ATR-IR device to monitor ruminal fermentation

An apparatus for monitoring nutrition, especially fermentation in a rumen of a ruminant, is designed to be orally applied to the ruminant and to stay permanently in the rumen. The apparatus includes: a) at least one sensing unit for sensing a characteristic value of dissolved carbon dioxide in the liquor of rumen and/or reticulum; and b) at least one first communication unit for the wireless communication of data with a respective second communication unit outside the ruminant. The sensing unit includes at least one attenuated total reflectance (ATR) sensor.