1α-Hydroxylated Cholecalciferol Compounds Act Additively with Microbial Phytase to Improve Phosphorus, Zinc and Manganese Utilization in Chicks Fed Soy-Based Diets1

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Abstract
Seven experiments were conducted to determine the efficacy of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] and microbial phytase in improving P, Zn and Mn utilization of chicks fed P, Zn- and Mn- deficient soy protein diets containing surfeit levels of cholecalciferol. Efficacy of 1α-hydroxycholecalciferol (1α-OH D3) was also studied. A dose titration study indicated that supplemental phytase at 1200 units/kg diet would increase bone ash by at least 65% when added to a corn-soybean meal diet containing 0.43 g P/100 g (0.1 g nonphytate P/100 g). These responses were similar to those obtained from supplemental P (0.1 g/100 g) as KH2PO4 or from added 1,25-(OH)2D3 (10 µg/kg). Dietary addition of both 1200 units phytase and 10 µg/kg 1,25-(OH)2D3 elicited bone ash responses that were near 100%. When chicks were fed a Zn-deficient soy-concentrate diet (13 mg Zn/kg), diet supplementation with 1,25-(OH)2D3 or phytase increased growth rate by 40% and tibia Zn content by > 100%; adding 1,25-(OH)2D3 together with phytase increased tibia Zn content by 160%. Utilization of both Zn and Mn contained in the corn-soybean meal diet also was markedly enhanced by supplemental phytase, 1,25-(OH)2D3, or the combination. The cholecalciferol analog 1α-OH D3 was found to improve dietary P utilization maximally (70% bone ash response) at a dose of 20 µg/kg diet, and effects were additive when 1α-OH D3 was fed in the presence of phytase. We hypothesize that 1α-OH D3 is absorbed and transported to the liver where it is converted to 1,25-(OH)2D3, after which a portion is transported back to the gut where it stimulates intestinal phytase and activates intestinal P absorption. Use of 1α-OH D3 as a feed additive for animals, with or without phytase supplementation, has great potential to materially lower necessary levels of P (also Zn and Mn) supplementation and, in so doing, to reduce P pollution caused by excessive levels of P in animal waste products.

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