Review: Homeostatic boundaries to dietary Zn, Cu and Mn supply in cattle

Zinc, Cu, and Mn are critical to sustain animal physiology, and therefore, dietary supplementation to domestic cattle is common practice. However, recent evidence rises concerns about current supplementation levels, as they may exceed dietary tolerance for these nutrients, impairing health and productivity. This generous supplementation has been justified to mitigate the uncertainty in basal mineral supply and its availability. Furthermore, current dietary reference recommendations assume conservative and fixed absorption coefficients. This mathematical simplification greatly limits our ability to precisely define supplemental levels. Apparent absorption efficiency, which comprises absorption and endogenous excretions, is the main mechanism animals use as homeostatic adaptation to a variable dietary supply. The first part of this review summarises quantitative evidence illustrating homeostatic competence of cattle to adapt apparent absorption efficiency, demonstrating a much greater than assumed resilience to low dietary supply. Also, adaptations to high dietary supply are described. These homeostatic adaptations are complex and nutrient-specific, but in all cases, they are tightly orchestrated by the animal's trace metal status, sharply increasing nutritional efficiency when status is low and decreasing it under abundant supply. Identifying the lower and upper boundaries of this homeostatic competence serves much better to prevent dietary deficiencies and excess in trace metal supply by supplementation. The second part of this review is therefore dedicated to estimating the lower boundaries of homeostasis defined as the maximal apparent absorption efficiency that can be achieved under constrained dietary availability. Among the three trace minerals evaluated, Cu and Mn supplies appear the most susceptible to nutritional challenge, despite adaptive upregulation of apparent absorption. Finally, the third and last part of the review intends to identify the upper boundaries of dietary trace metal regulation, that is, the supply levels above which homeostasis is overwhelmed resulting in unregulated trace metal retention. Dry matter intake plays a critical role in this risk, specifically in dairy cows. Indeed, except for Zn, trace metal demands for lactation are negligible. Thus, when DM intake increases to support milk production, dietary trace metal intakes increase disproportionally to their requirements, resulting in a risk for unregulated absorption that may not be compensated by endogenous excretion. Therefore, considering dietary trace metal adequacy in terms of daily intake is better than only considering dietary concentrations. Defining lower and upper boundaries for dietary supply allows for a more adequate prevention of risk for deficiency or excess supply.