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 (1,2). Therefore, changes in δ13C in milk components can provide information about the source of precursors for milk synthesis. We have recently used C3 and C4 plant feeding in a Charolais x Holstein F2-crossbred population (3). Since F2-crossbred cows showed an overall low milk production, we have investigated δ13C of individual milk components in Holstein cows with a much higher milk production level than in the F2-crossbred cows to test the hypothesis that changes in δ13C of individual milk components during C3 and C4 plant feeding depend on the level of milk production.