Effects Of Dietary Cadmium On Performance,Egg Quality,Liver And Kidney Damage Of Laying Hens

Cadmium is widespread in the environment,causing damage to the liver and kidney of animals,affecting the metabolism of trace elements in animals,and causing cadmium residues in animal products,resulting in food safety problems.It is of great theoretical and practical value to elucidate the effect of cadmium on animal performance and health and its possible biological mechanism.In this experiment,the effects of different doses of cadmium in diet on the performance of laying hens,the cadmium residual in eggs,the damage of cadmium to the liver and kidney of laying hens and its possible mechanism were studied.72 Hy-Line Brown laying hens of 300 days old were randomly divided into4 treatment groups:0 mg/kg,15 mg/kg,30 mg/kg and 60 mg/kg cadmium（calculated by Cd,added in the form of 2Cd Cl₂·5H₂O）.Each treatment has 6 repetitions,3 chickens for each repetition,and the experiment lasted 6 weeks.During the experiment,the egg production rate,feed intake and feed conversion ratio were recorded.The eggs were collected at the end of the 3rd and 6th week to determine the egg quality,and the eggs at the end of the 2nd,4th and 6th week were collected to determine the cadmium residual in the eggs.At the end of the 6th week,the chickens were slaughtered.Blood,kidney and liver samples were collected for the determination of serum biochemical indexes,liver and kidney antioxidant capacity,pathological sections,transmission electron microscopy of kidney,and transcriptome sequencing and verification.The results provide scientific basis for the prevention and control of cadmium damage to laying hens.The main results are as follows:1. Impact on production performanceAt the end of the 3rd week,30 mg/kg and 60 mg/kg Cd significantly decreased the feed conversion ratio（P<0.05）.At the end of the 6th week,60 mg/kg Cd significantly reduced the egg production rate and the feed conversion ratio（P<0.05）.2. Egg quality and cadmium residue in eggsAt the end of the 3rd week,60 mg/kg Cd significantly increased eggshell strength（P<0.05）,15 mg/kg and 60 mg/kg Cd significantly increased eggshell thickness（P<0.05）.At the end of the 6th week,60 mg/kg Cd significantly reduced the eggshell thickness（P<0.05）.In the 1st to 2nd weeks,the 3rd to 4th weeks and the 5th to 6th weeks,the cadmium residue in yolk increased significantly with the increase of cadmium content in feed,showing a significant linear correlation（P<0.001）.When the content of cadmium in the diet reached 60 mg/kg,the content of cadmium in the yolk was 9.0μg/kg,which was lower than the corresponding food hygiene standard.3. Renal damage caused by cadmiumCompared with the control group,30 mg/kg and 60 mg/kg Cd significantly increased the serum GLU content（P<0.05）.The results of pathological sections and transmission electron microscopy showed that in each group,the changes of glomeruli and renal tubules,nuclear and mitochondrial deformations were observed.15 mg/kg,30 mg/kg and60 mg/kg Cd significantly reduced the activities of GPX and CAT in kidney（P<0.05）.60 mg/kg Cd significantly reduced T-AOC（P<0.05）.The effect of Cd on residue and other trace elements in kidney showed that there was a linear relationship between cadmium residue in kidney and dietary cadmium（P<0.05）.Cadmium can also affect the content of microelements in kidney.30 mg/kg and 60 mg/kg Cd can reduce the content of Fe and increase the content of Zn in kidney（P<0.05）.In order to explore the toxic mechanism of cadmium on kidney,the control group and 60 mg/kg Cd group were used for transcriptome sequencing.Compared with the control,410 genes in the kidney of Cd treated laying hens changed more than 1.5 times（P<0.05）,221 genes were up-regulated and 189 genes were down-regulated.Combined with the functional analysis of GO and KEGG,some genes related to apoptosis and MAPK/Wnt/NFκB/TNF inflammatory signaling pathways were selected for fluorescent quantitative PCR verification.These genes include apoptosis related genes GADD45B and CDC14A,MAPK/Wnt/NFκB/TNF inflammatory signaling pathway related genes GHR,CACNA1D,FGF22,WNT5B,G6PC2,CREB3L1,EPHA2 and LPAR3.Compared with the control group,the m RNA levels of CACNA1D,CDC14A,GADD45B,G6PC2and GHR in the kidney after cadmium treatment were significantly down-regulated（P<0.05）.The m RNA expression levels of EPHA2,LPAR3,WNT5B,CREB3L1 and FGF22were significantly up-regulated in cadmium treated group（P<0.05）.The results of fluorescent quantitative PCR were consistent with those of sequencing.It can be seen that cadmium in diet plays an important role in kidney damage by regulating the MAPK/Wnt/NFκB/TNF pathway involved in inflammation and some genes involved in apoptosis.4. Effect of cadmium on the liver of laying hensThe activity of ALT and AST in serum was significantly increased by 60 mg/kg Cd in diet（P<0.05）.The results of pathological sections showed that cadmium in each dose group caused damage to the liver structure of laying hens,such as lymphocyte infiltration,bile duct epithelial hyperplasia and vasculitis.30 mg/kg and 60 mg/kg Cd significantly reduced the content of GSH in the liver（P<0.05）;the activity of GPX and the content of MDA in each dose group were significantly reduced（P<0.05）.Cadmium has an effect on the m RNA expression of antioxidation,metal binding and inflammation related genes in the liver.60 mg/kg Cd significantly down-regulated the m RNA expression of sod1,gpx1 and cat（P<0.05）.The m RNA expression level of mt4L and mt3 in each dose group was significantly down-regulated（P<0.05）.15 mg/kg and 60 mg/kg Cd significantly up-regulated the m RNA expression levels of TNF-α（P<0.05）.Therefore,the mechanism of hepatotoxicity of cadmium（above 15 mg/kg）in the diet may be due to the regulation of the expression of antioxidant related genes,metal binding and inflammatory factors by cadmium,thus changing the redox state of the liver and causing damage to the liver.Based on the above results,the conclusions of this experiment are as follows:（1）When the cadmium content in the diet reaches 60 mg/kg,the egg production rate and feed conversion ratio will be reduced,but there was no significant adverse effect on egg quality.（2）There was a positive correlation between the cadmium residue in yolk and the cadmium content in the diet.When the cadmium content in the diet reached 60 mg/kg,the cadmium content in the yolk was lower than the corresponding food hygiene standard.（3）Cadmium（15 mg/kg～60 mg/kg）in diet can cause kidney damage in laying hens,which may be related to the regulation of apoptosis and the expression of genes in MAPK/Wnt/NFκB/TNF inflammatory pathway.（4）Cadmium in the diet of 15 mg/kg～60 mg/kg can cause liver damage in laying hens.The mechanism of cadmium hepatotoxicity may be to regulate the expression of antioxidant and inflammatory factors.