| Growth performance degradation of broilers caused by immunological stress has become increasingly prominent, seriously restricting the sustainable development of broiler industry. Therefore, the nutritional regulation on immunological stress has become a hot topic in the animal immunonutrition. Astragalus polysaccharide(APS) attracted more attention in recent studies because of its low resistance, low side effects and strong immunomodulation. This study is composed of three trials, starting with the construction of LPS-induced immunological stress model, investigating the effect of dietary APS on growth performance, immune function, energy and protein metabolism, and aiming to provide a theoretical basis for the application of anti-immune stress additives.Experiment 1 Effect of different immune status on growth performance, body temperature and digestive enzyme activity in broilers. A total of 320 one-day-old broilers(Cobb500) were randomly assigned to 4 treatments with 8 pens per treatment and 10 birds per pen(Group 1: no vaccination, Group 2: conventional vaccination, Group 3: cyclophosphamide(CPM) stress and Group 4: lipopolysaccharide(LPS) stress). The results showed that there were no significant differences in ADG and ADFI between NV group and CV group. However, compared with these two groups, the injection of LPS resulted in significantly decreased ADG and ADFI at random intervals during the study(P < 0.05), while the significant decreases were also observed in chickens treated with CPM only during later period(P < 0.05). The F/G showed no significant differences among the four groups(P > 0.05). The body temperature increased rapidly, peaked in 4 h after administration and returned to baseline levels in 16 h after LPS administration on day 16. No significant differences were observed among the four groups on lipase activity(P > 0.05). However, both LPS and CPM markedly decreased amylase activity in jejunum, as well as a decreased trend on trypsin activity in LPS group. The data suggested that under our experimental conditions, NV can achieve the same performance as CV, whereas LPS and CPM can induce growth inhibition, which may be due to the raised energy requirement for maintenance and reduced starch utilization.Experiment 2 Effect of APS on growth performance and inflammatory response in broilers exposed to immunological stress. A total of 360 one-day-old AA broiler chicks were randomly assigned to 4 treatments with 6 replicate pens per treatment and 15 birds per pen. The main factors were immune stress(LPS or saline) and dietary APS(0 or 3 g APS/kg feed). The results showed that there was an LPS × APS interaction in ADG during week 1–3(P = 0.045) and weeks 4–6(P = 0.047) and ADFI during weeks 1–6(P = 0.041), indicating that LPS induced growth inhibition was attenuated by dietary APS. Compared with the unchallenged birds, LPS significantly increased spleen index on 14 days(P < 0.001) and 35 days(P = 0.023). All immune organ indexes showed no significant interaction between LPS challenge and dietary APS. Compared with the chickens treated with saline, LPS challenge resulted in the significantly increased mRNA abundance of IL-1β(P = 0.012) and IL-6(P = 0.032) at 35 days of age, as well as an uptrend on TNF-α. However, the elevation of cytokine induced by LPS challenge in chickens received APS was not as great as that of injected birds fed control diet. Furthermore, LPS interacted with APS on jejunal IL-1β(P = 0.013) and IL-6(P = 0.024) on day 35. A marked elevation of TLR4 and NF-κB expression was observed after LPS administration regardless of APS supplementation, while pretreatment of birds with APS inhibited the upregulation. Similar to the response in cytokines, indirect interactions were observed between LPS and APS on TLR4(P = 0.040) and NF-κB(P = 0.046) at 35 days of age. These beneficial effects of APS may be attributed to suppressing the expression of proinflammatory cytokines through reducing the TLR4 and NF-κB genes transcription, and therewith improving growth performance of broilers.Experiment 3 Effect of APS on energy and protein metabolism in broilers exposed to immunological stress. This test shared the same design with Experiment 2. When challenged with LPS, the levels of plasma cortisol, glucose, and insulin was higher than that of unchallenged birds(P < 0.05), while T3 was significantly downregulated(P < 0.001) at 14 days of age. There was an LPS × APS interaction on insulin on 14 days(P = 0.018). Significant interactions between diet and stress were observed in the body temperature at 6 h(P = 0.003), 8 h(P < 0.001), and 10 h(P = 0.005) post-injection, which indicated that dietary APS treatment partially attenuated the increase in body temperature induced by LPS challenge. Both LPS challenge and dietary APS upregulated avUCP(P < 0.05) in chest muscle of chickens. It showed significant LPS × APS interactions in mRNA abundance of avUCP in chickens on 14 days(P < 0.001) and 35 days(P = 0.008). LPS challenge increased the expression of y+LAT2 significantly in duodenal mucosa, while LPS × APS interactions on mRNA abundance of y+LAT2 were observed in duodenum(P = 0.017) and ileum(P = 0.042) at 14 days of age. LPS injection increased mRNA abundance of α1-acidglycoprotein(AGP; P < 0.001) and hemopexin(HPX; P = 0.004), while existing an interaction between LPS challenge and dietary APS(P = 0.037) for the mRNA abundance of HPX. The AID values of all AA, except for methionine, arginine, isoleucine, leucine, aspartate, and glutamate, were decreased(P < 0.05) by LPS administration. Moreover, chickens injected with LPS showed higher(P < 0.05) IEAA, including lysine, arginine, histidine, and alanine. Consequently, LPS decreased(P < 0.05) the SID of the following AA: cysteine, threonine, valine, glycine, serine, proline, and alanine. However, the lower AID and SID in broilers injected with LPS were numerically enhanced by dietary APS. A two-week trial of protein deposition showed that immunological stress significantly reduced the protein deposition(PD) and protein deposition rate(PDR) of left pectoral muscle(P < 0.05), and significantly increased the PD and PDR of spleen and bursa PDR( P < 0.05). There was an LPS × APS interaction on PD(P = 0.045)and PDR(P = 0.010)of spleen, demonstrating that excessive deposition of protein to spleen can be inhibited by dietary APS. The data suggested that supplementation with APS can partially suppress the raised energy requirement for maintenance, inhibit protein redistribution from skeletal muscle to immune system, improve amino acid digestibility and totally improve energy and protein metabolism of broilers. |
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