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The main target of the study was to identify reference values for the ultrasonographic backfat measurement to be used in dairy herd management. For this purpose, body condition was evaluated from 46111 dairy cows on 78 different farms. In early lactation milk yield and fertility are competitive concerning their energy demands. Besides producing milk the cow has to cope with the involution and regeneration of the uterus. Furthermore the reinitiation of cyclic ovarian activity and a new conception have to be realised. In this essay some descriptors of fertility have been related to the development of body condition in course of lactation as an indicator of energy balance. Intervals to first artificial insemination and to conception as well as calving intervals extend with an increasing energy deficit in early lactation. At the same time the return to a positive energy balance is slipping into later lactation with a worsening of the reproductive indices. To achieve profitable reproductive performance, conditional nadir should not decline below 13 mm RFD on herd average.