Lowering dietary calcium and available phosphorus in broilers

L.B. Linares, S.M. Carroll, M.A. Silva, J.T. Halley and C. Fisher

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The aim of this trial was to investigate the effect of lowering dietary calcium (Ca) and available phosphorus (av.P) during Starter (0 to 10d), Grower (11 to 24d) and Finisher (25 to 45d) phases on performance of 4416 Ross-308 male broilers. Diets were wheat-soybean meal, corn gluten meal based, and no phytase added.
Birds received one of two dietary Starter treatments: 1.13% Ca; 0.50% av.P or 1.03% Ca; 0.45% av.P. At 10 days, birds were assigned to a Grower/Finisher treatment as part of a central composite design (CCD). The CCD used a combination of one of five dietary Grower treatments (0.64% to 1.04% Ca; 2.2:1 Ca:av.P) with one of five Finisher treatments (0.58% to 0.99% Ca; 2.2:1 Ca:av.P) creating 12 runs (8 treatments + 4 centre points) with 8 replicates per run arranged according to each of the Starter treatments.
Daily 23-hrs light was provided up to day 7, and 18-hrs light until depletion. Birds were brooded at 32°C, with temperature gradually reduced to 20°C at 27d, and remained at this level until depletion. Liveweight (LW) and mortality adjusted FCR (FCRadj) were recorded on days 10, 24, 32, 39 and 45. For lixiscope (x-ray) bone evaluation, five and four average weight live birds/pen were selected on days 26 and 40, respectively.

These birds were also evaluated for any visual leg defects, incidence and severity of hock burn and foot pododermatitis (FPD) at both ages. At 39 days, five average weight birds/ pen were processed and carcass characteristics were recorded. At 10 days, there was no significant effect of dietary treatment on LW or livability. The FCRadj at day 10 was significantly improved (P<0.001) by lowering dietary Ca (1.020 vs. 1.009).
There was a significant linear response to increasing dietary Ca in the Grower phase for 24 day LW (mean 1248.1g P<0.01) and FCRadj (mean 1.359; P<0.006). Dietary treatment had no significant effect on lixiscope scores, incidence of leg defects or FPD at day 26.

There was a significant linear response to lowering Ca levels in Grower diets on incidence and severity of hock burn (P<0.01) at 26 days. There was no significant effect of dietary treatment (P>0.05) on 32 day LW (mean 2041.3g).
At 32 days FCRadj (mean 1.498) was improved linearly, by increasing Ca levels during the Grower period only (P<0.013). At 39 days, LW (mean 2803.1g) increased linearly by increasing Ca levels in the Finisher period (P<0.004) and there was a positive Grower x Finisher diet interaction (P<0.003). However, FCRadj (mean 1.594) was unaffected (P>0.05) at this time point. At 39 days there was a positive linear response in 0.04% of eviscerated carcass yield (mean 70.85%; P<0.02), by increasing dietary Ca in Finisher phase. Neither Grower nor Finisher dietary treatment had a significant effect on lixiscope analysis or leg defects at 40 days.
However, there was a linear response to lowering Ca in Grower diets resulting in a reduction in severity of hock burn (P<0.05). At 45d, LW (mean 3528.5g) and FCRadj (mean 1.606) were not affected by the treatments (P>0.05). In conclusion, when using a Starter diet with higher content of calcium it is possible to adopt intermediate dietary levels of Ca in Grower (~0.80%) and Finisher (~0.70%) phases to 45 days without compromising broiler performance or skeletal characteristics.
From the Proceedings of the Australian Poultry Science Symposium