Abstract Although several nutritional tables report Zn requirements for pigs, dietary Zn concentrations that exceed these recommendations are often used to enhance growth performance in nursery pigs. However, excessive Zn supplementation has become a controversial and regulated practice in several countries due to its potential association with antimicrobial resistance at the intestinal level and environmental concerns, as demonstrated in controlled laboratory studies. In practical conditions, however, isolating the negative effects of high dietary Zn concentrations can be challenging because dietary composition, management practices, and environmental factors may interact and influence the response. Also, despite pharmacological concentrations of Zn being typically used only during the first three dietary phases, this period accounts for nearly 50% of total Zn excretion, highlighting the need to develop management and nutritional strategies that balance growth performance with environmental impact during the nursery period. The first step toward achieving this goal is to identify the key factors influencing Zn excretion and to develop reliable tools or models capable of estimating Zn output. Such tools would enable producers and nutritionists to quantify the environmental impact of dietary and management interventions, facilitating more sustainable decision-making in swine production systems. The current review compiled data from 51 published studies to identify factors associated with Zn excretion using a principal component analysis and developed predictive models for fecal Zn excretion utilizing a meta-regression analysis. Because urine Zn excretion was negligible (around 2% of the total Zn excretion) fecal and total Zn excretion did not differ. Therefore, as most available data report fecal Zn excretion, the analysis focused on estimating fecal Zn excretion. the principal component analysis indicated that the first two principal components (PC) accounted for 35.9% and 19.3% of the total database variance, respectively. Principal component 1 was primarily influenced by nutrient intake variables, including fiber, NE, nitrogen, and minerals such as P, Ca, and Fe, as well as body weight and feed intake. Principal component 2 was dominated by Zn-related variables including Zn intake, fecal Zn excretion, the difference between Zn intake and Zn requirements estimates from NRC (2012) and added dietary Zn. Variables such as Mn, Cu, and lactose intake moderately contributed to both components, whereas the use of antibiotics, exogenous enzymes other than phytase, and acidifiers had a low impact on the variation explained by both components. Although the PCA identified several variables that may influence Zn excretion, and their effects could be further evaluated through experiments designed for that purpose, Zn intake was the primary factor driving fecal Zn excretion. This observation was supported by an independent analysis in which 97% of the variation in fecal Zn excretion was explained by Zn intake. This strong relationship was expected because Zn homeostasis is primarily regulated at the intestinal level, where a group of Zn transporters located in the enterocytes control Zn absorption according to the Zn status of the pig. On top of the variables evaluated in the PCA, independent studies have indicated that a time component is involved in Zn excretion. Data suggest that during the first 10 d after weaning, fecal Zn excretion in pigs fed pharmacological concentrations of Zn is similar to that of pigs fed diets without supplemental Zn. This observation suggests that, during this period, pigs are saturating Zn pools in body tissues such as the liver, muscle, and bones, and only after the saturation of the Zn pools fecal Zn excretion is proportional to the Zn intake. Stepwise selection indicated that when pigs were fed at or below their Zn requirement estimate, Zn, P, and Fe intake significantly influenced fecal Zn excretion. However, when Zn intake exceeds the requirement estimate, Zn sources and Zn intake were the most influential variables. Regardless of the scenario, Zn intake was the primary determinant of fecal Zn excretion. The meta-regression analysis indicated that when pigs are fed at or below their Zn requirement, 61% of Zn intake is excreted in feces, increasing to 82% when Zn intake exceeded requirements. In conjunction, these results indicate that strategies to optimize Zn use should focus on reducing dietary Zn levels or implementing nutritional approaches that enhance Zn absorption at the intestinal level, such as dietary acidification, which has been shown to increase Zn apparent total tract digestibility in nursery pigs. In conclusion, while Zn excretion is affected by multiple dietary factors, their contribution under practical conditions is relatively low and Zn intake remains the primary determinant of fecal Zn excretion.
Arroyave et al. (Wed,) studied this question.