Dependence of the carbon-isotope contents of breath carbon dioxide, milk, serum and rumen fermentation products on the δ 13 C value of food in dairy cows

Abstract

Six dairy cows of two breeds were fed during three alternating periods with products from C 3 - and C 4 -plants to yield different natural 13 C enrichments of the diet (δ 13 C range: -28.0 to -13.7‰). The resulting changes in the 13 C enrichment of breath carbon dioxide, serum and milk of the animals followed the 13 C: 12 C of the food, in agreement with the individual biological half-lives of those products, and established isotope discriminations. Breath CO 2 was more enriched in 13 C than expected. This could be related to isotope discriminations during rumen fermentation. From these results an isotopic balance model for the breath CO 2 could be established.