Super-dosing phytase improves the growth performance of weaner pigs fed a low iron diet

This experiment was conducted to test the hypothesis that a super-dose of phytase would improve the performance of weaner pigs fed an Fe-deficient diet, through an increase in phytate bound-Fe bioavailability. A total of 234 pigs (initial BW 7.6 + SE 0.16 kg) were weaned at ∼28 d of age and blocked into mixed sex pens of 4 or 5 balancing for weight, sex and litter of origin. Treatments were arranged as a 3 × 2 factorial with 3 levels of phytase (0, 500 or 2500 FTU/kg feed) and 2 levels of Fe (50 [low; L-Fe] or 300 [high; H-Fe] mg/kg feed as FeSO₄) supplemented to an Fe-deficient wheat-SBM based diet. Diets were offered to 8 replicate pens for 20 d post-weaning. At the end of the experiment, 1 pig per pen was euthanized for the collection of blood, plasma, liver and ileal digesta. Pigs fed the L-Fe diet had a lower ADFI (P < 0.01) than those fed the H-Fe diet. Phytase at 2500 FTU/kg improved the ADG of L-Fe fed pigs to a level comparable with the H-Fe fed pigs; however, its effect was diminished when added to the H-Fe diet, resulting in a significant Fe x phytase interaction for ADG (P < 0.05). Reducing the dietary Fe level resulted in reductions in haemoglobin (P < 0.10), haematocrit (Hct; P < 0.001), plasma Fe and ferritin (P < 0.05), and liver Fe concentration (P < 0.001). Moreover, plasma transferrin was higher (P < 0.05) in L- Fe fed pigs. Supplementary phytase at 2500 FTU/kg tended to increase Hct concentration (P < 0.10) and numerically increased plasma Fe and ferritin but had no influence on other measured indices of Fe status. There was an Fe x phytase interaction for ileal InsP₆ degradation (P < 0.05), which mirrored that observed for ADG: phytase increased InsP₆ hydrolysis to a greater extent when added to the L-Fe than the H-Fe diet (68.2 vs 30.8%). These data demonstrate that high doses of phytase can effectively improve the performance of weaner pigs fed an Fe-deficient diet. The benefit of using high doses of phytase in diets rich in Fe is seemingly reduced, likely as a result of the ability of Fe to degrade phytase efficacy