| After entering the peak laying stage,the egg production rate of laying hens remains at a high level for a period of time,but since then it has been on a downward trend.Substantial studies have shown that digestive and antioxidant functions continuously decline with age.For laying hens,this degeneration may also be an important factor for the decreased egg production rate after the peak laying stage.Enzyme preparation(EP)can supplement endogenous enzymes and improve the digestibility of nutrients.Vitamin E(VE)plays an important role in alleviating oxidative stress and maintaining the balance of redox system of organisms.Therefore,this study first compared the digestive and antioxidant function of laying hens at different ages,and on this basis,further explored the nutritional regulation of EP and VE in the late-phase layers.The study included 3 parts:Experiment 1 was to study the age-related changes in intestinal barrier,digestive function,and egg quality of laying hens.A total of 192 Hyline Brown laying hens with different ages from the same parental generation were divided into 1 of 3 groups according to their ages,with each group being composed of 8 replicates of 8 birds with the same age.The ages of laying hens were 195-d(D195),340-d(D340),and 525-d-old(D525),respectively.The results showed that 525-d-old laying hens exhibited the higher serum D-lactic acid content,and villus height(VH)and villus height/crypt depth in the duodenum,jejunum,and ileum than 195-d-old laying hens,whereas 525-d-old laying hens showed the lower j ejunal mucosal sucrase and maltase activities than their 195-d-old counterparts(P<0.05).The activities of pancreatic amylase,trypsin in the jejunal and ileal chyme,and lipase in the ileal chyme of 525-d-old laying hens were lower than those of 195-d-old laying hens(P<0.05).There was a decrease of trypsin and lipase activity in the ileal chyme of 525-d-old laying hens when compared with those in D195 or D340 group(P<0.05).Compared with 195-d-old laying hens,the apparent digestibilities of dry matter and crude protein of 340-d-old counterparts,and dry matter,crude protein,and ether extract of 525-d-old counterparts decreased(P<0.05).Excreted threonine,methionine,and leucine concentrations of 525-d-old birds were higher than those of 195-d-old laying hens(P<0.05).Compared with 195-d-old laying hens,the mRNA expression of alanine-serine-cysteine-threonine transporter 1 and excitatory amino acid transporter 3 in the jejunal mucosa and cationic amino acid transporter 2 in the ileal mucosa of 340-d-old counterparts,and alanine-serine-cysteine-threonine transporter 1,excitatory amino acid transporter 3,and b0,+ amino acid transporter in the jejunal mucosa and cationic amino acid transporter 1 in the ileal mucosa of 525-d-old counterparts decreased(P<0.05).The expression of cationic amino acid transporter 1 and oligopeptide transporter 1 in the ileal mucosa of 525-d-old laying hens was lower than that of 340-d-old laying hens(P<0.05).Compared with 195-d-old hens,the weight of egg from 525-d-old hens increased,whereas protein height,eggshell strength,and eggshell thickness decreased(P<0.05).Experiment 2 was to investigate the age-related changes of serum reproductive hormone concentration,hepatic metabolism,and antioxidant function of laying hens at different ages.The experimental design was the the same as experiment 1.The results showed that the serum estradiol content of 525-d-old laying hens was lower than that of 195-d or 340-d-old counterparts(P<0.05).Compared with hens from D195 group,serum total protein and globulin levels reduced,whereas serum aspartate aminotransferase activity,and liver triglyceride and total cholesterol contents increased in the D525 group(P<0.05).The serum and liver malondialdehyde(MDA)concentrations of 525-d-old laying hens increased when compared with 195-d or 340-d-old laying hens(P<0.05).The 525-d-old hens exhibited the lower activities of serum and hepatic glutathione peroxidase,as well as hepatic superoxide dismutase(SOD)than their 195-d-old counterparts(P<0.05).The mRNA abundance of hepatic nuclear factor-erythriod 2-related factor 2 and glutathione peroxidase 1 of 340-d-old laying hens increased in comparison with 195-d-old counterparts(P<0.05).The birds showed a downregulation of nuclear factor-erythriod 2-related factor 2,NAD(P)H quinone oxidoreductase 1,and SOD1 mRNA expression in the liver from 340-d to 525-d-old(P<0.05).Experiment 3 was to explore the effects of EP and VE on the productive performance,egg quality,and intestinal integrity,as well as digestive and antioxidant function of laying hens during the late laying stage.A total of 288 560-d-old Hyline Brown laying hens were randomly allocated into 4 treatments for 56 days,and each treatment had 6 replicates with 12 birds each.The birds were fed a basal diet supplemented with 0(Control group),200 mg/kg EP,100 mg/kg VE,and combination of 200 mg/kg EP and 100 mg/kg VE,respectively.The results showed that VE increased glucose content and reduced alanine aminotransferase activity in the serum(P<0.05).EP increased VH and villus height/crypt depth in the jejunum(P<0.05);VE increased VH in the ileum,and reduced serum diamine oxidase activity(P<0.05).EP increased amylase activity in the pancreas and ileal chyme,and trypsin activity in the jejunal chyme,as well as sucrase and maltase in the jejunal mucosa(P<0.05).The increased serum SOD and CAT activities,and hepatic glutathione and glutathione peroxidase levels,as well as decreased hepatic MDA content were observed in the VE administration group(P<0.05).VE also increased the albumen height(P<0.05).In addition,an interaction on the Haugh unit was observed between EP and VE(P<0.05),whereas there is no interaction for other indices(P>0.05).It can be concluded as follows:1.The compromised intestinal barrier function,reduced digestive enzyme activity and apparent nutrient digestibility,and downregulated intestinal nutrient transporter expression were observed as the age increased,indicating that the digestive function of laying hens gradually declined after the peak laying period.2.As the age increased,the circulating estrogen concentration decreased,coupled with the disordered hepatic lipid metabolism and redox system,and all of these would indicate the degenerated liver function.3.200 mg/kg EP could increase the endogenous digestive enzyme activities of 560-dold laying hens,and 100 mg/kg VE could increase the antioxidant capacity and egg quality,and both of them could improve the intestinal integrity. |
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