## 2 Areas of Application of Rumen Boluses  
The use of rumen boluses in cattle farming is not widespread today due to the high cost of the sensors compared to external ones used in other smart herd diagnostic systems. In practice, they may have diagnostic value during the transition period in smaller farms to detect the ruminal adaptation to changes in close-up and lactation diets, as a number of production-related diseases may affect the pH within the rumen. The measurement of ionic activity has been employed to monitor the rumen status of cattle, particularly to detect sub-acute rumen acidosis (SARA). Several pH thresholds for SARA are reported, influenced by factors such as sampling location, sampling time, and measurement frequency. Considering its diurnal patterns [20], thresholds of a daily ruminal pH average <6.16 [21] and of a period with ruminal pH < 5.6 for more than 3 h/day [22] or ruminal pH < 5.8 for more than 5 h/day [21] were described for SARA risk cows using continuous measurements of the pH in the ventral rumen. Several wireless intravuminal sensors were developed in the last decade, and due to the high correlation between sensory pH data and those obtained with calibrated laboratory pH probes, ruminal boluses became accepted tools for ruminal-pH measurement [23]. However, due to their extremely short lifespan for pH measurement, rumen boluses are currently used primarily for research purposes. These measurement systems are noninvasive alternatives to surgery using a pH probe calibrated with a reference solution, a processing unit that records the pH signal, and a converter that converts the pH signal to radiofrequency and a receiver [24]. Monitoring systems developed for working farms generally measure the pH in the reticulum. Most of the publications presented the reticulo-ruminal pH [25; 26], while in other papers, authors distinguish between the ruminal and the reticular pH values [27]. Since the ruminal and reticular pH values have been proven to be different [28], the sensor localization was considered when establishing the diagnostic limit for SARA. The pH is higher in the reticulum; therefore, instead of ruminal pH! 5.8, a reticular pH of!6.04 was suggested for no more than 6 h/da [29]. In a recent study, a refined ruminal pH indicator was recommended in long-term SARA diagnosis based on individual reticulo-ruminal pH patterns, because commonly used pH SARA indicators were not able to distinguish SARA due to the great interindividual variability in ruminal pH, the calibration drift of the pH probe (0.025 pH units/week), and certain high-frequency noises causing false-negative pH peaks [30]. By correcting the absolute pH value with these factors, SARA detection could be improved significantly. Normalized pH kinetics were smoothed by applying a 180 min moving average, resulting in filtered normalized kinetics. Using the normalized pH indicators, it was more accurate to model the effects of feeding high- vs. low-starch diets as normalized pH parameters increased significantly [30]. In a more recent study, significant associations between reticular pH and chewing behavior (feed intake time, the frequency and time spent ruminating, and number of rumination boluses) and milk chemical composition (fat and fat/protein ratio) were found [31]. Still, the continuous on-farm reticular pH measurement has serious limitations, due to the limited lifespan of the pH boluses and costs involved. Besides the assessment of ruminal pH, bolus sensors are also able to measure ruminal-temperature changes, reflecting a shift in animal physiological states [32; 33; 34; 35]. It was shown by several authors that a decrease in ruminal temperature reflects drinking and feed-intake events, and its increase coincides with increased body temperature [36; 37; 33]. Monitoring changes in the ruminal temperature and the cows' activity can facilitate the early detection of estrous [38] and inflammatory conditions [39], and is, therefore, a valuable approach to support herd management.