Invited Review: Somatotropin and Lactation Biology.

Abstract

Purpose of this review is to update the human and animal safety findings on bovine somatotropin (bST) since 1993 as well as to discuss ST action and its impacts on sustainability. Bovine somatotropin (bST) 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 impact 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 somatotropin's direct effects involve adaptations in a variety 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 insulin-like growth factor (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 impact 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 FDA 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.