Effect of second iron injection on growth performance, hematological parameters, and fecal microbiome of piglets fed different dietary iron levels

This experiment was conducted to evaluate the effects of a second iron injection for suckling pigs fed diets with different dietary iron levels in the nursery period on growth performance, hematological parameters, serum and liver trace mineral content, fecal score, microbiome, and metabolites. A total of 70 newborn pigs from 7 litters were assigned to either 1 or 2 iron injections within the litter and received the first i.m. iron injection (200 mg) at 2 to 3 d of age. Pigs assigned to the second injection treatment received an additional iron injection 5 d after the first injection. At weaning (days 27 to 30 of age), pigs within iron injection treatments were divided into 2 nursery diet treatments for a 27-d growth period. Treatments were 1) no additional iron injection + nursery diets with 100 ppm iron (NC), 2) second i.m. iron injection (200 mg) + NC diets, 3) no additional iron injection + nursery diets with 200 ppm iron (PC), and 4) second i.m. iron injection (200 mg) + PC diets. The second iron injection increased liver iron content at weaning (P = 0.08, tendency), and serum iron, hemoglobin, and hematocrit levels until day 13 postweaning (P < 0.05). In the nursery period, pigs receiving the second iron injection had a greater final body weight (P = 0.08, tendency), overall growth rate (P = 0.08, tendency) and feed intake (P < 0.05), and lower fecal score (P < 0.05) indicating firmer feces compared to those receiving 1 iron injection. There was no major effect of dietary iron level or interaction with the iron injection treatment in any measurements except that the pigs fed the PC diets had greater hemoglobin and hematocrit levels (P < 0.05) at day 27 postweaning and a lower fecal score (P = 0.08, tendency) in the late nursery period than those fed the NC diets. The second iron injection reduced fecal bacterial alpha-diversity based on Faith's phylogenetic diversity at weaning (P < 0.05), while the second iron injection and dietary iron levels resulted in dissimilarity in the fecal bacterial community based on Unweighted Unifrac analysis (P < 0.05; at weaning by iron injection and day 27 postweaning by dietary iron level). In conclusion, the second iron injection for suckling pigs improved postweaning growth performance and hemoglobin levels and affected the fecal microbiome, whereas an additional 100 ppm of dietary iron supplementation increased hemoglobin levels and altered the fecal microbiome in the late nursery period but did not affect postweaning growth.