Invited review: Somatotropin and lactation biology

Purpose of this review is to update the human and animal safety findings on bST since 1993, as well as to discuss somatotropin (ST) action and its effects on sustainability. Bovine ST is a naturally produced hormone which is a key regulator of growth and milk production. Beginning in the 1930s and continuing until today, investigations have examined bST's effect on animal-related factors such as nutrition, bioenergetics, metabolism, health, and well-being, and consumer-related issues such as product safety, milk quality, and manufacturing characteristics. Overall, bST homeorhetically orchestrates (both directly and indirectly) the coordination of key physiological processes involved in lactation. Bovine ST's direct effects involve adaptations in a variety of tissues and altered metabolism of all nutrient classes-water, carbohydrates, lipids, protein, and minerals. Mechanistically, this includes modifying key enzymes, intracellular signal transduction systems, tissue response to homeostatic signals, and diversity of receptor subtypes. Indirect effects are mediated by the IGF system. Collectively, IGF governs cellular changes within the mammary gland, resulting in increased rates of milk synthesis and enhanced maintenance of secretory cells. The responses to bST are modulated by environmental and management factors, especially an animal's nutritional plane. This modulation is a principal component in allowing bST to play a key role in regulating nutrient utilization across a range of physiological states. Recombinant bST (rbST) was developed in the early 1980s, and commercial rbST use in the United States began in 1994. Utilizing rbST markedly increases milk yield and improves feed efficiency and farm income; thus, it was rapidly adopted by many dairy producers. Despite reducing the environmental footprint of milk production and having no effect on cow health in well-managed dairies, milk consumption, or human safety concerns, many within the processing, grocery, and retailer industries began labeling and promoting "rbST-free" dairy products as a marketing strategy. The Food and Drug Administration was concerned this represented an implied health issue, so they required products labeled as "rbST-free" to also include the statement that "no significant difference has been shown between milk derived from rbST-treated and non-rbST treated cows." Many Cooperatives had an aggressive strategy to market "rbST-free" milk to compete with "organic" milk and suggested producers would receive higher milk prices if they voluntarily stopped using rbST. The net effect was American farmers ceased using the technology. However, rbST continues to safely increase farmer revenue and to minimize the carbon footprint of dairy production in many parts of Asia, Africa, and South America. Overall, bST is a homeorhetic control which orchestrates metabolic processes affecting nutrient partitioning and animal productivity, and it is naturally higher in genetically superior animals. The intrinsic biology of endogenous bST can be harnessed with the use of exogenous rbST to safely and sustainably improve animal performance.