Effect of dietary calcium and phosphorus restriction during grower period on growth performance, carcass traits, blood and bone parameters and broiler chickens adaptation in finisher period

Introduction: In recent years, increasing feed costs in broiler production encouraged nutritionists to reduce feeding expenditure, along with maintaining optimal performance of broiler chickens and minimizing environmental pollution. Phosphorus and calcium are two important minerals in the poultry diets, which is necessary to accurately feeding these minerals in the poultry nutrition. Environmental contamination, as a result of over-feeding of phosphorus by poultry, is a matter of concern, which has urged researchers to seek solutions such as reducing dietary phosphorus concentrations without adversely affecting the growth performance. Also, due to the interaction effect of phosphorus and calcium in the gastrointestinal tract, the ratio and balance between these two elements is also important. It has been argued that broiler chicks, when fed by diets containing lower levels of phosphorus and calcium, absorb these materials with higher efficacy and thus reduce their excretion from the gastrointestinal tract. Yan et al. (2005) reported that feeding broiler chickens with diets containing reduced levels of phosphorus and calcium during starter phase, and then using diets containing sufficient levels of phosphorus and calcium, caused better utilization and bone mineralization. Birds respond to dietary phosphorus and calcium content, by increasing the expression of mRNA encoding calcium and phosphorus transporters in the small intestine.More studies are needed to optimize the levels of phosphorus and calcium in the early stages of growth, as well as fine-tuning the appropriate time period for decreasing them with the aim of designing nutritional strategies that increase the utilization of phosphorus and improving the growth performance and mineralization of the bones. Therefore, the aim of this study was to determine the effect of phosphorus and calcium restriction during grower phase and its effect on growth performance, blood and bone parameters and adaptation response in broiler chicks.Materials and Methods: A total of 648 one-day-old male broiler chicks of the Ross 308 strain were used in this study. The chicks were randomly distributed into floor pens (1.2 m × 1m). During the starter period (1-10 d), all birds were fed with a standard diet containing recommended nutrients of the Ross 308 strain. Then, the experimental diets in the grower period (11-24 d) were included: 1) standard diet as control 2) diets with 15% reduction in available phosphorus (aP) and calcium (Ca) and 3) diets with 30% reduction in aP and Ca. In this period, the control treatment included 6 replicates of 12 chicks, and the other two treatments included 24 replicates with 12 chicks each. On d 25 of age, each treatment group (except control) was divided into 4 treatment groups including 0, 10, 20 and 30% reduction in aP and Ca levels for the finisher period diets; so that a total of 9 dietary treatments with 6 replicates and 12 birds per pen were formed. Average body weight (BW), daily feed intake (DFI), daily weight gain (DWG) and feed conversion ratio (FCR) were measured at the end of grower and finisher periods. On day 24, one bird from each pen, weighing closest to the mean body weight was selected and slaughtered, and the carcass parts, as well as internal organs were weighted and expressed relative to live body weight. Blood samples were taken from wing vein of 5 chicks in each treatment on day 24 and serum Ca, Pi and ALP levels were analyzed. Percentage of ash, Ca, Pi and breaking strength of tibia, were measured at the ages of 24 and 42 days. Data were analyzed as a completely randomized design using the General Linear Model (GLM) procedure in SAS software (SAS, 2009). Statistical significance of differences among treatments was assessed using Duncan’s test when the F-test from the ANOVA was declared significant (P< 0.05). The probability level between 0.05 and 0.1 was considered as a marginal trend toward significance. Linear and quadratic contrast was also investigated in response to dietary calcium and phosphorus reduction at the end of each experimental period (grower and finisher).Results and Discussion: The results of this experiment showed that none of growth performance parameters were affected by the treatments during the grower, finisher and the whole experimental period. The relative liver weight was increased in a linear trend with dietary calcium and phosphorus reduction at 24 d of age. Similarly, serum alkaline phosphatase level was linearly increased with decreasing of calcium and phosphorus reduction at 24 d of age. Tibia ash, calcium and phosphorus percentage were significantly affected by experimental treatments; so that they were decreased in a linear trend when dietary calcium and phosphorus decreased at 24 and 42 d of age. Tibia breaking strength was not significantly affected at 24 d of age; nevertheless, it had a trend to be significant and was decreased in response to decreasing dietary calcium and phosphorus at 42 d of age. The different response between growth performance and bone characteristics can be due to the fact that calcium and phosphorus requirements are higher for maximum bone function than soft tissues growth. In fact, bone contains 99% and 80% of the body's calcium and phosphorus, respectively. Both act as the main component of hydroxyapatite during the hardening of soft tissue in combination with the organic bone matrix to increase the mechanical strength of bone.Conclusion: The present study showed that reducing the percentage of dietary calcium and phosphorus, despite their significant effect on the blood and bone characteristics of broilers chicken had no significant effect on broilers growth performance. In general, available phosphorus and calcium can be reduced by 15% during the grower period and up to 10% in the finisher period. However, further reduction in the percentage of calcium and phosphorus of diet can lead to adverse effects on the measured traits.