Novel supplemental guidelines for Zn, Cu, and Mn, in bovines, by integrating net requirements, native dietary occurrence, and the boundaries of homeostatic regulation

A long-standing objective in ruminant nutrition is to ensure an adequate supply of the key trace metals Zn, Cu, and Mn, while accounting for bioavailability, nutritional interactions, and not exceeding tolerance. In practice, however, dietary supplementation often by far exceeds reference nutritional guidelines, largely as a precaution against the multiple uncertainties of trace mineral supply. Current reference recommendations are determined using fixed and highly conservative uptake coefficients, an approach that overlooks both the opportunities and limitations of absorptive regulation, the primary homeostatic mechanism controlling trace metal balance. While generous supplementation should mitigate deficiency risks, it also introduces the less recognized risk of exceeding tolerable availability. Recent complete balance studies demonstrate that homeostatic downregulation of trace metals can be overwhelmed at dietary levels commonly observed in practice, underscoring the need for a new framework to define supplementation. We therefore developed an approach that integrates 3 elements: (1) stochastic analysis of basal dietary supply; (2) stochastic analysis of net nutrient requirements; and (3) probabilistic assessment of the likelihood that dietary supply falls outside the lower or upper boundaries of homeostatic regulation for these metals in cattle. This last element addresses the main limitation of current dietary guidelines by considering the dynamic range of absorptive regulation: maximum uptake rates achievable during upregulation and minimum uptake rates attainable during downregulation, instead of assuming fixed absorption coefficients. As a result, this framework defines ranges of adequate dietary supply rather than a single minimum requirement. The probability distribution of nutrient supply was derived from variation in dry matter intake, diet composition, and trace mineral occurrence in feedstuffs, whereas the distribution of net trace metal requirements was defined by the variability of the components of a classic factorial requirement approach applied to different animal conditions. These distributions were then compared against earlier defined boundaries of homeostatic regulation. This risk-based approach supports preventive supplementation of Cu, Zn, and Mn for dairy cattle across most physiological and productive states. At the same time, it highlights the risk to exceed tolerance thresholds under current supplementation practices, even at levels commonly considered safe. Overall, this analysis introduces novel reference guidelines to trace metal nutrition, demonstrating the opportunity to improve adequacy and reduce risk by reevaluating conventional supplementation strategies.