The impact of reducing the levels of Ca and P on broiler performance, Ca and P digestibility and bone parameters

The release of P into the environment is a target that must be achieved not only for economic and environmental reasons, but also to increase the sustainability of animal production. The impact of dietary Ca on P digestibility, and consequently the level of the inclusion of inorganic P sources in the feed, plays an important role by hindering the phytase to hydrolyse phytate-P (Sommerfeld et al., 2018). The aim of this trial was to investigate the impact of lowering the inclusion of limestone (and consequently Ca) combined with lowering P on broiler performance, Ca and P digestibility and bone quality.

Eighteen pens with 24 male Ross 308 male per pen were fed a feed containing an intrinsically heat stable 6-phytase (OptiPhos® Plus) at 1000 FTU/kg (only considering a 1.76 g/kg available P (aP) matrix value) and an endo-1,4- beta-xylanase (Hostazym® X) (on top; 1500 EPU/kg) and were split over 2 treatments. Birds of one treatment were fed a starter feed (day 1-10) containing 0.85 % Ca and 0.45 % aP, a grower feed (day 10-21) containing 0.70 % Ca and 0.36 % aP and a finisher feed (day 21-35) containing 0.60 % Ca and 0.30 % aP. Birds of the other treatment were fed the same feed except with Ca and aP lowered to 0.65 and 0.40 %, 0.50 and 0.31 % and 0.40 and 0.29 % in starter, grower and finisher respectively. As a result, the grower and finisher feed did not contain any added inorganic P. Technical performance was measured for every feeding phase. At day 21, the tibiae from 3 birds per pen were removed and analysed for bone strength. Afterwards, the 3 tibiae were pooled into 1 sample for the determination of ash after fat extraction and drying. At day 35, a mixed faecal sample was taken from 5 birds per pen for the determination of the total tract digestibility of Ca and P (TiO2 as marker).

The birds outperformed the breed standard (close to 2.7 kg bird weight at 35 days with an FCR of 1.4). Lowering the Ca and P level in the feed did not have a negative effect on performance. On the contrary, it increased end weight by 22 g numerically (from 2675 to 2697 g, P > 0.05). However, the bone ash decreased slightly from 47.5 to 46.8 %, while bone strength decreased from 548 to 532 N (in both cases numerically). On the contrary, lowering the Ca and aP levels in the feed significantly increased the Ca digestibility (51.7 to 66.4 %) and P digestibility (77.3 to 83.2 %). Based on feed intake and analysed Ca and P levels in the feed, this yielded a significantly lower digestible Ca uptake (0.49 vs 0.62 g per bird per day) but increased the digestible P intake significantly from 0.53 to 0.57 g per bird per day. This trial demonstrated that lowering the Ca and P levels in the feed with 0.2 % and 0.05 % in starter, grower and finisher which contains 1000 FTU/kg phytase does not have a negative impact on performance, increases Ca and P digestibility, and has a limited impact on bone ash and bone strength. However, it indicates that there are different Ca and P requirements for technical performance and for bone development as suggested by David et al. (2023).