Method for monitoring nutrition, especially fermentation in a rumen of a ruminant, wherein a characteristic value of carbon dioxide inside the rumen and/or reticulum is determined. A bolus system (1) is provided for measuring dissolved carbon dioxide in the rumen of the ruminant, such as a dairy cow, using a near dispersive infrared sensor (2). Dissolved C02 is correlated with pH and rumen acidosis can be detected. The bolus (1) is permanently implanted in the rumen and data are transmitted wirelessly. The system can be used in a milking parlor.
