| Abstract | Among the agriculture-based industries, poultry sector is expanding very fast. Genetic improvements in broiler strains during past two decades requires to reassess the nutrient requirements. Among poultry dietary ingredients, phosphorus (P) is ranked as the third most expensive ingredient. Besides, P resources are finite and excess use of P not only impact on production cost but also poses a serious threat to environment by pollution of excess P in the excreta. In this regard, it is necessary to find the way for efficient use of P in poultry diets. In poultry, optimum P utilization is closely related with dietary Ca content. Vitamin D3 (VD3) regulates the homeostasis of both P and Ca. At the molecular level, around 90% of intestinal active P transport is performed by sodium dependent phosphate cotransporter type IIb (NaPi-IIb). In the present study, four experiments were conducted to access the changes in dietary contents of non-phytate phosphorus (nPP), Ca, both nPP and Ca, and VD3 on broiler growth efficiency, bone characteristics and intestinal NaPi-IIb expression in starter and grower phases of broilers.
In the first experiment, restriction of dietary nPP content on growth, bone characteristics and duodenal NaPi-IIb expression were observed. Each treatment group was treated with one of the diets formulated with 0.33, 0.37, 0.41, 0.45, 0.49 and 0.53% of nPP up to 14 days. During 15 to 31 days, birds were treated with one of the diets containing 0.23, 0.27, 0.31, 0.35, 0.39 and 0.43% of nPP. Level of Ca was kept the same across all treatments. Dietary nPP level influenced on body weight gain and feed intake in both growth phases (P < 0.001) while effect on feed per gain ratio was seen only in second phase. Toe ash content, tibia ash content and tibia bone breaking strength responded to treatments (P < 0.01) at 14 days. Only tibia ash % was significantly improved at 31 days (P < 0.001). Growth and bone parameters linearly improved when dietary nPP level was increased (P < 0.05). Above dietary nPP 0.41 % and 0.31 % for first phase and second phase, respectively, no significant improvement was seen. Duodenal NaPi-IIb mRNA overexpressed with decrease in dietary nPP in both phases (P < 0.05). In lowest nPP group, relative expression of NaPi-IIb was 2.2 folds higher in the first phase and 3.6 folds higher in the second phase compared to respective highest nPP groups of each phase. No significant change in NaPi-IIb expression was seen above 0.37 % of dietary nPP for 14 days and 0.31 % of dietary nPP for 31 days. Dietary requirement of nPP 0.41 % for 0 to 14 days and 0.31 % for 15 to 31 days, were adequate for optimal growth and bone parameters. Duodenal NaPi-IIb worked independently of Ca in respond to dietary nPP.
Influence of dietary Ca concentrations on growth, bone characteristics and duodenal NaPiIIb expression of starter and grower broilers were observed in the second experiment. Six dietary treatments with different Ca levels of 0.76%, 0.84%, 0.92%, 1.00%, 1.08% and 1.16% in basal diet were given from 0 to 14d. Dietary Ca levels of 0.66%, 0.74%, 0.82%, 0.90%, 0.98% and 1.06% were provided into the basal diet from 15 to 31d of age. Levels of dietary nPP was fixed at the recommended levels of 0.45% and 0.35% for starter and grower diets, respectively in all treatments. Body weight gain and feed intake were negatively influenced by increase in dietary Ca content (P <0.001) at 14d. At 31d of age, growth efficiency was same across all treatments. However, changes in dietary Ca levels significantly influenced (P< 0.05) on toe ash % and tibia ash % at 14d and 31d, respectively. At 14d tibia ash % is highest in the group provided 1.00% of Ca. Increase in 1.7 folds of NaPi-IIb (P< 0.001) was seen in the birds with highest levels of dietary Ca at 1.16% compared to those with lowest dietary Ca group. There was no significant change in NaPiIIb expression when there was increase in dietary Ca from 0.76 to 1.08. Increase in dietary Ca while receiving adequate dietary nPP adversely affected the body weight gain at 14d.
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Imbalance in Ca and nPP ratio might have played the reduction in feed intake at 14d. Changes in dietary Ca levels did not influence the growth performance at 31d when dietary nPP was provided at adequate amount. Dietary Ca levels as low as 0.76% for starters and 0.66% for growers gave the desirable growth performance and bone characteristics. Upregulation of NaPi-IIb with increase in dietary Ca might be the reason for imbalance Ca and nPP metabolism in the body.
The third experiment was aimed to understand the changes in both dietary Ca and nPP levels at starter and grower phases on broiler growth performance, bone characteristics and expression of NaPi-IIb. Each treatment group was treated with one of the diets supplemented with Ca:nPP 0.77:0.33, 0.82:0.37, 0.91:0.41, 1.00:0.45, 1.09:0.49 and 1.18:0.53 % in the diets. During 15 to 31 days, birds were treated with one of the diets containing Ca:nPP 0.59:0.23, 0.69:0.27, 0.80:0.31, 0.90:0.35, 1.00:0.39 and 1.11:0.43 % in diets. The ratio of Ca:nPP was kept the same for the diets in each phase. Lowering the both Ca and nPP in both 14d and 31d of age negatively impacted on the body weight and feed intake (P < 0.001). Increasing dietary Ca and nPP in both phases improved toe and tiba ash % significantly (P < 0.001). Improvement in tibia bone breaking strength was observed only at 31d of age. Depletion was not significantly affected by Ca and nPP levels in both growing phases. Expression of duodenal NaPi-IIb was upregulated significantly (P < 0.001) at 31d of age. Compared to highest Ca:nPP group, NaPi-IIb in birds received lowest Ca:nPP expressed 5.6 folds higher. For growth performance, birds received Ca:nPP at 0.91:0.41 for starter and 0.80:0.31 for grower phase did not significantly differ from those received higher Ca:nPP. However, for bone characteristics, dietary concentration of Ca:nPP at 1.00:0.45 for broilers at starter phase and 0.90:0.35 for grower phase were recommended. It was assumed that requirement of both Ca and nPP for optimum growth efficiency in broiler was lower than the requirement of Ca and nPP for optimum bone characteristics. Expression of NaPi-IIb is negatively correlated to increase in dietary Ca:nPP. This study further strengthened the previous findings on role of Ca and nPP in broiler growth, bone and NaPi-IIb expression. Moreover, this is one of the very few studies that determined the role of NaPi-IIb in broilers above three weeks of age.
In the last experiment, the influence of dietary VD3 levels on growth, bone performance NaPi-IIb expression in broiler chicken were studied. Each of the six treatment groups received diet containing different amounts of VD3 (80, 200, 500, 1250, 3125 or 7813 IU per kg of diet) from a day old to 31 days of age. Dietary phosphorus and calcium were kept the same across all treatments in each phase. Body weight gain at 14d and feed intake at 31d of age were influenced (P < 0.05) by VD3. At 14 d, increase in dietary VD3 content linearly improved toe ash % and tibia ash % (P < 0.05). In both phases, relative expression of duodenal NaPi-IIb linearly increased with increase in dietary VD3 content (P < 0.01). At 14 d, highest expression of 3.2 folds was observed in birds treated with VD3 at 7,813 IU/ kg of feed. At 31 d, birds that received VD3 levels of 3,125 and 7,813 IU/ kg of feed showed 2.9 folds higher in NaPi-IIb expression compared to those fed lowest level of VD3 at 80 IU/ kg of feed. When dietary calcium and phosphorus were maintained at the standard requirement, increase in dietary VD3 did not improve growth performance. For optimum growth and bone characteristics, dietary inclusion of VD3 at 500 IU/kg was adequate for both starter and grower broiler diets.
Overall, when dietary nPP was met or exceeded the requirements of the birds, changes in Ca alone, Ca:nPP ratio and VD3 were less capable of altering the bird’s growth performance. Tibia ash content positively responded to increase in dietary nPP or VD3. Expression of intestinal NaPi-IIb genes were influenced by dietary nPP and VD3 levels. Dietary Ca or vi Ca:nPP ratio did not affect NaPi-IIb expression when nPP was adequate or exceeded. This study contributes in understanding of intestinal NaPi-IIb expression in both starter and grower broilers in response to different level of dietary nPP, Ca, Ca and nPP, and VD3. |