Relative availability of iron in mined humic substances for weanling pigs

Humic substances include several biological active and inactive compounds that are commonly used for improving soil fertility. Use of humic substances in swine diets is a novel concept. Humic substances contain 8,700 mg/kg of iron but its bioavailability is unknown. This study was conducted to test the bioavailability of iron in humic substances for nursery pigs. One hundred twenty five pigs (Newsham, Colorado Springs, CO) were not given supplemental iron while nursing for 21 d. Pigs were weaned on d 21 and allotted to one of five treatments (four control treatments with different levels of supplemented iron; 0, 30, 70 and 88 mg/kg from FeSO₄ and one treatment with 70 mg/kg iron from humic substances). Pigs were fed diets for 5 wk ad libitum and water was accessible freely. Body weight and feed intake were measured weekly. Blood samples were taken from pigs on d 28 to measure the number of red blood cells and hemoglobin concentration. Pigs fed a diet with the humic substances grew faster (p<0.05) during the first week postweaning, but performance was not different during the entire 5 wk period. Feed intake and gain/feed were the same among treatments. The slope ratio technique was used to estimate relative iron bioavailability. The concentration of blood hemoglobin did not respond to dietary iron levels using this model. However, the number of red blood cells (106/μl) was modeled by 4.438+0.017x 'iron (mg/kg) from FeSO ₄'+0.012x'iron (mg/kg) from the humic substances'. Based on the comparison between the slopes (0.012 from humic substances and 0.017 from FeSO₄), iron in humic substances was 71% as available as the iron in FeSO₄. The slopes for dietary feed intake of FeSO₄ and the iron in humic substances did not differ (p>0.05). Humic substances can replace FeSO₄ as an alternative iron source for pigs at 71% relative bioavailability.