Effects Of Protein Level On Growth,intestinal Health And Nutrient Deposition Of Piglets Fed Antibiotic-free Diets

The current and subsequent effects of dietary CP levels on growth performance,intestinal health,colonic microbes and metabolism,nutrient digestion and absorption,and body composition deposition of piglets fed antibiotic-free diets were evaluated in the present study.This study may provide a theoretical basis for protein nutrition of weaned piglets after the withdrawal of in-feed antibiotics.A total of 250 newly weaned piglets（21 d of age;6.39±0.03 kg body weight（BW）;Duroc×Landrace×Yorkshire）were allotted to 1 of 10 treatments（with 5 animals per replicate and 5replicates per treatment）in a 2×5 factorial arrangement with 2 antibiotic supplementations（with or without antibiotics）and 5 CP levels（16,18,20,22 and 24%）.Antibiotics contained 30 mg/kg bacitracin methylene disalicylate,75 mg/kg chlortetracycline and 300 mg/kg calcium oxytetracycline.Treatment diets were fed for 14 d（weaning transition period）,after which all piglets were fed the same nursery diet（19%CP）without antibiotics during d 15 to 25 kg BW（nursery period）until they reached an average BW of approximately 25 kg to determine if carryover effects of treatment diets existed.The main results were shown as follows:（1）Growth performance and organ index.Increasing dietary CP level linearly improved BW on d 14,and the ADG,ADFI and feed efficiency from d 0 to 14（P<0.05）,while in-feed antibiotics had no significant effect on the growth performance of weaned piglets（P>0.05）.Although decreasing CP level linearly reduced the growth performance of weaned piglets from d 0 to 14（P<0.05）,there was no difference in the experimental days and ADG of piglets from d 0 to 25 kg among groups（P>0.05）when the nursery diet switched to the normal CP level during d 15 to 25 kg.Increasing dietary CP level linearly improved the absolute weight of the liver,lungs,kidneys,and intestines（P<0.05）,but had no significant effect on their relative weight（P>0.05）,while in-feed antibiotics had no significant effect on the organ index of weaned piglets（P>0.05）.There was no significant difference in organ index among groups at 25 kg BW of piglets（P>0.05）,regardless of the dietary CP level and antibiotic supplement or not during d 0 to 14.（2）Intestinal health,colonic microbiota composition,and metabolites.Dietary CP level had no significant effect on intestinal morphology of weaned piglets（P>0.05）,while in-feed antibiotics significantly decreased crypt depth of duodenum（P<0.05）.There was no significant effect of CP level on diarrhea rate,p H of gastrointestinal contents,m RNA expression of tight junction protein and inflammatory factors in the intestinal mucosa of weaned piglets fed diets with or without antibiotics（P>0.05）.In-feed antibiotics significantly decreased the relative abundance of harmful bacteria such as Alloprevotella and Sutterella（P<0.05）at d 14,and then in-feed antibiotics withdrawal during d 15 to 25 kg BW significantly increased the relative abundance of harmful bacteria such as Alloprevotella and Oscillibacter（P<0.05）.In-feed antibiotics mainly down-regulated the compounds involved in amino acid metabolism（P<0.05）,which reduced protein fermentation in the hindgut,and then in-feed antibiotics withdrawal during d 15 to 25 kg BW up-regulated the compounds involved in amino acid metabolism（P<0.05）,which increased protein fermentation in the hindgut.Compared with 20%CP diet,16%CP diet significantly decreased the relative abundance of harmful bacteria such as Alloprevotella and Faecalicoccus（P<0.05）,and then significantly decreased the relative abundance of harmful bacteria such as Marvinbryantia and Neisseria（P<0.05）when the nursery diet switched to the normal CP level during d 15 to 25 kg.Compared with 20%CP diet,16%CP diet mainly up-regulated the compounds involved in tryptophan metabolism（P<0.05）,which are beneficial to the regulation of host immune and metabolic homeostasis,and then up-regulated the compounds involved in glycerophospholipid metabolism and fatty acid esters of hydroxy fatty acids metabolism（P<0.05）,which contributes to anti-inflammatory and strengthen the intestinal barrier.（3）Digestion,absorption and metabolism of nutrients.Increasing dietary CP level linearly improved serum urea nitrogen concentration（P<0.05）and linearly decreased serum triglyceride concentration（P<0.05）of weaned piglets（P>0.05）.Increasing dietary CP level linearly decreased serum threonine,valine,methionine,isoleucine and lysine concentrations（P<0.05）,linearly increased the serum tyrosine concentration（P<0.05）,and significantly increased the m RNA expression of B⁰AT1 in the jejunum and ileum mucosa of weaned piglets（P<0.05）,while in-feed antibiotics had no significant effect on serum biochemical parameters,serum free amino acid concentration and m RNA expression of amino acid transporters in the intestinal mucosa of weaned piglets（P>0.05）.Dietary CP level and antibiotics had no significant effect on apparent digestibility of nutrients in weaned piglets（P>0.05）.There was no significant difference in apparent digestibility,biochemical parameters and serum amino acid concentration of nutrients among groups at 25 kg BW of piglets（P>0.05）,regardless of the dietary CP level and antibiotic supplement or not during d 0 to 14.（4）Body composition deposition.Increasing dietary CP level linearly increased body protein content（P<0.05）,and linearly decreased body ash content,lipid:protein and ash:protein ratio（P<0.05）,while in-feed antibiotics had no significant effect on the body composition of weaned piglets（P>0.05）.Increasing dietary CP level linearly improved body water,protein and ash deposition rates（P<0.05）,and decreased body lipid deposition rate（P<0.05）,while in-feed antibiotics had no significant effect on body composition deposition rates of weaned piglets（P>0.05）.Although decreasing CP level linearly reduced the body protein content and body protein deposition rate of weaned piglets from d 0 to 14,there was no difference in body protein content at25 kg BW,and body protein deposition rate of piglets from d 0 to 25 kg among groups（P>0.05）when the nursery diet switched to the normal CP level during d 15 to 25 kg.（5）CP and energy utilization efficiency.Increasing dietary CP level linearly improved daily CP and metabolized energy（ME）intake,the CP intake per unit weight gain and the protein deposition per unit ME intake（P<0.05）,and linearly decreased the body protein and lipid deposition per unit CP intake,the ME intake per unit weight gain and the lipid deposition per unit ME intake of weaned piglets（P<0.05）.In-feed antibiotics had no significant effect on CP and energy utilization efficiency of weaned piglets（P>0.05）.（6）CP and energy requirement.Based on ADG,the optimal dietary CP level and daily CP requirement were 23.01%and 100.90 g/d for weaned piglets fed antibiotic-free diet,respectively.Based on body protein deposition（PD）,the optimal dietary CP level and daily CP requirement were24.00%and 101.70 g/d for weaned piglets fed antibiotic-free diet,respectively.Results showed that the factorial method[ME intake（MJ/kg）=0.444 MJ/kg BW^0.75+0.0444 MJ/g body protein deposition（g/d）+0.0523 MJ/g body lipid deposition（g/d）]could be used to partition ME intake of weaned piglets.In conclusion,although decreasing CP level linearly reduced the ADG and PD of weaned piglets from d 0 to 14,compensatory growth and compensatory PD may occur in piglets when the nursery diet switched to the normal CP level during d 15 to 25 kg.Moreover,decreasing CP level was conducive to the colonic microbial composition and its metabolites of weaned piglets,and continued to improve the stability of the intestinal microecosystem of piglets when the nursery diet switched to the normal CP level during d 15 to 25 kg.The present study may provide a theoretical basis for the study of the CP requirements of piglets during the weaning transition period under the background of in-feed antibiotic prohibition and the feasibility of the nutritional strategy of protein restriction during the weaning transition period followed by protein realimentation during the nursery period.