Continuous Ruminal pH Measurement: Validation, Opportunities, and Limitations

Microbial fermentation of feed in the rumen drives net energy and metabolizable protein supply for ruminants emphasizing the importance for a comprehensive understanding of fermentation characteristics. However, ruminal fermentation does not occur in steady-state and, hence, data collection must occur frequently to adequately characterize the response to diet, management, and host-factors. In addition, substantial regional stratification occurs in the rumen; thus, further requiring clear information for the region of pH measurement. Given the importance of fermentation, indwelling systems have been designed to evaluate ruminal pH in real-time. These systems have been validated and have remarkable accuracy and precision relative to samples measured using spot sampling approaches. Initial continuous pH measurement systems required cattle to be tethered to support hard-wire connections between the pH electrode, pH meter, and data-logger. While these systems provided novel information, application was. More recently, indwelling pH systems have been developed that enable measurement in group-housed cattle. This advancement has allowed for application of pH measurement under loose-housed dairy, feedlot, and grazing conditions. Surprisingly, ruminal pH measurement in feedlot cattle has revealed much lower risk for low ruminal pH than would have been previously thought. However, as these systems are smaller and lighter than the previous indwelling systems, differences in their inherent design and method of application (oral dosing vs. via a ruminal cannula) affect interpretation of the results. In particular, the ability to orally dose pH systems and the subsequent measurement of reticular pH has drawn questions regarding the relationship between reticular and ruminal pH. Most studies conducted have demonstrated that reticular pH is greater and less responsive than ruminal pH; however, this relationship may not hold true for cattle fed high-concentrate diets. While there is a greater opportunity to measure ruminal pH with indwelling devices, challenges to the relevance of pH as a sole predictor of fermentation conditions is warranted. Advancements in ability to measure reticular and ruminal pH have improved our understanding of how management, diet, and host-factors affect ruminal pH, but, conclusions based on pH data alone should be regarded with caution.