The transfer of nutrients derived 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. Therefore, changes of δ13C in milk components can be used as a tool to estimate the dietary carbon fraction contributing to milk components and the degree of substrate mobilisation from body tissues. We hypothesized that milk yield is a factor determining the extent and dynamics of carbon transfer from the diet and thus have investigated nutrient transformation in dairy and beef cows that greatly differ in their potential for milk production.