Requirement of Digestible Calcium at Different Dietary Concentrations of Digestible Phosphorus for Broiler Finishers (Days 25-35 Posthatch)

Calcium (Ca) and phosphorous (P) are two essential minerals for the skeletal growth and other biological functions in poultry. Maintaining an appropriate balance between Ca and P is necessary as these minerals are closely associated in their absorption and post absorptive utilisation. Because of the recent works on the measurement of digestible Ca in feed ingredients, initiative has been taken to determine the standardised ileal digestible (SID) Ca and SID P requirements in broiler starters (day 1-10 post-hatch; David et al., 2021) and broiler growers (day 11-24 post-hatch; David et al., 2022) for growth, bone mineralisation and mineral utilisation. These results showed that the recommended SID P for the maximum growth performance, bone mineralisation, and Ca and P utilisation of broiler starters and growers were 5.0 and 3.5 g/kg, respectively. At respective SID P, the requirement of SID Ca for the maximum weight gain of broiler starters and growers was 3.32 and 3.05 g/kg, respectively, and for the maximum tibia ash was 4.51 and 3.69 g/kg, respectively. Corresponding SID Ca to SID P ratio that maximised the weight gain in broiler starters and growers was 0.66 and 0.87, respectively, and that maximised the tibia ash was 0.90 and 1.05, respectively. Therefore, the objective of the current follow-up study was to determine the requirements of digestible Ca and digestible P for the maximisation of weight gain and tibia ash in broiler finishers (days 25-35 post-hatch).

The experimental protocol was approved by the Massey University Animal Ethics Committee. Fifteen experimental finisher diets based on maize-soybean meal were formulated in a 5 × 3 factorial arrangement with diets containing five concentrations of digestible Ca and three concentrations of digestible P. Diets were formulated to contain 2.0, 2.5, 3.0, 3.5 and 4.0 g/kg SID Ca (corresponding to 3.7, 4.6, 5.5, 6.4 and 7.3 g/kg total Ca, respectively) and 2.5, 3.5 and 4.5 g/kg SID P (corresponding to 3.4, 4.9 and 6.4 g/kg total P, respectively). A total of 720, day-old male broilers (Ross 308) were fed broiler starter crumbles (4.4 g/kg SID Ca; 5.0 g/kg SID P) from day 1 to 10 and the broiler grower pelleted diet (3.69 g/kg SID Ca; 3.5 g/kg SID P) from day 11 to 24 post-hatch. The concentration of SID Ca for broiler starter and grower diets were based on the previous findings (David et al., 2021; 2022) and the concentration of other nutrients were based on Ross (2019). The concentration of total Ca ranged from 0.47 to 0.94 times the requirement for total Ca (Ross, 2019). All experimental diets were isoenergetic and isonitrogenous. Each diet was separately mixed and pelleted. On day 25, the birds were weighed and allocated (mean ± SD, 1.31 ± 0.02 kg) to 90 grower cages (eight birds per cage). The experimental diets were offered ad libitum to six replicate cages of broilers from day 25 to 35 post-hatch. The birds had free access to water. Body weight was recorded on a cage basis at the start and end of the experimental period and the weight gain was calculated. On day 35, right tibia from six birds per replicate was removed and processed as described by David et al. (2021). The tibia ash concentration was determined using AOAC (2016) procedures. Data were analysed using the General Linear Model (GLM) procedure of SAS (2019), with cage serving as the experimental unit. Two sets of analyses were conducted. First, as a factorial arrangement of treatments examining the effects of dietary concentrations of SID Ca and SID P and their interaction. The effects were considered significant at P < 0.05. Second, if the interaction or main effects were significant, then the estimates for the second-order response surface model were determined using the GLM procedure of SAS (2019) and these estimates were used to calculate the maximum response and the SID Ca concentration required for maximum response.

All birds remained healthy during the experiment. Table 1 and Figure 1 present the body weight gain and the concentrations of tibia ash of birds fed the diets containing different SID Ca and SID P from day 25 to 35 post-hatch. There was an interaction (P < 0.001) between SID Ca and SID P for weight gain. At the lowest SID Ca (2.0 g/kg) concentration, increasing concentration of SID P reduced weight gain. In contrast, increasing SID P concentrations increased the weight at the highest SID Ca concentration (4.0 g/kg), but had no effect on the weight gain at 2.5 and 3.0 g/kg SID Ca concentrations. At 3.5 g/kg SID P concentration, the weight gain was higher (P < 0.05) at 3.5 g/kg SID Ca. Predictions for SID Ca at maximum response was not calculated for weight gain due to the linear Ca response. The findings suggest that the requirement of SID Ca for the weight gain depends on the dietary SID P concentration. Accordingly, the SID Ca concentration of 2.0, 3.5 and 3.0-4.0 g/kg can be recommended for broiler finishers when the dietary SID P is 2.5, 3.5 and 4.5 g/kg, respectively. However, based on the factorial analysis, a concentration of 3.5 g/kg SID Ca and 3.5 g/kg SID P can be recommended for broiler finishers as the weight gain was higher at this combination. The equivalent total Ca (6.4 g/kg) value of the current estimate is lower than the current Ross (2019) Ca recommendation (7.8 g/kg total Ca) for 25-39 day old birds.

Based on factorial analysis, there was no interaction (P > 0.05) between SID Ca and SID P for the tibia ash. Tibia ash increased (P < 0.05) with increasing concentration of SID P. Similar to weight gain, the predictions for SID Ca at maximum response was not calculated for tibia ash due to the linear Ca response and therefore the findings suggest that the requirement of SID Ca for the tibia ash depends on the dietary SID P concentration.

Based on factorial analysis, dietary concentrations of 3.0-3.5 g/kg SID Ca and 3.5 g/kg SID P can be recommended for the optimum tibia ash in broiler finishers. The estimated SID Ca (3.0-3.5 g/kg) requirement for tibia ash is lower than the current Ross (2019) total Ca recommendation for finishers (7.80 g/kg total Ca equivalent to 4.25 g/kg SID Ca) and that reported for broiler starters (4.51 g/kg SID Ca) and growers (3.69 g/kg SID Ca), demonstrating a reduction of 22-33% and 5-19%, in broiler finishers compared to starters and growers, respectively. Unlike our previous studies on broiler starters (David et al., 2021) and growers (David et al., 2022), the digestible Ca requirement for maximum tibia ash was not greater than that for maximum weight gain in broiler finishers, suggesting the low Ca demand for bone development in broiler finishers.

In conclusion, the requirements of SID P for the weight gain and tibia ash of 25-35 day old broilers is 3.5 g/kg. At 3.5 g/kg SID P, weight gain and tibia ash are maximised at 3.5 and 3.0-3.5 g/kg SID Ca concentrations, respectively.