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 corn-soybean meal-based diets in a 2 × 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 mono-calcium 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.