Abstract
The hypothesis that dietary inclusion of microbial phytase improves apparent calcium (Ca) digestibility thereby allowing a lower dietary Ca inclusion without compromising growth performance was tested. One-day-old male Ross 308 broilers (25 birds/pen, 9 pens/treatment) were assigned to 8 experimental diets containing one of 4 dietary Ca to retainable P (rP) ratios (1.3, 1.8, 2.3, and 2.8) with (1,000 FTU/kg) or without microbial phytase. On d 21 to 23, digesta from different intestinal segments of 8 birds per pen were collected to determine apparent Ca and P digestibility. Mid duodenal mucosa was collected for expression of Ca (CaBP-D28k, PMCA1) and P (NaPi-IIb, PiT-1, PiT-2, and XPR1) transporters by RT-qPCR. Dietary phytase inclusion in low Ca/rP diets increased Ca digestibility in the distal ileum (Pinteraction = 0.023) but not the proximal or distal jejunum. Broilers receiving the lowest Ca/rP displayed the lowest body weight gain, highest feed conversion ratio (P < 0.001), and lowest tibia strength, irrespective of dietary phytase inclusion. Incremental dietary Ca/rP linearly reduced P digestibility to a greater extent in the absence of phytase in the distal jejunum and ileum (Pinteraction = 0.021 and 0.001, respectively). Incremental dietary Ca/rP linearly reduced serum P more in phytase-free diets (Pinteraction < 0.001), and lowered duodenal expression of P transporters NaPi-IIb, PiT-2, and XPR1 (P = 0.052, 0.071 and 0.028, respectively). Incremental dietary Ca/rP linearly increased (P < 0.001) serum Ca irrespective of phytase inclusion, accompanied by a lower (P < 0.001) duodenal expression of Ca transporters CaSR, CaBP-D28k and PMCA1 and Ca-pore forming claudins CLDN-2 and CLDN-12. Dietary phytase increased (P = 0.026) NaPi-IIb but reduced (P = 0.029) CLDN-2 expression. Incremental Ca/rP reduced Ca and P digestibility, increased serum Ca, lowered serum P and inhibited mRNA levels of Ca and P-related transporters, indicating that these transporters and CLDN contribute to the observed effect of dietary Ca and phytase on Ca and P absorption. Despite the improvement in Ca digestibility, dietary phytase did not restore the compromised growth performance and tibia strength of broilers fed a Ca-deficient diet, leading to rejection of the hypothesis.
Highlights
Phosphorus (P) is the third most expensive ingredient in farm animal diets (Woyengo and Nyachoti, 2011) and plays an important role in many biological processes such as muscle contraction, energy production, cell signaling, and bone formation (Chang and Anderson, 2017).Ó 2021 The Authors
Dietary Ca content quadratically affected body weight gain (BWG), feed intake (FI), and feed conversion ratio (FCR) (P < 0.001, 0.004 and < 0.001, respectively), with the maximum values of BWG and FI being observed in broilers fed a dietary Ca/retainable P (rP) of 1.8
No significant Ca to retainable P ratios (Ca/rP) £ phytase interactions for BWG, FI, and FCR were observed during the entire experiment (d 0−36)
Summary
Phosphorus (P) is the third most expensive ingredient in farm animal diets (Woyengo and Nyachoti, 2011) and plays an important role in many biological processes such as muscle contraction, energy production, cell signaling, and bone formation (Chang and Anderson, 2017).Ó 2021 The Authors. Inositol phosphate bound P (IP-P) is the major form of P storage (60−80%) in most cereal grains and oil seeds (Rodehutscord et al, 2016). This IP-P is poorly digestible in non-ruminant animals such as pigs and broilers. It is generally accepted that reduction of dietary calcium (Ca) content improves IP degradation and IP-P digestion (Sommerfeld et al, 2019). Over-reduction of dietary Ca content may compromise growth performance and tibia breaking strength (Hu et al, 2020) because a minimum supply of dietary Ca is required to suffice post-absorption P utilization and bone development. Improving dietary Ca digestibility seems to be a promising approach to reduce dietary Ca inclusion
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