255 Anchor Talk: Consider intrinsic factors that regulate homeostasis to optimize sustainability of mineral supplements for swine

Abstract Decades of research has focused on methodologies to improve the efficiency of mineral use while assuring sufficient intake to meet animal well-being and production needs. A primary focus of these efforts is P, as P is the third most expensive category of nutrients supplemented in swine diets. Bioavailability of P is more variable than other nutrients in feed ingredients commonly fed to swine. Due to complexities involved in assessments of P bioavailability efforts shifted to defining P digestibilities for each ingredient. Efforts to refine P digestibilities, based on intake and fecal excretion, even with corrections for endogenous losses, have failed to provide a constant coefficient for feed ingredients that can be included in diet formulations, as the digestibility estimates vary with animal age, production phases and inclusion of enzymes such as phytase. An under-appreciated contribution to inaccuracies of digestible P relates to intrinsic regulation of Ca and P homeostasis by the animal. Regulation of Ca and P homeostasis by PTH and 1,25(OH)2D3, must now include a third mineralotropic hormone, FGF23. Applying insights for intrinsic roles of FGF23 will allow more efficient use of nutrients, especially P and vitamin D. FGF23 regulates activation of vitamin D and controls renal P excretion. Over-supplementation of P signals osteocytes (bone cells embedded in a mineralized matrix) to release intact FGF23 which targets the kidney to block reabsorption of P for maintenance of soft tissue homeostasis, resulting in an inefficient use of dietary P. Intact FGF23 also inhibits hydroxylation of 25-OH D3 to form 1,25(OH)2D3. These endocrine pathways illustrate major roles provided by kidneys and bone to allow adaptation to variable Ca and P inputs, thus maintaining homeostasis. Recent efforts to assess P requirements of sows during gestation and lactation illustrate the sensitivity of using renal excretion of Ca and P to assess requirements. Based on 24-h collections, urine Ca:P ratios 1.5 were consistent with deficient P intake, ratios from 0.5 to 1.5 reflected P adequacy, and ratios 0.5 indicated excessive P intake. As a practical method, spot urine samples collected at variable times throughout the day reliably predicted Ca:P ratios in 24-h urine collections. Thus, a simplified approach using Ca to P ratio in spot urine samples can be used to assess P intake adequacy. This practical approach can be applied in commercial sow herd to allow herd-based assessment of P status. In conclusion, within herd assessments, based on urine Ca:P ratios, can be used to determine if P supplements are deficient, adequate, and excessive. As such, significant progress can be made to provide a balance between the need to adequately meet animal need with efforts to minimize negative environmental impacts from over-supplementation of P.