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Phytase is commonly supplemented in laying hen diets to increase the bioavailability of P and Ca. Densitometry of each specific bone fraction and assessment of the circulating concentrations of bone biochemical markers can help to understand the effects of dietary phytase on bone biology. Little information is available on the effects of dietary Ca and avP levels and phytase supplementation on bone metabolism in laying hens. The objective of this study was to evaluate the effects of moderate and severe reductions of dietary Ca and avP and phytase supplementation on bone densitometry; serum osteocalcin, pyridinoline, PTH, Ca, and P concentrations.
One hundred H&N Nick Chick Single-comb White Leghorn hens were obtained at 68 wk of age (woa) from a flock of healthy laying hens at the Poultry Research Centre of the University of Alberta. The hens were housed individually and fed a typical commercial laying hen diet adequate in avP and Ca from 66 to 68 woa. From that group, 100 hens with at least 93% egg production during the 2-wk pre-experimental period were selected for the trial. Each of 5 dietary treatments was randomly assigned to 20 individually housed hens from 68 to 78 woa. The mash diets included a positive control (PC) with 0.35% avP and 3.5% Ca, and the PC with avP and Ca levels moderately reduced by 0.187 and 0.159% of the diet, respectively (NC1; 53 and 4.5% less than the PC, respectively) or severely reduced by 0.231 and 0.275% of the diet, respectively (NC2; 66 and 7.9% less than the PC, respectively). Other diets were the NC1 or NC2 diets supplemented with Buttiauxella sp. phytase (BSP) at 600 FTU/kg (NC1 + BSP or NC2 + BSP, respectively). Egg production was assessed daily. On a 2-wk basis, individual hen BW and average daily feed intake were determined. At 78 woa, all hens were euthanized, and the left femurs were excised for subsequent bone densitometry assessment using QCT.
From 68 to 71 woa, egg production was not affected by dietary treatment. Egg production was not different between PC, NC1, and NC1 + BSP hens except at 77 woa, when egg production by the NC1 and NC2 hens was increased by BSP supplementation. However, the NC2 diet reduced egg production relative to all other treatments from 72 to 78 woa (P < 0.001). Hen BW was not affected by diet at the start of the experiment (68 woa) nor at 70 woa (P = 0.001). Eggshell breaking strength was maintained by NC1 and NC2 relative to PC and the respective BSP-supplemented diets at all ages except at 70, 76, and 78 woa, however, was higher in NC2 + BSP than NC2 at 68 woa (P = 0.001). At each of 70, 76, and 78 woa, eggshell breaking strength was decreased by the NC2 relative to the PC and NC2 + BSP treatments. Egg specific gravity was not different between diets at 68 and 74 woa but was lower for NC2 than in PC and NC2 + BSP at 70, 76, and 78 woa (P = 0.009). At 72 woa, NC2 had lower specific gravity than PC but was intermediate in NC2 + BSP. There was no diet effect on serum osteocalcin, PTH, P, or Ca concentrations. Proximal metaphysis and mid-diaphysis total BMD and mid-diaphysis trabecular space BMD were each lower (P ≤ 0.002, 0.004, and 0.001, respectively) for NC1 and NC2 than PC, NC1 + BSP, and NC2 + BSP. Proximal metaphysis trabecular space BMD was decreased by NC1 and NC2 relative to PC, but supplemental BSP completely alleviated the adverse effect of dietary mineral reductions in the NC1 diet only (P = 0.002).
Phytase in the diet of laying hens may allow for further reductions in avP and Ca levels beyond those in the NC1 diet to decrease feed costs relative to the current practice, with no adverse effects on performance and bone health of the birds.