Study On Calcium Requirement Of Broiler Chicks Fed A Corn-soybean Meal Diet From 1 To 21 Days Of Age

The experiment was conducted to select specific sensitive functional indicators in order to evaluate dietary calcium(Ca)requirement of broilers fed a corn-soybean meal diet from 1 to 21 days of age by investigating the effect of dietary Ca levels on growth performance,blood indices,tibia and middle toe bone characteristics,and mRNA and protein expression levels of enzyme and protein involving in calcium utilize of broilers,which can provide a scientific basis for the rational supplement of calcium in broiler production.A single-factor completely randomized design was used in this experiment.A total of420 commercial 1-day-old Arbor Acres male broilers were randomly assigned to 7 treatment groups of 6replicate cages(10 chicks per cage)according to body weight.The dietary Ca levels were 0.60,0.70,0.80,0.90,1.00,1.10 and 1.20% Ca,respectively,in accordance with the dietary Ca recommendation1.0% for broilers from 0 to 3 weeks of age prescribed in NRC(1994)and the feeding standard for chickens in China(2004).The nonphytate phosphorus(NPP)level was 0.39% in the diets of all treatments.All broilers were allowed ad libitum access to experimental diets and water.The trial lasted21 days,and maintained on a 24-h light schedule.The results showed that:(1)Dietary Ca levels had no effect on average daily feed intake,feed/gain and mortality rate(P>0.05),however,as dietary Ca levels increased,the average daily gain significantly decreased(P<0.01).(2)Dietary Ca levels did not affect serum and tibia Ca or P content(P>0.05),but affected alkaline phosphatase(ALP)activity in serum and tibia(P<0.05).(3)Dietary Ca levels had no effect on ash percentage and bone mineral content(BMC)of middle toe(P>0.05),but affected tibia ash percentage,BMC,bone strength and bone mineral density(BMD)of tibia and middle toe(P<0.05),and the Ca content in middle toe ash increased linearly as dietary Ca levels increased(P<0.05).(4)Dietary Ca levels did not affect mRNA expression levels of ALP and osteoprotegerin(OPG)in tibia(P>0.05),but significantly decreased the mRNA expression levels of osteocalcin(OC)and bone morphogenetic protein-2(BMP-2)(P<0.05).(5)The Protein expression levels of OPG,OC,BMP-2 in tibia were not affected by dietary Ca levels,but the ALP protein expression levels changed quardratically as dietary Ca level increased(P<0.05).These results indicate that the daily gain of broilers decreased with the increase of dietary Ca levels,and the Ca requirement to meet the best growth was about 0.60%,serum and tibia ALP activity,tibia ash percentage,tibia strength,tibia and middle toe BMD,and protein expression levels of ALP in tibia were specific sensitive indices to evaluate dietary Ca requirement of broilers fed a corn-soybean meal diet from 1 to 21 days of age.The optimal dietary Ca level obtained from the best broken-line and quadratic curve model was 0.80-1.00%.Based on all the above results,in order to meet all the Ca metabolic and bone development requirements of broilers,the dietary Ca requirement of broilers fed a corn-soybean meal diet from 1 to 21 days of age was suggested to be 1.00%,which was in accordance with the recommendation(1.0%)of NRC(1994)and the feeding standard for chickens in China(2004)for broilers of the same period.The aforementioned dietary Ca requirements can make broilers obtain optimal growth and bone development,therefore,they have important sense to maintain the health andefficient growth of modern broilers.