The impact of limestone source and particle size on the digestibility of dietary calcium and phosphorus in broilers

Abstract

Standard industry practice is to formulate broiler diets to a total calcium (Ca) and available or digestible phosphorous (P). The limitation with this is that inherent characteristics of the limestone source (LS) and particle size (PS) of limestone added to feed may affect phytate P utilisation and dietary Ca and P digestibility. The aim of the experiment was to evaluate the limestones in South Africa in order to give nutritionists a better understanding of the Ca and P digestibility associated with South African limestones. The study included both in vitro and in vivo experiments. Laboratory work was conducted in order to determine the particle size of the limestones and the solubility of the limestone used in the experiment. Limestones were evaluated based on particle size and solubility and compared to one another. To achieve this, two broiler digestibility trials were conducted, both including different limestone sources and two particle sizes. The aim of the first digestibility trial was to determine the effects of LS, limestone PS and the inclusion of phytase on Ca and P digestibility. Digestibility trial 2 aimed at examining the effects of different levels of phytase on Ca and P digestibility associated with limestone at two different particle sizes. In Digestibility Trial 1 two groups of 600 Ross 308 male broilers were fed experimental diets in two sets (blocks) from days 19-21 and 23-25, respectively, in order to determine the effects of LS, PS, and phytase on apparent ileal digestibility (AID) of Ca and P. A randomised block design was applied with a 3x2x2 arrangement of treatments. The twelve treatments included three limestone sources, each with two particle sizes (0.8 mm and commercial) and two phytase levels (0 and 1000 FTU/kg feed). The experiment was repeated twice (block) with five cages per treatment per block. Each block consisted of 100 cages with 6 birds/cage. Digesta from the distal half of ileum was collected, freeze dried, and analyzed for Ca, P, and Cr marker to determine the apparent ileal digestibility (AID) of Ca and P. Data was analysed as a full factorial using Proc Mixed (SAS Institute 2012) with block included as a random effect. Results showed a tendency for AID of Ca to be altered by LS and phytase (P<0.01), with no interaction of LS, PS and phytase. A significant three-way interaction of LS, PS, and phytase was observed for AID of P. In the absence of phytase, a larger limestone PS consistently increased AID of P. With phytase added, the effect of PS on AID of P differed between LS. The increment in AID of P from added phytase was also dependent on the LS (P<0.01). Digestibility Trial 2: The optimal phytase dose for broiler chickens using the Buttiauxella phytase strain has previously been determined, although the optimal dose of phytase using different particle sizes of limestone has not been quantified. Using only one source of limestone, the objective of the second digestibility trial was to determine whether or not the optimal phytase dose for broiler chickens changes when using different limestone particle sizes. Two groups of 600 Ross 308 male birds were fed from days 19-21 and 23-25, respectively, in order to determine the effects of limestone particle size and phytase concentrations on Ca and P digestion. The experiment had a randomised block design and 2x4 arrangement of treatments using a single limestone source with two particle sizes (0.8 mm and Commercial) and four phytase (0, 250, 1000 and 2000 FTU/kg) concentrations. The experiment was repeated twice (block) with five replications/treatment per block. Each block consisted of 100 cages with six birds per cage. Digesta from the distal ileum was sampled in order to determine disappearance of Ca and P on days 21 and 25 of age. Data collected showed a highly significant (P<0.0001) positive response in P digestibility when a 0.8 mm particle size was used. Increasing the concentration of phytase in the diets using the same limestone source showed a positive response (P<0.001) for both Ca and P digestibility. The use of phytase up to 2000 FTU demonstrated that the negative effects of fine limestone can be overcome by increasing the phytase dose above that which is currently recommended (1500 FTU). Digestibility Trial 1 showed that the use of limestone in broiler diets would have to be evaluated according to the source of the limestone being used as well and the particle size of the limestone being used. Data from Digestibility Trial 2 will enable nutritionists and scientists to more accurately calculate the optimal phytase dose for differing limestone particle sizes being used in broiler feed.