A 100-Year Review: Regulation of nutrient partitioning to support lactation.

We have seen remarkable advances in animal productivity in the last 75 years, with annual milk yield per cow increasing over 4-fold and no evidence of nearing a plateau. Because of these gains in productive efficiency, there have been dramatic reductions in resource inputs and the carbon footprint per unit of milk produced. The primary source for the historic gains relates to animal variation in nutrient partitioning. The regulation of nutrient use for productive functions has the overall goal of maintaining the cow's well-being regardless of the physiological or environmental challenges. From a conceptual standpoint, it involves both acute homeostatic controls operating on a minute-by-minute basis and chronic homeorhetic controls operating on a long-term basis to provide orchestrated adaptations that coordinate tissues and body processes. This endocrine regulation is mediated by changes in circulating anabolic and catabolic hormones, hormone membrane receptors and intracellular signaling pathways. The coordination of tissues and physiological systems includes a plethora of hormones, but insulin and somatotropin are 2 key regulators of nutrient trafficking. Herein, we review the advances in our understanding of both conceptual and actual regulation of nutrient partitioning in support of milk synthesis and identify examples of the challenges and future opportunities in dairy science.