Differences between pH of indwelling sensors and the pH of fluid and solid phase in the rumen of dairy cows fed varying concentrate levels

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Feeding of diets rich in easily fermentable carbohydrates has become common practice in dairy cattle nutrition, increasing the risk of subacute rumen acidosis (SARA). Due to only nonpathognomonic signs like intermittent drop of feed intake, decreased milk fat production or intermittent diarrhoea, as well as delayed clinical signs such as laminitis and vena cava syndrome, the diagnosis of SARA is mainly based on monitoring paraclinical parameters (Plaizier et al., 2008). Among them, the measurement of the ruminal pH has been suggested the most accurate method (Enemark, 2008), whereby the risk of SARA is known to increase when ruminal pH drops below 5.8 for more than 5–6 h/day (Zebeli et al., 2008a) or below 5.6 for more than 3 h/day (Plaizier et al., 2008).
Permanent, indwelling pH measurement systems have become more popular during the last years to monitor pH and ensure early diagnosis of SARA. The eCow bolus, a telemetric pH and temperature sensor, is one of those systems (Mottram et al., 2008). The oral insertion with a balling gun also allows usage in nonfistulated cows. In comparison with single measurements, the indwelling systems provide continuous pH data and can therefore detect diurnal pH changes, thus helping in diagnosing SARA better than single measurements via rumenocentesis or rumen cannula. However, the pH thresholds of SARA are based on the pH in the rumen itself, more precisely on measurements in the ventral liquid phase (Zebeli et al., 2012), whereas the indwelling boli commonly end up staying in the reticulum, thus providing pH data from the reticulum, not the rumen (Mottram et al., 2008). In the rumen, digesta separates into the ventral fluid phase with loose and small‐sized particles (free‐rumen liquid, FRL), a middle thick‐packed rumen mat, containing large feed particles and their associated rumen liquid (PARL), and a dorsal gas cap (Zebeli et al., 2012). Due to higher nutrient availability for microbial degradation and greater microbial density in the rumen mat, concentration of volatile fatty acids (VFA) is higher in PARL, leading to a lower pH in the PARL than in the FRL (Zebeli et al., 2008b, 2012). These circumstances demonstrate that a single rumen‐sampling location is not enough to accurately reflect the whole reticuloruminal‐environment. Although pH difference between the reticulum and the rumen have been described before (Klevenhusen et al., 2014; Falk et al., 2016), to the authors best knowledge, there was no study performed to evaluate the eCow bolus as a continuous measurement system regarding the pH differences with two rumen locations (i.e. FRL and PARL) in cows fed both roughage‐based and high‐concentrate diets in a long‐term feeding experiment. The objective of this study was to evaluate the reproducibility of the permanent pH measurement system eCow to predict pH values in the FRL and PARL under different feeding conditions. Further, this study quantified the relationship between pH measurements via boli and the pH of different locations in the rumen aiming to estimate rumen pH from reticular pH and hence improve SARA diagnosis using wireless sensors.
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