Summary of the Standardised Ileal Digestible Calcium Requirements of Broilers from Hatch to Day 42 Post-Hatch when Considering Breed and Gender

I. INTRODUCTION

Calcium (Ca) is an important nutrient for skeletal development, muscle contraction, nerve impulses, acid-base balance, and a cofactor for blood clotting and endogenous enzymes. Due to the involvement of Ca in numerous functions in the body, accurate supply of dietary Ca is essential for optimal growth, feed efficiency and nutrient utilisation. Oversupply of dietary Ca or an imbalance between dietary Ca and phosphorus (P) has been linked to significant reductions in growth performance and nutrient digestibility (Amerah et al., 2014; Mutucumarana et al., 2014). Accurate supplementation of both dietary Ca and P may provide further improvements in growth and efficiency as well as responsible use of resources and reduce nutrient excretion into the environment.

Recently, a series of studies were conducted to estimate the standardised ileal digestibility (SID) or apparent ileal digestibility (AID) of Ca in various feed ingredients for broilers (as reviewed by Walk et al., 2021a). With this information it is now possible to formulate broiler diets using SID Ca coefficients in feed ingredients. Over the last two years studies have been conducted to estimate the SID Ca requirements of broilers from hatch to day 42 post-hatch (David et al., 2021, 2022; Walk et al., 2021b, 2022a, 2022b). The objective of this work was to combine the results from studies designed to estimate the SID Ca requirements of broilers and provide an updated SID Ca requirement for the starter, grower, and finisher phases of broiler production.

II. METHOD

A series of studies (n = 12) were conducted between 2020 and 2022 to determine the SID Ca requirement of broilers from hatch to day 42 post-hatch. In all studies, four to six graded concentrations of SID Ca were fed (Table 1) to broilers using P-adequate (77%) diets or diets formulated with graded levels of SID P (23%). The studies were conducted from hatch to day 10/17 (starter, n = 54), day 11/17 to 21/31 (grower, n = 57), or day 22/32 to 42/44 (finisher, n = 32). This resulted in a total of 143 data points in the experimental model.

Data from all studies were combined into one excel file. This included an arbitrary study number, animal husbandry and diet information, experimental treatments, formulated and analysed nutrient concentrations, particularly total Ca and P, and the mean results from growth performance, AID of Ca and P, tibia ash percent and weight, and apparent total tract retention (ATTR) of Ca and P. The analysed total Ca concentration in the experimental diets was used to estimate the SID Ca within each experiment. The studies included genetics from Ross 308 (74%), Arbor Acres Plus (11%), or Cobb 500 (15%) broilers and used predominantly males (85%) or straight-run birds.

Table 1 - Experimental design, mean, minimum, and maximum value of key variables.

Data were analysed using JMP Pro v. 16. Prior to statistical analyses, data were checked for outliers using the explore outliers platform. Means were analysed using the fit model platform and separated by feeding phase. The full statistical model included the linear and quadratic effects of SID Ca (as a continuous variable), broiler breed, and the interactions. Nonsignificant model effects, that were not part of a significant interaction, were removed and the parameter estimates recalculated using the reduced model. The SID Ca requirements were determined using the maximum response values from the linear or quadratic equations. Significance was accepted at P < 0.05 and trends discussed at P < 0.10.

III. RESULTS

The formulated and analyzed total Ca concentration in the experimental diets was in good agreement (R2 = 0.91) and this meant the formulated and estimated SID Ca concentrations were in good agreement as well (R2 = 0.90). There was a significant effect of trial site on all response variables, and it accounted for a large proportion of the variation in the statistical model. Therefore, the statistical model was updated to include location (random variable) nested within broiler breed.

During the starter, grower and finisher phases, there was no significant (P > 0.10) SID Ca × breed effect on body weight gain, feed intake, feed conversion ratio (FCR), tibia ash weight or percent, ATTR of P, or apparent ileal digested P. However, there was a main effect of breed on almost all parameters measured. This means there are differences in performance, bone ash, and P digestibility or P retention among the broiler breeds tested, but the estimated SID Ca requirement is the same, regardless of the broiler breed. However, the apparent ileal digested Ca or ATTR of Ca was influenced by a significant SID Ca × breed interaction. In the starter phase, the apparent ileal digested Ca was greater in Ross 308 birds compared with Arbor Acres birds and increased to a greater extent (linear, P < 0.05) in Ross 308 birds (38 vs 45%) as the SID Ca concentration in the diet increased (SID Ca × breed, P < 0.05). In the grower phase, the ATTR of Ca was greater in Arbor Acres birds and decreased (linear, P < 0.05) to a greater extent compared with Ross 308 birds (SID Ca × breed, P < 0.05).

The linear and quadratic effect of graded levels of SID Ca within each feeding phase are presented in Table 2. The estimated SID Ca requirement, based on growth performance, apparent ileal digested Ca or P, tibia ash percent and weight, and ATTR of Ca or P, within each feeding phase, is presented in Table 3. There were limited effects of the graded levels of SID Ca during the finisher phase and therefore the estimated SID Ca requirement was not determined (data not shown).

Table 2 - Summary of the statistical analyses (P-values) used to estimate the standardized ileal digestible Ca requirements of broilers from hatch to day 42 post-hatch.

IV. DISCUSSION

The results from the current meta-analysis were highly variable and significantly influenced by study location and broiler breed. These factors were included in the statistical model to account for variation between the studies, but this may have resulted in an over-fit model and reduced the applicability of these results outside of this dataset. The impact of breed or lack of a significant interaction between the SID Ca requirement and breed also requires careful consideration as the majority (74%) of the birds in the dataset were of Ross 308 genetics. Additional data will improve the model and predictability in the future.

The influence of dietary Ca on growth performance and bone ash appears less consistent compared with the impact of dietary P. This is reflected in the lower R2 in the published SID Ca studies (David et al., 2021, 2022; Walk et al., 2021b, 2022a, 2022b) compared with those evaluating non-phytate P responses on growth or tibia ash (Vieira et al., 2015; Yi et al., 1996). This phenomenon is most likely due to the birds’ ability to adapt to excess or deficiencies of dietary Ca, especially in long-term performance studies, through hormonal regulation of calcitonin, parathyroid hormone, and vitamin D via the kidneys and the bones (Shafey, 1993). The combination of these factors and the birds’ own ability to adapt to dietary Ca may have all contributed to the variation noted in the meta-analysis. This was particularly true for the finisher phase, in which very few response variables were influenced by the graded levels of SID Ca in the diets; and this limited the ability to estimate the SID Ca requirement using the meta-analysis.

However, for the starter or grower phase, body weight gain, FCR, or tibia ash were responsive to the graded levels of SID Ca in the diets. In this regard, the SID Ca requirement was estimated using the maximum response from quadratic regressions. The quadratic response may over-estimate the requirement compared with a broken-line or a quadratic broken-line (Walk et al., 2022a), but is a good model to estimate toxic or negative effects of both an over- or an under- supply of dietary nutrients (Pesti et al., 2009). In conclusion, a meta-analysis of the SID Ca requirement studies was conducted and the estimated SID Ca requirement for broilers was 0.488 or 0.452% in the starter or grower phase, respectively.

Table 3 - Summary of the estimated standardized ileal digestible (SID) Ca requirements of broilers from hatch to day 42 post-hatch using the maximum response from quadratic regressions.