Interaction between metabolic challenges and productivity in high yielding dairy cows.

The onset of lactation in modern dairy cows is characterized by a negative energy balance, due to a drastic increase in energy requirements for milk yield and a simultaneous depression in dry matter intake around parturition. Prioritization of milk yield over maternal body functions is a universal biological strategy in all lactating mammals to buffer the newborn's nutrition from fluctuations in the dam's energetic status. Consequently, in case of an energy deficiency, the dam will mobilize fat and protein reserves in order to safeguard milk yield. During decades of one-sided selection for milk yield, man has exploited the cow's potential to prioritize mammary energy supply without an equivalent progress in dry matter intake capacity. Consequently, genetic selection for milk yield has widened the gap between energy expenditure and energy intake, and has increased the cow's inclination to respond to energy deficiencies in the transition period by aggressive body tissue breakdown. Chronically elevated concentrations of non-esterified fatty acids and ketone bodies have been demonstrated to affect multiple organ systems including the immune system, the reproductive axis and the liver and are, in contrast to absolute milk yield, closely and consistently related to the final incidence of reproductive disorders.