Nutrient partitioning and metabolic rate during lactation is determined by the synthesis rate of milk components. Dairy and beef cows differ markedly in milk production and also show differences in the endocrine control of nutrition partitioning (Veerkamp et al., 2003). Charolais and German Holstein cattle differ markedly in their properties of nutrient transformation and in endocrine regulation of nutrient flow. Charolais are known for their distinct nutrient accretion and low milk yield. In contrast, Holsteins are known for high milk secretion and low tissue accretion of nutrients (Kühn et al., 2002). In Dummerstorf at the Leibniz Institute for Farm Animal Biology (FBN), an experiment was initiated using segregating F2 offspring from the P0 founder breeds Charolais and German Holsteins. The segregating family structure of cattle bred allows the investigation of nutrient flow regulation in lactating F2 offspring with a combined dairy and beef genetic background (Kühn et al., 2002; Hammon et al., 2007). 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 (Metges et al., 1990). Therefore, changes in δ13C in milk components can provide quantitative information about the source of precursors for milk synthesis. In this study we have measured δ13C values in main milk components in response to C3 and C4 plant diets and have tested the hypothesis that changes of δ13C values are related to milk production level and differ among F2-crossbred families |
