Investigating the impact of exogenous enzymes and phosphorus-induced appetite regulation in broiler chickens
For this dissertation, four experiments were conducted to evaluate the effect of dietary addition of exogenous protease and amylase enzymes on growth performance and nutrient utilization in broiler chickens. An additional fifth experiment was designed to determine the role of central and peripheral appetite regulators in birds fed diets deficient in dietary phosphorus (P). This arose from consistent reports in literature of a direct effect of dietary P concentration on feeding response in broiler chickens.
Experiment 1 examined the growth performance and protein utilization responses of broiler chickens to purified trypsin inhibitors (TI) and exogenous protease additions. Experimental diets were arranged as a 2 × 2 factorial with factors being dietary TI (1,033 or 10,033 TIU/g) and exogenous protease (0 or 15,000 PROT/kg). Protease supplementation improved BW gain (P < 0.01) and gain to feed ratio (P < 0.05) of birds. The relative weight of pancreas increased (P < 0.05) with added TI on d 14 and 21 but was reduced (P < 0.001) with protease supplementation. Apparent ileal digestibility (AID) of all amino acids (AA), except methionine, decreased (P < 0.001) with added TI, but increased (P < 0.05) with protease supplementation. Duodenal trypsin and chymotrypsin activities were reduced (P < 0.05) with added TI but increased (P < 0.01) with protease supplementation. It was concluded that dietary addition of purified TI negatively affects nutrient utilization by broiler chickens and that the efficacy of the exogenous protease might be independent of dietary TI concentration. A follow-up experiment was conducted (Experiment 2) to evaluate the impact of TI and exogenous protease supplementation on endogenous AA loss in broiler chickens. Four diets were arranged as a 2 × 2 factorial with factors being dietary TI (0 or 8,000 TIU/g) and exogenous protease (0 or 15,000 PROT/kg). There was no effect of TI, exogenous protease, or their interaction on growth performance of birds. Endogenous nitrogen (N) loss and all AA (except Cys) increased (P < 0.05) due to added dietary TI. Exogenous protease had no effect on endogenous loss of N and all AA. The AID of Ca, Fe, Mg, Mn, and Cu was reduced (P < 0.05) by added dietary TI. Protease supplementation improved the AID of Cu (P < 0.01) and K (P < 0.05). Secretion of crude mucin and sialic acid (g/kg DM intake) increased (P < 0.05) with increased dietary TI and was not recovered by protease supplementation. It was concluded from this study that TI increases the endogenous loss of AA, reduces the digestibility of minerals in broiler chickens, and that exogenous protease had no effect on endogenous AA flow, irrespective of added dietary TI.
In Experiment 3, the responses of broiler chickens fed corn-soybean meal-based diets to dietary α-amylase supplementation during 4 growth phases were evaluated. Birds were assigned to 8 treatment diet in a 2 × 4 factorial arrangement of 2 dietary levels of α-amylase supplementation (0 or 80 kilo-Novo alpha amylase units (KNU) per kg diet) and 4 post hatching growth phases (d 0 to 11, d 11 to 21, d 21 to 42, or d 42 to 56). Body weight gain and feed efficiency of birds improved (P < 0.01) with α-amylase supplementation. There were main effects of α-amylase, growth phase and interaction (P < 0.01) on AID of starch. The total tract retention (TTR) of starch increased (P < 0.05) with amylase supplementation but was not different across growth phases. Amylase supplementation improved (P < 0.05) gross energy utilization in birds, and specifically, during d 11 to 21 post hatching, the viscosity of jejunal digesta and pancreatic amylase activity increased (P < 0.01) with amylase supplementation. The conclusion from the study was that the growth phase of birds may affect the response to exogenous amylase. Following the result of this study, Experiment 4 was conducted to evaluate the effect of amylase supplementation on starch and energy digestibility at various intestinal sites in broiler chickens. Experimental diets comprised 3 concentrations of α-amylase supplementation (0, 80, or 160 KNU/kg diet) and sampling was done on 4 intestinal sites: anterior jejunum (AJ), posterior jejunum (PJ), anterior ileum (AI) and posterior ileum (PI). There were linear and quadratic (P < 0.01) responses of increasing α- amylase supplementation on starch and energy digestibility at the PJ and AI, with only linear effects on TTR of starch (P < 0.05). A linear increase in starch disappearance and digestible energy (kcal/kg) was observed (P < 0.01) with digesta flow from AJ to PJ with increasing amylase supplementation, which may be related to the observed decrease in the viscosity of the jejunal digesta (P < 0.05). Results from this experiment demonstrate the efficacy of exogenous amylase to improve starch, and energy digestibility in broiler chickens, with the highest impact observed in the posterior jejunum.
A final study (Experiment 5) was conducted to evaluate the impact of dietary phosphorus (P) concentration on hypothalamic molecular regulation of appetite by broiler chickens. Birds were randomly assigned to 3 experimental diets which contained 1.2 (P-deficient), 2.8 (P-marginal) or 4.4 (P-adequate) g/kg non-phytate P (nPP). A decrease in feed intake and BW gain was observed (P < 0.001) in birds fed the P-deficient diet. There was upregulation (P < 0.05) in the mRNA expression of Sodium-phosphate cotransporter (NaPi-IIb), anorexia-related hypothalamic cholecystokinin receptor (CCKAR) and melanocortin receptors (MC3R and MC4R) in birds fed P-deficient diets, whereas cholecystokinin (CCK) mRNA was downregulated (P < 0.01). It may be concluded that a deficiency in dietary P decreases feed intake in broiler chickens by altering the expression of anorexigenic genes in the gut and hypothalamus.