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Dairy cows have been selected during the last century primarily for milk production, which has been increased by a factor 3-5 per lactation during this period without a concomitantly adequate increase of dry matter intake (DMI). This discrepancy between input and output is caused by a negative or minutely positive genetic correlation between milk yield and DMI and leads, in high-producing dairy cows in early lactation, to a severe and long-lasting negative energy balance (NEB) with distinct hormonal and metabolic alterations. Milk production during this period is regulated by homeorhesis with high priority for this trait, which is relatively uncoupled from DMI, and hence with possible restrictions of other functions. The extent and duration of the current NEB is a health risk and is probably one of the reasons for genetic correlations between milk yield and disease. The gap between input and output is closed by the mobilization of reserves characterized by a rapid increase of non-esterified fatty acids (NEFA) above the acute requirement, in turn leading to ectopic fat disposition in the liver and other organs. Therefore, fat liver and ketosis occur during early lactation within a phase of the priority of the homeorhetic (genetic) regulation of milk production at insufficient DMI. Ketosis is correlated with an impairment of fertility. The correlation between an early cause (ketosis) and a later effect (impaired fertility) cannot be explained satisfactorily, but possible epigenetic alterations look promising for future research. The revealed connection between homeorhesis, fat liver and ketosis, and the impairment of fertility provides an approach for discussions of these topics as a complex. The convergence between these issues should furthermore be extended to other production diseases. Since the genetic background of this interaction must not be neglected, the current breeding system should include further health traits with a predominant emphasis on parameters of metabolism and energy balance.

Transition Period of the Dairy Cow Revisited: II. Homeorhetic Stimulus and Ketosis With Implication for Fertility