Effects of genetic selection for milk yield on energy balance, levels of hormones, and metabolites in lactating cattle, and possible links to reduced fertility

Selection for a higher milk yield increases metabolic load via a higher yield per se and/or via physiological processes that facilitate milk yield, and it is difficult to differentiate between these two. Here, we aim to identify important pathways that contribute to the reduction in fertility following selection for higher yield. The associations between milk yield and fertility may run via pleiotropic effects, i.e. via functional pathways (for example related to intake), or linkage of genes and may involve changes in levels of hormones and metabolites. A number of studies have investigated the effects of genetic merit for milk yield on fertility, feed intake, energy balance and levels of metabolic and fertility hormones or metabolites. Differences in genetic merit were associated with differences in: (1) feed intake; (2) energy balance; and (3) plasma levels especially of GH, IGF-I, prolactin, progesterone, insulin, glucose, NEFAs and ketones. In the discussion we focus on the possible roles that energy balance, the growth hormone axis, and glucose together with insulin may have in the reduced fertility that is associated with high yield. The overall conclusion is that many minor pathways probably contribute, but that reduced metabolic fuel availability, rather than direct effects of hormone concentrations, is an important cause of poorer fertility with increasing genetic merit. (C) 2003 Elsevier B.V. All rights reserved.