Effects Of Dietary Selenium Sources And Selenium Levels On Histomorphology Of Digestive Tract,Cecal Fermentation Function And Bacterial Flora Structure In Meat Donkeys

Selenium sources include inorganic selenium and organic selenium.Previous studies have concluded that adding an appropriate level of organic selenium to the diet can promote the fattening performance and nutrient digestion and metabolism of meat donkeys better than inorganic selenium,but high doses of organic selenium do not.Conducive to the growth and development of meat donkeys.On the basis of previous research,this thesis mainly studies the effect of selenium source and selenium level on the tissue morphology of digestive tract,cecal fermentation function and bacterial flora structure.The influence mechanism of metabolism,guiding the scientific addition of selenium in meat donkey diets provides data theoretical support.The experiment adopted a single-factor randomized trial design.Twenty-four meat donkeys（males）with an age of 1 year and a similar weight（120±25kg）were randomly divided into SS group,SY1 group and SY2 group,with 8donkeys in each treatment group.The SS group was supplemented with 0.3 mg/kg inorganic selenium（sodium selenite）in the basal diet,and the SY1 and SY2 groups were supplemented with 0.3 mg/kg and 0.5 mg/kg organic selenium（yeast selenium）in the basal diet,respectively.All donkeys were house-fed and fattened,and the experiment was divided into pre-fat（1-6 weeks）,middle（7-12 weeks）and late（13-18 weeks）.After the test,all the test donkeys were fasted for 12 hours and water for 2 hours before being slaughtered,and the tissue samples of the stomach,small intestine,cecum,and colon as well as the contents of the cecum and colon were collected.The test mainly consists of four parts:Experiment 1 mainly studied the effects of dietary selenium sources and selenium levels on the tissue morphology of the digestive tract of meat donkeys.The results showed that compared with the SS group and the SY2 group,the SY1 group significantly increased the villus length of the duodenum and jejunum and the villi-gland ratio of the small intestine（P≤0.05）.The width of ileal villi in SY1 group was significantly higher than that in SS group and SY2 group（P=0.002）,and the results of duodenal crypt depth were opposite（P<0.0001）.The thickness of duodenum,ileum and cecum mucosa in SY1 group was significantly higher than that in SS group and SY2 group（P=0.018,P=0.001,P<0.0001）.Compared with the SS group,the thickness of the muscularis mucosa of the duodenum and cecum in the SY group was significantly increased（P=0.018,P=0.024）;the thickness of the muscularis mucosa of the jejunum in the SY1 group was significantly higher than that in the SS group（P=0.042）.The number of cecal glands in SY1 group was higher than that in SS group and SY2 group（P=0.001）.This shows that compared with inorganic selenium,0.3mg/kg organic selenium promotes the development of the tissue morphology of the digestive tract of meat donkeys;but 0.5mg/kg organic selenium has an inhibitory effect.Experiment 2 mainly studied the effects of dietary selenium sources and selenium levels on the activities of enzymes in the stomach and small intestine of meat donkeys.The results showed that the gastric lipase activity of meat donkeys in SS group and SY1 group was significantly higher than that in SY2 group（P=0.024）.The lipase activities of jejunum and ileum and theα-amylase activities of duodenum,ileum and pancreas in SY1 group were significantly higher than those of the other two groups（P≤0.05）.Compared with the SS group,the pancreatic lipase activity in the SY group was significantly increased（P=0.001）.Compared with the SS group and the SY1 group,the duodenal and jejunal trypsin activities in the SY2 group were significantly decreased（P=0.015,P<0.0001）.Compared with inorganic selenium,0.3 mg/kg organic selenium could increase the digestive enzyme activities in the gastrointestinal tract of meat donkeys;with the further increase of selenium levels,the above-mentioned digestive enzyme activities decreased.Experiment 3 mainly studied the effects of dietary selenium sources and selenium levels on the fermentation function of the cecum and colon of meat donkeys.The results showed that the concentration of ammonia nitrogen in the cecum and colon of the SY1group was significantly higher than that of the other two groups（P=0.019,P=0.016）.Compared with the SS group,the bacterial protein concentration in the cecal fluid of the SY group was significantly increased（P=0.013）;the bacterial protein concentration in the colonic fluid of the SY1 group was significantly higher than that of the SS and SY2 groups,and the SS group was significantly higher than the SY2 group（P=0.001）.The concentrations of acetic acid,propionic acid,butyric acid and total volatile fatty acids in the cecum and colonic fluid of the SY1 group were significantly higher than those of the other two groups（P<0.05）;the cecal fluid acetic acid concentration of the SS group was significantly lower than that of the SY2 group（P=0.0002）.The valeric acid concentration,ethylene-propylene ratio and the isobutyric acid concentration in the colonic fluid of the SY1 group were significantly higher than those of the other two groups（P<0.0001,P=0.010,P<0.0001）;the SS group had significantly higher concentrations of isobutyric acid in the colonic fluid higher than that in the SY2 group（P<0.0001）.Experiment 4 mainly studied the effect of dietary selenium source and selenium level on the bacterial flora structure in the cecum of meat donkeys.The results showed that the Sobs,Chao and Ace indices of bacterial flora in the cecum of the SY1 group were significantly higher than those of the other two groups（P=0.001,P<0.0001,P=0.0003）.Compared with the SY1 group,the cecal flora structure of the meat donkey in the SS group was significantly different,and the difference in the SY2 group was smaller.At the phylum level,the relative abundances of Fusobacterium and Cyanobacteria in the cecum of the meat donkey in the SY1 group were significantly higher than those in the other two groups（P=0.006 and P=0.004）;compared with the SS group,the relative abundance of Proteobacteria in the SY1 and SY2 groups The relative abundance of Actinobacteria was significantly increased（P=0.004）in the SY1 group（P=0.027）.At the genus level,51genera such as Prevotella and Lachnospira were significantly enriched in the cecum of the meat donkey in the SS group,and 37 genera including Lactobacillus,norank＿f＿＿norank＿o＿＿WCHB1-41 and Vibrio succinate were significantly enriched in the SY1 group.In the SY2 group,9 species of bacteria,including Sarcinus and Lachnospira,were significantly enriched.The results of correlation analysis showed that the relative abundances of Lactobacillus,norank＿f＿＿norank＿o＿＿WCHB1-41,Vibrio succiniformis,Prevotella and Sarcinus in the cecum of meat donkeys were closely related to NH₃-N,MCP,acetic acid,propionic acid,butyric acid,valeric acid and TVFA and ethylene-propylene ratio were significantly positively correlated,while Lachnospiraceae was the opposite.This indicated that 0.3mg/kg organic selenium had better effect on promoting the fermentation function of the cecum and colon of meat donkeys than inorganic selenium,but the organic selenium level was less than 0.5mg/kg,which was related to the change of cecal flora structure.To sum up,0.3mg/kg organic selenium was better than inorganic selenium in promoting the tissue morphology and enzyme activity of meat donkey digestive tract,cecal fermentation function and regulating bacterial flora structure,but 0.5mg/kg organic selenium was better than inorganic selenium.On the contrary,it had a negative impact on the above indicators.