Mineral Composition of Calcium Sources used by the Australian Poultry Feed Industry

Limestone is composed primarily of CaCO₃ and is commonly used as a source of calcium (Ca) in poultry diets. Pure CaCO₃ has a molecular weight of approximately 100g/mole and so is around 400 g/kg Ca. However, CaCO₃ sources used in animal feeding are typically only 370-380 g/kg Ca, sometimes less, due to the presence of other minerals such as Mg or Fe. Furthermore, the bioavailability of Ca to the bird is affected by many factors including the source of Ca, solubility, gut retention time, particle size as well as the presence of other minerals (Oso et al., 2011). A paucity of information detailing the mineral composition of commonly used Ca sources is available to the Australian poultry industry. This paper reports on the determined mineral composition of 15 samples of Ca sources commonly used in the Australian poultry feed manufacturing industry. 

Fourteen samples of limestone (CaCO₃) in the form of powder (N=13) and grit (N=1) were obtained from various feed mills and distributors throughout Australia. Samples were wet acid digested using nitric acid and hydrogen peroxide (Peters et al., 2003) prior to the determination of Ca, P, Mg, Fe, Cu, K, Mn, Na, Sr and Zn concentration by Inductively Coupled Plasma-Optical Emission Spectroscopy using a Perkin Elmer OPTIMA 7300 (Perkin Elmer Inc, Waltham, MA, USA). All samples were analysed as received in triplicate. Results are sample means presented as g/kg for Ca and Mg and as mg/kg for Fe, K, Mn, Na, Sr and Zn. The concentration of P is reported as mg/kg for all samples except those reported as g/kg. 

The results for the determined mineral composition are shown in Table 1. For all samples, the concentration of Cu was below detectable limits and is not reported. All samples contained higher than expected Ca concentrations and when combined with the micro-mineral results this suggests that the samples were not pure CaCO₃. Calcium concentrations ranged from 410.8 g/kg to 392.2 g/kg and are broadly in keeping with those found by Reid and Weber (1976).

Sample A (305.3 mg/kg) contained more P than all the other limestone samples and this was approximately 5 times greater than the amount of P contained in sample K (56.5 mg/kg). Concentrations of Fe differed widely between the highest (sample D, 1506 mg/kg) and the lowest (sample N, 109 mg/kg). It is possible that the source of Fe in some of the samples may be attributed to purity of the limestone sample as well as contamination from metal fatigue in mining equipment that was used during the quarrying process.

Greater concentrations of Na and Sr were present in sample C when compared to all of the other samples while increased K was found in samples C and D. Concentrations of Mn varied by 237 mg/kg when comparing sample H (253 mg/kg) to sample O (16 mg/kg). Concentrations of Mg were found to range from approximately 1.0 g/kg to 5 g/kg. Corn-soy diets typically contain 1 to 5g of Mg and are generally below the toxicity limits of birds. However, contamination of mineral sources with Mg and/or the use of dolomitic limestone has been shown to increase diet Mg concentration and result in wet litter in broilers and reduced egg shell quality in layers (Leeson and Summers, 2001). The quality of limestone may become more pertinent for layers where higher concentrations of limestone are used. Zinc concentrations displayed large variation between the lowest (sample D, 0.57 mg/kg) and the highest (sample N, 19 mg/kg).

A further consideration when formulating diets with varying limestone purity is the chelating capacity of phytate and the resultant reduction in phytase efficacy. Phytate is able to chelate mineral ions including Ca, Zn, Fe, Mg, Mn and Cu (Tamim and Angel, 2003). However, the solubility and stability of these phytate-mineral complexes are pH dependent with most being relatively soluble below pH 3.5 while maximal insolubility occurs between pH 4 and 7 (Selle et al., 2000). Importantly, the capacity of these minerals to inhibit phytate- P hydrolysis by phytase varies as Zn >> Fe > Mn > Fe > Ca > Mg (Maenz et al., 1999; Angel et al., 2002). In vitro work by Tamim and Angel (2003) reported that phytate phosphorus hydrolysis was reduced with the addition of micro-minerals when compared to no minerals being added but no effect was found in vivo. The authors postulated that the reason for the discrepancy in results may relate to the concentration of minerals used in the in vivo study not being sufficient to elicit a response.

Though the overall concentrations of minerals other than Ca within the limestone sources were relatively low, their presence may be important when formulating diets. For example, a diet formulated with unknown mineral composition of limestone may result in higher concentrations of micro-minerals than expected. This would be more important when samples have higher mineral concentrations than those with lower mineral concentrations. It may also be that the potential effects of the limestone mineral composition may be additive to the mineral composition of the water used on farm and these need to be considered together.

The work reported here suggests that the mineral composition of limestone is highly variable between sources and it is plausible that further variation exists between other sources that have yet to be analysed. Further investigation has been proposed to quantify these differences. Based on the results of this study it may be prudent to conduct regular analysis of limestone samples for mineral composition as a part of regular quality control practices. 

The authors gratefully acknowledge Ruth Consolidated Industries for the provision of limestone samples. 

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