Studies On Standardized Phosphorus Availabilities In Fecdstuffs, Available Phosphorus Requirement And Mechanism Of Phosphorus Absorption In The Small Intestine Of Broiler Chicks

Five studies were conducted in this dissertation to investigate the standardized phosphorus (P)availabilities in feedstuffs, non-phytate P and standardized available P requirement, as well as theabsorption mechanism of inorganic P in the ligated small intestine segments of broiler chicks.Firstly, three experiments were conducted to develop a procedure for estimating the standardizedmineral (calcium, Ca; phosphorus, P; copper, Cu; manganese, Mn; zinc, Zn) availabilities (SMA) in themost commonly used model ingredient (soybean meal) for broilers. In experiment1, twelve24-d-oldcommercial AA male chicks were used to determine the length needed to empty the total tract of feedresidues during the pre-experimental fasting period. The chicks were randomly allotted to four replicatecages of three chicks per cage. Birds were fed the corn–soybean meal basal diet for2h, then feeds wereremoved and excreta samples were collected for12,24,36or48h after feed withdrawal. The resultsindicated that the excreta collection time of24h after feed withdrawal was sufficient to empty the totaltract of feed residues. In experiment2, forty-eight22-d-old commercial AA male chicks were used todetermine SMA in soybean meal. The chicks were randomly allotted to one of the two treatments(mineral-free or soybean meal diet group) with six replicate cages of four chicks per cage. After3-dacclimation, chicks were fasted for24h and then fed mineral-free or soybean meal diet for4h, andexcreta samples were collected for28h (24h after feed withdrawal) or52h (48h after feed withdrawal).The results showed that the extended excreta collection time of52h was adequate for estimating SMA.The standardized availability values of Ca, P, Cu, Mn and Zn in soybean meal were51.1,50.2,36.3,30.8and49.7%, respectively. In experiment3, a similar bioassay was conducted with forty-eight36-d-oldcommercial AA male chicks to measure SMA in the same soybean meal. Forty-eight34-d-old chickswere randomly allotted to one of the two treatments (mineral-free or soybean meal diet group) with sixreplicate cages of four chicks per cage, then birds recovered and acclimated for3d. The standardizedavailability values of P (51.59%), Cu (36.37%), Mn (31.94%) and Zn (43.37%) were similar (P>0.12)to the results of experiment2except for Ca (41.49%), indicating that the age of birds had no effect onthe standardized availabilities of P, Cu, Mn or Zn in soybean meal. The results from this study suggestedthat this simple and rapid procedure could be used to determine SMA in feedstuffs (e.g. soybean meal)for broiler chicks.Secondly, two experiments were conducted to estimate standardized phosphorus (P) availabilities(SPA) of corn, soybean meal (SBM) and corn-soybean meal (C-SBM) diet in broilers chicks, and verifythe additivity of SPA for feed formulation of broilers. Ninety-six22-d-old commercial AA male broilerswith similar bodyweight were used in each experiment. The chicks were randomly allotted to one of thefour treatments (P-free, corn, SBM or C-SBM diets group) for six replicate cages of four chicks per cage.After3-d acclimation, chicks were fasted for24h and then fed P-free, corn, SBM or C-SBM diets,respectively, for4h in experiment1or72h in experiment2, and collected excreta for52h (48h afterfeed withdrawal) in experiment1or120h (48h after feed withdrawal) in experiment2. In experiment1, the results showed that the excreta collection time of52h (48h after feed withdrawal) was adequate forthe estimation of SPA. The basal endogenous P loss (EPL) of chicks fed the P-free diet was estimated tobe123mg/52h per bird. The values of SPA corrected by basal EPL were37.6and50.5%for corn andSBM, respectively. The determined value of SPA for the C-SBM diet was very close (P>0.79) to thepredicted summation of SPA from corn and SBM (44.4vs.43.5%). In experiment2, the results showedthat the excreta collection time of96h (24h after feed withdrawal) was sufficient for the estimation ofSPA. The basal EPL of chicks fed the P-free diet was estimated to be85.4mg/96h per bird. The valuesof SPA corrected by basal EPL were40.2and52.9%for corn and SBM, respectively. The determinedvalue of SPA of the C-SBM diet was lower (P <0.001) than the predicted summation of SPA from cornand SBM (39.7vs.46.0%), which might be due to the effect of higher total P intake. The results from thecurrent study demonstrated that the P-free diet could be used for measuring basal EPL in broiler chicks,and then estimating the SPA values of plant feedstuffs for broiler chicks. However, the additivity of SPAin the diet formulation needs to be further studied.Thirdly, two experiments were conducted to estimate standardized phosphorus (P) availabilities(SPA) of dicalcium phosphate (DCP), monocalcium phosphate (MCP) and monopotassium phosphate(KCP) in broilers chicks. Ninety-six22-d-old commercial AA male chicks with similar bodyweight wereused in each experiment. The chicks were randomly allotted to one of the four treatments (P-free, DCP,MCP or KCP diets group) with six replicate cages of four chicks per cage. After3-d acclimation, chickswere fasted for24h and then fed P-free, DCP, MCP or KCP diets, respectively for4h in experiment1or72h in experiment2, and collected excreta for52h (48h after feed withdrawal) in experiment1or120h (48h after feed withdrawal) in experiment2. In experiment1, the basal endogenous P loss (EPL) ofchicks fed the P-free diet was estimated to be109mg/52h per bird. The values of SPA corrected bybasal EPL were68.7,69.8or76.6%for DCP, MCP and KCP, respectively. In experiment2, the resultsshowed that the excreta collection time of96h (24h after feed withdrawal) was sufficient for theestimation of SPA. The basal EPL of chicks fed the P-free diet was estimated to be49.2mg/96h perbird. The values of SPA corrected by basal EPL were71.8,70.6and78.3%for DCP, MCP and KCP,respectively. The results from the current study demonstrated that the P-free diet could be used formeasuring basal EPL in broilers, and then estimating the SPA values of inorganic P sources for broilerchicks.Fourthly, one experiment was conducted to investigate the effects of dietary non-phytatephosphorus (NPP) levels on performance, bone characteristics and tissue gene expression of Pmetabolism-related function gene, so as to determine the non-phytate P and standardized available Prequirement of broiler chicks fed a corn-soybean meal basal diet from1to21days of age. A total of540one-day-old commercial AA male chicks were assigned to one of nine treatments (eight replicate cagesof ten chicks per cage) in a completely randomized design. The diets included a basal corn-soybean mealdiet (0.08%of NPP) supplemented with0.10,0.15,0.25,0.30,0.35,0.40,0.45, or0.50%of inorganic Pin the form of CaHPO4·2H2O, respectively. All diets contained the same Ca content of1.0%. The resultsindicated that average daily gain, tibia bone strength, tibia ash percentage, tibia bone mineral content (BMC), bone mineral density (BMD) and middle toe ash percentage were sensitive indices forestimating P requirement of broilers, and tibia BMC and BMD were new specific and sensitive criteriafor estimating the optimal dietary NPP requirement for broiler chicks. Serum and tibia ALP activities,tibia ash P concentration, tibia Ca and P ratio, middle toe ash P concentration, duodenum NaP-IIb andtibia TNAP mRNA levels were suitable for determining dietary P bioavailabilities for broiler chicks. Inaddition, the duodenum NaP-IIb mRNA level was significantly (P <0.03) affected by dietary NPP level.Based on above sensitive response indices, the estimated NPP requirements were0.34,0.39,0.37,0.37,0.39and0.36%, respectively. According to the above results, the NPP requirement was0.39%, and thestandardized available P requirement was0.37%for broiler chicks from1to21days old fed acorn-soybean meal basal diet.Finally, two experiments were conducted to study the kinetics of inorganic P absorption in theligated duodenum, jejunum, and ileum intestinal segments of broiler chicks and whether NaP-IIbtransporter was involved. Experiment1was conducted to investigate the P absorption with time, so as todetermined appropriate post-perfusion time in the subsequent experiment. Ninety-six commercial AAmale chicks with similar bodyweight were randomly allotted to one of the six treatments (sixpost-perfusion time points) with eight replicate cages of two chicks per cage. The intestinal loops wereperfused with solutions containing6mmol P/L as KH2PO4, and samples were collected at0,2.5,5,10,20or40min post-perfusion. The results showed that P absorption increased in an asymptotic response topost-perfusion time within40min in the all ligated segments, and P absorption in the duodenum washigher (P <0.04) than that in the other2segments at20min post-perfusion. The results indicated thatthe duodenum might be the main site of P absorption for chicks. In addition, the absorption at20minwas greater than87%of the maximum absorption in each segment. For this reason, the subsequentexperiment was carried out at20min post-perfusion. Experiment2was carried out to study the kineticof P absorption and the mRNA levels of NaP-IIb in the different ligated segments, in order to explain themechanisms of P absorption. Fifty-six commercial AA male chicks with similar bodyweight wererandomly allotted to one of the seven treatments (seven perfusion solutions) with eight replicate cages ofone chick per cage. Intestinal loops were perfused with solutions containing0,1.5,3,6,12,24, or48mmol P/L as KH2PO4. The mRNA levels of NaP-IIb in three intestinal loops in the0,6or48mmol P/Lgroups were analyzed. The kinetic curves showed that P absorption in duodenum depended on a saturatecarrier mediated process, whereas P absorption in the jejunum and ileum occurred with a non-saturatediffusion process. Moreover, the NaP-IIb mRNA levels in the duodenum were higher (P <0.02) thanthat in the ileum or jejunum in the0,6or48mmol P/L groups, further indicating that P absorption in theduodenum occurred mainly by a saturate carrier mediated process.In conclusion, series of present studies developed a procedure to determine the SPA in feedstuffs forbroilers chicks, and according to this procedure the estimated SPA values of corn, soybean meal,corn-soybean meal diet, DCP, MCP or KCP were determined as37.6,50.5,44.4,68.7,69.8and76.6%,respectively. The results of current studies also indicated that tibia BMC and BMD were the specificsensitive criteria for estimating NPP requirement of broiler chicks. Based on the new criteria, the optimal NPP requirement of broiler chicks was0.39%, and the standardized available P requirement was0.37%for broiler chicks from1to21days old fed a corn-soybean basal diet. In addition, P absorption in theduodenum depended on a saturate carrier mediated process, in which NaP-IIb transporter might beinvolved, whereas P absorption in the jejunum and ileum occurred with a non-saturate diffusion process.The above new findings in the current studies had important theoretical and practical significances forreasonable and efficient use of P in feed formulation to accurately meet available P requirement inbroiler production, so as to decrease feed cost and reduce environmental pollution from P excretion ofbroilers.