potential of a solution you can monitor the enzymatic activity of the solution, thus through reduction potential you can detect how easily a certain nutrient is digested. With this in mind the solubility of certain nutrients can be reflected in the values for reduction potential.