Effects And Underlying Mechanisms Of Compound Organic Acids Utilized As Alternatives To Antibiotics Or Zinc Oxide In Weaned Piglet Diets

In this thesis,three large-scale animal feeding trials were conducted to investigate the effect of compound organic acids,composed of short chain fatty acids?SCFAs?and medium chain fatty acids?MCFAs?,on growth performance and health status of weaned piglets fed compound organic acids-supplemented diets under different feeding conditions or under the premise that antibiotics?AB?or zinc oxide?ZnO?was removed from the diets.Subsequently,digestion and metabolism trials and comparative slaughter trials were conducted to investigate the underlying mechanisms of compound organic acids working as replacement of AB or ZnO,by determining apparent ileal digestibility?AID?of gross energy,calcium,phosphorus and amino acids using the exogenous indicator?Cr₂O₃?method,the expression of genes encoding the proteins responsible for glucose or amino acids transport and cytokines involved in inflammatory responses in the small intestine using quantitative polymerase chain reaction?qPCR?method,the gut microflora diversity and relative abundance of main bacteria species using the Denaturing Gradient Gel Electrophoresis?DGGE?techniques and PCR method.The objective of this study is to provide experimental data support for compound organic acids to be utilized as the replacement of antibiotics or ZnO in weaned piglet diets,and to provide theoretical basis for green healthy production of pigs in the post-antibiotic era.A total of four trials conducted in this study are described as follows:Exp.1 Effects of dietary supplementation with compound organic acids on growth performance of weaned piglets under different feeding conditionsThis study was conducted to determine the effects of different doses of compound organic acids on the growth performance of weaned piglets under different rearing conditions,with two large-scale feeding trials designed.The same experimental diets were used in the two experiments.It was divided into three treatments according to the experimental diets:treatment 1?T1?,negative control?NC?group,fed basal diets without antibiotics?AB?and without zinc oxide?ZnO?,T2,NC+3SF3 group,fed basal diets supplemented with 3.0 g/kg SF3?34%calcium formate,16%Calcium lactate,7%citric acid and 13%medium-chain fatty acids?,and T3,NC+5SF3 group,fed basal diets supplemented with 5.0 g/kg SF3.Two trials under different feeding conditions were conducted using these dietary treatments.In trial 1,a total of 180?84 females and 96 castrated males?of 21 day old weaned piglets?Duroc×large white×Yorkshire,DLY?were randomly divided into 3 groups according to body weight?6.64+0.97?and sex.Each group had 6 replicate pens?10 pigs/pen?,of which 4 pens housing half males and the other two pens housing 6:4 of male:females.All pigs were fed a prestart diet for a week after weaning.Then they were replaced with experimental diets NC,NC+3SF3 or NC+5SF3.The experiment lasted for two weeks,and the ambient temperature before and during the experiment was suitable and relatively stable?26-32??.In trial 2,a total of 128?half female?of 21 days old weaned piglets were randomly divided into 3 groups according to their weight and sex.There were 6,5 and 5 replicate pens for NC,NC+3SF3 and NC+5SF3treatment,respectively,with 8 pigs?half female?in each pen.The experimental diets NC,NC+3SF3 or NC+5SF3 were fed directly from the day of weaning.During the experiment,the room temperature continued to drop,and the temperature at 7:00 a.m.dropped from 26.5?on the day of weaning to 19?on the sixth day of weaning.The trial lasted for one week and was terminated due to extensive diarrhea.Results showed that at relatively stable room temperature?Trial 1?,there was no significant?P>0.05?difference in body weight?BW?,average daily gain?ADFI?,and feed/gain?F/G?among dietary treatment groups during any of experimental stage.However,in Trial 2,although there was no significant?P>0.10?difference in feed intake among treatments,the weight of piglets fed NC+5SF 3 diet increased positively on the seventh day after weaning,while the weight of piglets fed NC and NC+3SF3 diet increased negatively.Moreover,the frequency of diarrhea increased from the second day after weaning to the seventh day after weaning.The diarrhea frequency of piglets in NC group?56.4%?and NC+3SF3 group?62.8%?was more than 55%,while that in NC+5SF3 group was 43.4%,which was significantly?P<0.05?lower than that in NC+3SF3 group and tended?P<0.10?to be lower than that in NC group.At the end of the trial,the frequency of diarrhea in NC+5SF3 group?20.5%?was significantly?P<0.05?lower than that in NC group?26.5%?and NC+3SF3 group?27.8%?.The results of this study show that the post-weaning rearing condition has an important influence on the response of weaned piglets to SF3 supplementation.When there is one-week adaptation after weaning,and continuously-appropriate ambient temperature,adding organic acid mixture SF3 has no effect on piglet growth performance.However,in case of direct feeding after weaning,and the room temperature is lower than the appropriate level or under other extreme feeding conditions,the dietary supplementation of SF3 has a positive effect on reducing diarrhea in weaned piglets.In a nutshell,under conditions of complete absence of antibiotics and zinc oxide,organic acid mixture SF3 itself is not enough to effectively protect the intestinal health of piglets,which suggests the potential scenario to apply SF3 under commercial production conditions.It also shows that scientific application of SF3 and research on mechanisms of action for SF3 is worthy of further studying.Exp.2 Effects of Compound Organic Acids Replacing Antibiotics or Zinc Oxide on Growth Performance of Weaned PigletsThe experiment was designed by a completely randomized trial with four treatments.The control group was fed with diets supplemented with antibiotics?AB,colistin sulfate and enramycin,each 20 mg/kg diet?and zinc oxide?ZnO,2.5 g/kg diet?.The test 1?T1?group was fed diets swupplemented with compound organic acids SF2?containing 46%formic acid,16%milk?at 3 kg/t diet.T2 group?ZnO+SF3?was fed diets without antibiotics but supplemented with SF3 at 3 kg/t diet.T3 group?AB+3SF3?was fed diets without ZnO but supplemented with SF3 at 3 kg/t diet.A total of 240 of 21 day old weaned piglets?DLY?were randomly divided into 4 groups according to their weight and sex.All pigs were allowed to use the prestart diet for one week after weaning,and then fed the experimental diets?no ZnO was added to the diets for the second two weeks?.Each dietary treatment had 6 replicate pens with 10 piglets?half female?per pen.All pigs had free access to feed and water throughout the experimental period.The results showed that,compared with the control group fed ZnO+AB diet,1)the end body weight,average daily gain and feed intake of the pigs fed ZnO+SF2 diets increased significantly?P<0.05?in the first week,but the ratio of feed to gain increased significantly?P<0.05?at week 2 and at the second two weeks.2)the pigs fed ZnO+SF3 showed increased body weight and average daily gain in week 1 and higher?P<0.05?feed intake in the first two weeks,but there was no significant difference in feed intake;3)the pigs fed the AB+SF3 diet showed increased?P<0.05?body weight,average daily gain and feed intake in the first two weeks,while the feed/gain was not different.Compared with the pigs fed ZnO+SF2 diets,the average daily gain of the pigs fed ZnO+SF3 diets increased significantly?P<0.05?and the feed to gain ratio decreased significantly?P<0.05?in week 2.The results showed that under commercial production conditions,dietary supplementation of complex organic acids consisting of short-chain fatty acids?SCFA?and medium-chain fatty acids?MCFA?was more conducive to improving the health status of piglets,promoting the growth of piglets and improving feed conversion efficiency than that of organic acids consisting solely of SCFA.Exp.3 Study on the mechanism of compound organic acids substituting antibiotics on growth performance of Weaned PigletsOn the basis of Experiments 1 and 2,digestion and metabolism trial and comparative slaughter tests were conducted to determine the effects of SF3 replacing antibiotics at 5 kg/t diet on immunity and nutrient digestion and absorption of weaned piglets.Fourteen28-day-old weaned piglets?DLY?were selected and fed separately in metabolic cages.All pigs were fed the control diet?ZnO+AB?for one week after weaning,and then 7 of them were fed the ZnO+5SF3 diet without antibiotics.Each diet replicated 7 times?n=7?.All pigs were housed in temperature?26??control room,and had free access to feed and water.The experimental period lasted two weeks.Result showed that compared with the control group,the SF3-fed piglets had greater?P<0.05?apparent ileal digestibility of calcium and dry matter,while also demonstrating greater?P<0.05?overall gross energy digestibility,up-regulated?P<0.05?jejunal expression of SGLT1 and TGF-?,down-regulated?P<0.05?jejunal expression of TNF-?,along with higher?P<0.05?ileal Lactobacillus content,lower?P<0.05?total bacteria content,lower?P<0.05?plasma TNF-?but higher?P<0.05?Ig-G levels.The results showed that dietary SF3 supplementation and substitution of antibiotics could promote the expression of proteins related to jejunal glucose transport and improve the digestibility and absorption of dietary nutrients in weaned piglets.At the same time,SF3could help to improve the biodiversity of intestinal microflora,regulate the expression of intestinal immune-related genes,increase plasma antibody levels and enhance Immunity of weaned piglets.This provides a partial explanation for the effect of SF3 on alleviating diarrhea in piglets as observed in Experiment 1.Exp.4 Study on the mechanism of compound organic acids substituting Zinc oxide on growth performance of Weaned PigletsIn order to further explore the mechanism of SF3 substituting for zinc oxide to improve the performance of piglets,the effects of SF3 substituting for zinc oxide on nutrient digestibility,immunity and intestinal microflora of weaned piglets were studied through digestion and metabolism experiments and comparative slaughter tests.Fourteen crossbred piglets?28-day old,9.05±1.38 kg?were housed individually in metabolism cages,fed the CON?AB+ZnO?diet for one week and then fed the CON?AB+ZnO?or SF3?AB+3SF3?diet for two weeks with 7 pigs?4 female and 3 male?per diet.Comparing with the CON-fed piglets,the SF3-fed piglets had higher?P<0.05?AID of the main part of AAs in diet,lower?P<0.05?plasma tumor necrosis factor alpha?TNF-??but higher?P<0.05?plasma immunoglobulin-G concentrations,higher ileal and rectal Lactobacillus content?P<0.05?,and upregulated?P<0.05?jejunal AA transporters?EAAT3and CAT2?expression and regulatory cytokine?transforming growth factor beta?expression but downregulated?P<0.05?jejunal proinflammatory cytokine?TNF-??expression.The results of this experiment showed that adding SF3 to replace ZnO in the diet can promote the expression of jejunal amino acid transport related carriers and increase the rate of digestion and absorption of dietary amino acids in weaned piglets.At the same time,SF3could help to improve the biodiversity of intestinal microflora and promote the proliferation of lactic acid bacteria,and regulates intestinal immune-related gene expression,increases plasma antibody levels and enhances piglet immunity.These results provide the mechanistic explanations on the role of SF3 in reducing the disease incidence of piglets and improving production performance in the Exp.1 and 2.In this thesis,a complete and in-depth discussion on the results from the previous four experiments leads to the following conclusions:1.Organic acid mixture SF3,as an alternative for antibiotics or zinc oxide,has the effect to improve growth performance and reduce the disease incidence of piglets.The stable environmental temperature and smoothly transition after weaning when carrying on the replacement of antibiotics or zinc oxide by SF3,are the key elements to ensure the SF3 effects on growth enhancement and animal health.2.The improvement on the growth performance of piglets is related to the improvement of digestibility of nutrients.To be more specific,SF3 increased the apparent digestibility of dry matter,total energy and calcium,which was associated with increased expression of the jejunum-glucose cotransporter SGLT1 and decreased intestinal acidity;SF3 increased the apparent ileal digestibility of the main amino acids in the diet,which is partially related to the up-regulation of CAT2,a jejunal amino acid transporter.3.The role of SF3 on strengthening the ability of piglets to resist diarrhea and reduce the incidence of diarrhea is related to the piglets'immunity improvement.Furthermore,the up-regulation of regulatory cytokines and the down-regulation of pro-inflammatory cytokines may be related to the proliferation of intestinal bacteria Lactobacillus after feeding with organic acids.The increase in the concentration of lactobacillus and diversification of microbial communities in the intestinal tract of piglets may be associated with the pH decrement in the gastrointestinal tract following SF3 addition.