Effects And Mechanisms Of Leucine On Production Performance And Starch Digestion In The Small Intestine Of Lactating Goats

The degradation of starch in the rumen,small intestine and large intestine successively produces volatile fatty acids(VFA),glucose and VFA,it can be seen that the sites of digestion and utilization of starch in ruminants significantly affect the energy efficiency of starch.Therefore,Yao Junhu et al.put forward the theoretical formula of dietary starch energy efficiency(SEF): SEF(%)= 0.7 RDS + 1.0 SIDS + 0.4 LIDS,in which RDS,SIDS and LIDS are the percentage of starch degradation in the rumen,small intestine and large intestine in total starch intake,respectively.However,the starch digestibility of ruminants’ small intestine is about 60%.The lack of starch digestibility limits the energy value of starch.Studies have shown that insufficient pancreatic α-amylase secretion is the key factor limiting the digestion and utilization of starch in the small intestine of ruminants.The research of our team on young dairy cows and dairy goats shows that leucine can stimulate the secretion of pancreatic α-amylase through m TOR signal pathway.However,the above studies are limited to the cell or tissue level,or the experimental animals are in the state of fistula surgery,which can not fully reflect the role of leucine in the normal physiological state of animals.In addition,it is still unknown whether the increase of pancreatic α-amylase secretion will improve the starch digestibility of small intestine in ruminants,and the effect of leucine on the production performance of lactating goats remains to be studied.Therefore,in this study,rumen-protected leucine(RP-Leu)was added to the diet of lactating goats,and lysine,the main limiting amino acid of ruminants and substrate for protein synthesis,was used as a control to evaluate leucine’s effects on production performance and starch digestion in the small intestine of lactating goats,so as to provide a theoretical basis for the application of functional amino acids in lactating goats.Exp.1 Effects of leucine on production performance and immune indices of lactating goatsThis study was conducted to evaluate the effects of RP-Leu on production performance and immune indices of lactating goats.45 lactating goats(body weight: 43.11 ± 5.38 kg,first parity,day in milk: 135 ± 15 days,milk yield: 2.06 ± 0.45 kg/d)were random Ly assigned into one of the following dietary treatments:(1)CON: basal diet;(2)LEU: basal diet + 10.6 g/d RP-Leu;(3)LYS: basal diet + 12 g/d RP-Lys.The experimental period lasted 45 days after 15 days of gradual adaptation to the feeding system,the weight of dairy goats was weighed at the beginning and end of the experimental period for 2 consecutive days,the milk yield of lactating goats was recorded for 2 consecutive days every week as the milk yield of that week,and rumen fluid,blood,rectal feces,and milk samples were collected for analysis after the experiment.After the experiment,the experimental goats were mixed into a large group,and the follow-up situation of the experimental goats were continuously tracked,including the lambing rate,the number of lambing,body weight change and other indicators.The results showed that compared with CON group,the diet supplemented with RP-Leu increased the average daily gain of dairy goats(P < 0.05),but had no effect on DMI、milk yield and milk composition(P > 0.05),and tended to improve the dairy efficiency(BW)(0.05 ≤ P < 0.10);The addition of RP-Lys in diet had no effect on growth performance and lactation performance(P > 0.05).In reproductive performance,there was no difference in body weight tracking,weight gain and lamp birth weight among the three groups(P > 0.05).Compared with CON group,the lambing rate of LEU group increased by 28%.The addition of RP-Leu and RP-Lys in diet had no significant effect on glutamic oxaloacetic transaminase,glutamic pyruvic transaminase,total protein,albumin,globulin,Ig A,IL-6,IL-1β,IFN-γ and TNF-Α of lactating goats(P > 0.05).This study showed that the dietary addition of RP-Leu increased the ADG of lactating goats,and had no effect on lactation and immune indicators.Exp.2 Effects of leucine on starch digestion in the small intestine of lactating goatsAs the main limiting amino acid of ruminants,lysine has no effect on the production performance of lactating goats,indicating that leucine has other functions in addition to being a protein substrate.Therefore,based on Exp.1,this study focused on the effects of leucine on the starch digestion ability in the small intestine of lactating goats,so as to explain the possible reasons for the increase of ADG of lactating goats,and the effect of lysine will not be discussed.The experimental design and animal feeding are the same as Exp.1,total digestive apparent digestibility of nutrients,rumen and rectal fecal fermentation parameters,rectal fecal microbes,blood biochemical parameters,serum free amino acids,milk hydrolyzed amino acids and other indicators were detected.The results have shown that the addition of RP-Leu did not affect apparent total tract digestibility of starch and rumen fermentation parameters in lactating goats(P > 0.05).The feces of goats fed diet supplemented with RP-Leu reduced molar proportion of butyrate,and increased p H value and molar proportion of acetate compared with that of goats fed the basal diet(P < 0.05).Gram-positive bacteria Firmicutes and gram-negative bacteria Bacteroidetes are the bacteria with the highest abundance in the level of rectal fecal microbiota,and the total abundance of the two bacteria accounts for about90% of the total bacterial abundance.Dietary supplementation of RP-Leu increased the relative abundance of Christensenellaceae,Christensenellaceae＿R-7,Ruminococcaceae and Ruminococcus,while reduced the relative abundance of Lachnospiraceae and Anaerostipes in goat’s feces(P < 0.05).In addition,the addition of RP-Leu in diet increased the content of serum low density lipoprotein cholesterol(P < 0.05)and tended to increase the content of total cholesterol and high density lipoprotein cholesterol(0.05 ≤ P < 0.10)Compared with CON,dietary supplementation of RP-Leu increased the concentration of BCAA、Ile、Thr、Leu and EAA,decreased the concentration of Pro(P < 0.05),and tended to increase the concentration of Trp and TAA in the serum(0.05 ≤ P < 0.10).Goats fed with RP-Leu has no effect on milk amino acid composition of lactating goats(P > 0.05).In conclusion,dietary supplementation of RP-Leu probably improved starch digestibility in the small intestine of lactating goats showing as reduced hindgut starch fermentation and unchanged starch intake,apparent total tract digestibility of starch and rumen fermentation parameters.Leucine can increase the content of glucogenic amino acids in lactating goats’ serum,which will improve the body’s amino acid metabolism and nutritional supply.This study showed that the addition of RP-Leu to the diet of middle and late lactation stage’s lactating goats could promote the starch digestion of small intestine,improve the nutritional status of amino acids and fat condition,and may improve the reproductive performance of the next birth.