Ferrous Glycinate (Fe-Gly) Supplementation Improves Growth Performance by Modulating Intestinal Immunity and Microbiota in Weaned Piglets

Iron is an essential nutrient for piglets, but iron sources vary greatly in bioavailability, and their effects on intestinal health remain unclear. In this study, 21-day-old weaned piglets were used to compare the effects of different iron sources (ferrous sulfate (FeSO4), ferric ammonium citrate (FAC), and ferrous glycinate (Fe-Gly)) on growth performance, intestinal inflammation, and gut microbiota. Compared to the FeSO4 group, the Fe-Gly group significantly increased the body weight of piglets at 35 days (p < 0.05), promoted the average daily feed intake (ADFI) and average daily gain (ADG) of piglets from day 21 to 35 (p < 0.01), and also markedly reduced the diarrhea rate of piglets (p < 0.01). Meanwhile, although FAC increased growth performance-related indicators (ADG, ADFI) in piglets, there was no significant statistical difference compared with FeSO4 (p > 0.10). Moreover, Fe-Gly supplementation significantly elevated serum iron levels and total iron-binding capacity (p < 0.01), while significantly reducing the iron content in colonic chyme (p < 0.0001). Both the Fe-Gly and FAC significantly improved the anti-inflammatory and antioxidant capacities of the piglets (p < 0.01). Interestingly, Fe-Gly primarily increased the abundance of Lactobacillus, thereby reducing the abundance of harmful bacteria such as Escherichia coli. Functional prediction using PICRUSt2 revealed that Fe-Gly supplementation tended to elevate the relative abundance of gut bacteria capable of carbohydrate metabolism and amino acid synthesis. In conclusion, this study demonstrated that dietary Fe-Gly supplementation improved systemic iron status, effectively reduce residual iron in the intestine, inhibit the proliferation of pathogenic bacteria in the gut, promote the growth performance and intestinal health of piglets, and reduce the diarrhea rate.