A system for monitoring digestive efficiency within the rumen of one or more ruminant animals comprises rumen boluses shaped and sized to be retained within the rumen dorsal sac and each comprising temperature, pH sensor, and redox sensors, and a wireless transmitter. A processor is arranged to derive from the sensor data one or more parameters indicative of animal digestive efficiency including any combination of one or more of hydrogen scale (rH), partial pressure of hydrogen (pp[H2]), oxygen fugacity (f(O2)) and free energy of the system (ΔG). A method and bolus are also claimed.