Nutrient partitioning during lactation is determined by the synthesis rate of milk components. Nutrient transformation distinctly differs between Charolais (accretion type) and German Holstein (secretion type) due to their different potential for milk production and body accretion. The transfer of nutrients originated from feed into milk can be estimated by feeding C3 and C4 plants, because C4 plants have a higher natural 13C enrichment (δ13C) than C3 plants (1). Therefore, changes in δ13C in milk components can provide quantitative information about the source of precursors for milk synthesis. We measured δ13C value in main milk components in response to C3 and C4 plant diets and have tested the hypothesis that changes of δ13C values differ among milk components and among F2-crossbred families.