Effects of the composition of the basal diet on the evaluation of mineral phosphorus sources and interactions with phytate hydrolysis in broilers.

The objectives of this study were to determine the availability of P from mineral phosphate sources by using different basal diets and measurement of P retention and prececal (pc) P digestibility as well as pc myo-inositol phosphate (InsP) degradation in broilers. Semi-synthetic and corn-soybean meal-based basal diets were used in experiment 1, and corn-based and wheat-based basal diets were used in experiment 2. Anhydrous monosodium phosphate (MSPa) or monocalcium phosphate monohydrate (MCPh) was supplemented to increment the P concentration by 0.05, 0.10, and 0.15% or by 0.075 and 0.150% in experiments 1 and 2, respectively. Titanium dioxide was used as an indigestible marker. Diets were pelleted through a 3-mm screen. In experiment 1, retention was measured based on total excreta collection from 20 to 24 d of age using 7 replicated birds per diet. In experiment 2, digesta from the terminal ileum was collected from 22-d-old broilers penned in groups of 19 with 5 replicated pens per diet. The P retention response to supplemented MSPa did not differ between the 2 basal diets in experiment 1. The response in pc P digestibility to MCPh supplements also did not differ between the 2 basal diets in experiment 2, as calculated by linear regression analysis. Hydrolysis of InsP6 measured on both the excreta and pc levels was high in the basal diets without a mineral P supplement. Mineral P supplementation significantly decreased (P < 0.05) InsP6 hydrolysis from the InsP-containing diets in both experiments. Thus, the choice of the basal diet did not affect the evaluation of the supplemented mineral P source. However, calculated values for mineral P sources need to be adjusted for the decline in hydrolysis of InsP contained in the basal diet that results from the P supplement.

Comparison of retention and prececal digestibility measurements in evaluating mineral phosphorus sources in broilers.

The objectives of this study were to compare measurements of retention and prececal (pc) digestibility in evaluating mineral phosphorus (P) sources in 3- and 5-wk-old broilers. A corn-soybean meal-based basal diet was used (0.35% P on DM basis). Anhydrous monosodium phosphate (MSP(a)) or anhydrous dibasic calcium phosphate (DCP(a)) was supplemented to increment the P concentration by 0.08%, 0.16%, and 0.24%. Titanium dioxide was used as the indigestible marker. Two retention trials with excreta collection from d 16 to 20 and d 30 to 34 were conducted (n = 8 birds per diet). Another 8 pens of 10 birds from the same hatch were allocated to each diet on d 11 or 25 each to measure pc digestibility in both age periods. After 10 d of feeding, these birds were euthanized and the content of a defined section of the terminal ileum was obtained. Percentage P retention and pc digestibility for MSP(a) and DCP(a) were calculated by linear regression analysis. In 3-wk-old broilers, P retention for MSP(a) was 70% and significantly higher (P < 0.001) than for DCP(a) (29%). Values determined for pc digestibility at the same age were very similar (67% for MSP(a) and 30% for DCP(a); P < 0.001). In 5-wk-old broilers, P retention was 63% (MSP(a)) and 29% (DCP(a); P < 0.001) and pc digestibility was 54% (MSP(a)) and 25% (DCP(a); P = 0.002). We concluded that both retention and pc digestibility can be used for evaluating mineral P sources in broilers based on a regression approach. In 3-wk-old broilers, results obtained with both approaches were the same. In 5-wk-old broilers, the ranking of the 2 P sources was also the same for both approaches. Values did not differ significantly between the 2 age periods, but further studies on the relevance of broilers' age in P evaluation are suggested.

Comparison and evaluation of bone measurements for the assessment of mineral phosphorus sources in broilers.

The main objective of this study was to compare different bone measurements in response to supplements of mineral P sources. Comparisons were also made with P retention and digestibility responses determined in a companion study and with blood inorganic phosphate (P(i)) responses. A corn-soybean meal-based basal diet was used (0.35% total P on DM basis). Anhydrous monosodium phosphate (MSP(a)) or anhydrous dibasic calcium phosphate (DCP(a)) was supplemented to increment the P concentration by 0.08%, 0.16%, or 0.24%. Each of the 7 diets was fed for 10 d starting 11 d (period 1) or 25 d posthatch (period 2). Bone ash and P were determined, and density criteria were measured using quantitative computed tomography. Responses were evaluated and compared based on linear regression analysis. In general, responses to MSP(a) had a greater slope than DCP(a) for all criteria studied. In period 1, differences between the slopes were significant (P < 0.05) for almost all bone criteria. In period 2, the slopes significantly differed for the amounts of ash and P of all bones studied, for tibia, tarsometatarus, and foot ash percentage, for total and cortical density of tibiae, but not for the other criteria. For the different bones, the ratio of slopes for MSP(a) and DCP(a) was very similar based on the amount of ash in both periods. Foot ash proved to be as sensitive as tibia ash for evaluation of mineral P sources in both periods. Ninety-four percent of the variance of the corticalis content based on quantitative computed tomography measurements could be explained by the amount of tibia ash in period 1. Blood serum P(i) and BW gain were not suitable for P evaluation. We concluded that the ranking of mineral P sources based on bone criteria differed from the ranking that was based on P retention or prececal digestibility. This underlines the need for developing a standard protocol of determination of available P in poultry.

A note on sampling digesta from the ileum of broilers in phosphorus digestibility studies.

The responses of broilers to increments of dietary P concentration in P retention and P prececal digestibility were studied in 2 experiments using Ross 308 broilers in their fourth week of age. The low-P basal diet was mainly based on corn, potato protein, and corn starch. Titanium dioxide was used as the indigestible marker. Monobasic calcium phosphate was added in 6 (experiment 1) or 5 (experiment 2) graded levels up to a maximum of 8.1 g of P/kg of the diet. Five (experiment 1) and 6 (experiment 2) replicated pens of 10 birds were used. Excreta were collected from trays underneath the pens. Birds were asphyxiated by carbon dioxide exposure and the ileum (from Meckel's diverticulum to 2-cm anterior to the ileo-ceca-colonic junction) was dissected. Digesta was collected from the entire section (experiment 1) or from 3 subsections of equal lengths (experiment 2). Excretion of P increased linearly with increasing dietary P concentration up to a level of about 5.2 g of P/kg of the diet and increased nonlinearly with further increase in dietary P. In contrast, P flow in the ileum increased linearly over the entire range of P intake. Up to the level of 5.2 g of P/kg of the diet, P excretion was slightly lower than P flow in the ileum, but responses in P excretion and P flow in the ileum were similar, suggesting that P excretion with urine is very low and unaffected by P intake when the birds are supplied with P below their requirement. Between the 3 subsections of the ileum, calculated digestibility was significantly different for the basal diet and the diet with the 3 lowest levels of P supplementation. We concluded that the response in P prececal digestibility to increments in dietary P concentration is linear over a wider range of dietary P than the response in P retention. When digesta is collected from the ileum for determination of P prececal digestibility, the first third of the ileum should not be considered.