Phosphorus and nitrogen utilization responses of broiler chickens to dietary crude protein and phosphorus levels

A study was conducted to investigate the effect of dietary CP levels on pre-cecal digestibility and total tract retention of phosphorus (P) in broiler chickens. A total of 384 14-day-old male broiler chickens were used in a randomized complete block design with 8 treatments and 6 replicates per treatment in a 7-d experimental period. There were 8 cornsoybean meal-based diets in a 2 x 4 factorial arrangement, which included 2 CP levels (10.7 or 21.5%) and 4 apparent total tract digestible P (ATTDP) levels (0.18, 0.32, 0.45, or 0.59%). Soybean meal and monocalcium phosphate were used to adjust the CP and ATTDP levels, respectively. At the end of the experiment, BW was recorded and digesta samples from the distal two-thirds of ileum and mucosa samples from the middle of the jejunum were collected. Total RNA also was isolated from mucosa samples and used for real-time PCR to determine the gene expression of sodium-phosphate co-transporter IIb (NaPi-IIb). Results showed that low dietary CP level limited the growth performance (P < 0.01), pre-cecal digestion, and total tract retention of P (P < 0.01), and NaPi-IIb gene expression (P < 0.05), compared with high dietary CP. Pre-cecal digestion and total tract retention of P (g/kg DM intake) linearly increased (P < 0.01) with increasing ATTDP levels in both low and high CP groups. In conclusion, this study suggests an interrelationship between N and P digestion such that CP deficiency decreased the growth performance of birds consequently reducing pre-cecal P digestion in broiler chickens. Total tract retention of CP and P are linked with each other and body tissue growth may be a driver of the deposition of these 2 nutrients. Supplementation of protein may be necessary in diets during P digestibility studies to ameliorate an effect of protein deficiency on P digestion and retention.