The impact of cow genetics on the somatotropic axis in pasture-based systems: Implications for production and reproduction.

Seasonal, pasture-based milk production imposes distinctive physiological demands on dairy cows: compact calving, prompt resumption of estrous cyclicity and completion of uterine involution, achieving peak milk production on a diet primarily composed of grazed grass with short-term variability in supply and quality, and re-establishing pregnancy during a short breeding period. Central to these adaptations is the somatotropic axis-growth hormone (GH), hepatic GH receptor (GHR), insulin-like growth factor-1 (IGF1), 6 IGF binding proteins (IGFBP1-IGFBP6), and the IGF binding protein acid labile subunit (IGFALS)-which orchestrates nutrient partitioning, lactation, and the return to cyclicity and potential to reestablish pregnancy. This review summarizes the insights from experiments conducted in Ireland and New Zealand comparing different cow genotypes managed under grazing systems (e.g., North American vs. New Zealand Holstein-Friesian cows; Holstein-Friesian cows with divergent fertility merit but similar milk merit) and integrates herd-scale postpartum phenotypes linked to reproduction. In seasonal-calving, pasture-based systems, breeding indexes to improve cow genetic merit (greater emphasis on fertility and milk solids) are associated with a more favorable somatotropic axis profile (greater serum IGF1 during lactation), more conservative nutrient partitioning (less BCS loss), and superior uterine health, estrous expression, luteal function, and conception, without compromising milk solids output per cow. The biology of the somatotropic axis in dairy cows has an important effect on nutrient partitioning and fertility under pasture-based systems of milk production.