Mechanism And Effects Of Selenomethionine And Bacillus Subtilis On Meat Quality In Broilers Under Heat Stress

Heat stress is a common phenomenon during breeding process of poultry,which can decrease the growth performance and meat quality.It seriously restricts the development of poultry industry.Previous study had shown that selenomethionine and B.subtilis could improve the meat quality of broilers.However,the mechanism is unclear.In this study,Ross308 broilers were used as the experimental object.Firstly,we established the heat stress model.Then,on the basis of this model,we systematically studied the effects of dietary selenomethionine and B.subtilis on the growth performance and meat quality of broilers,and elucidated the nutritional regulation mechanism at the tissue and molecular levels.1.The establishment of heat stress model and effects of dietary selenomethionine and B.subtilis on growth performance and meat quality of broilers under heat stressFive hundred and forty Ross 308 broilers（male）aged 21 days which were uniform weight and healthy were selected to conduct this study.Four hundred and eighty broilers were designed a 2×2×2 factorial arrangement.Temperature included normal temperature（22±2℃）and heat stress（32±1℃）;Selenium source included inorganic selenium（sodium selenite）and organic selenium（selenomethionine）;B.subtilis included addition（1×10⁹ cfu/kg）and no addition.According to factors and levels,broilers were randomly divided into 8 groups:normal temperature+sodium selenite+not add group（NC）,normal temperature+selenomethionine+not add group（NTS group）,normal temperature+sodium selenite+B.subtilis group（NTB group）,normal temperature+B.subtilis+selenomethionine group（NTBS group）,heat stress+sodium selenite+not add group（HS group）,heat stress+selenomethionine+not add group（HSS group）,heat stress+sodium selenite+B.subtilis group（HSB group）and heat stress+B.subtilis+selenomethionine group（HSBS group）,and to study the effects of temperature,selenium source and B.subtilis on heat stress index,growth performance and meat quality.We set up a pair-fed group（PF group,the feed intake was consistent with the HS group）to study the effects of feed intake on heat stress index,growth performance and meat quality by compared with NC and HS groups.Each group had 6replicates,and each replicate contained 10 broilers.The experiment was lasted for 21 days.The results as follow:（1）Compared with normal temperature,rectal temperature of broilers increased 0.45℃under high temperature（heat stress）,and heat stress extremely significantly increased the expression level of HSP60 gene in breast muscle（P<0.01）.Compared with NC and PF groups,HS group significantly increased the expression levels of HSP genes in breast muscle,thigh muscle,duodenum and jejunum,and significantly increased the concentration of serum corticosterone（P<0.05）.Feed intake had no significant effect on the above heat stress indexes（P>0.05）.Compared with inorganic selenium,organic selenium extremely significantly decreased the expression level of HSP60 gene in breast muscle（P<0.01）;Compared with without B.subtilis,B.subtilis extremely significantly reduced the expression level of HSP60gene in breast muscle（P<0.01）,and extremely significantly reduced the concentration of serum corticosterone（P<0.01）.Organic selenium and B.subtilis could alleviate the effect of heat stress by decreasing the expression levels of HSP genes.（2）Compared with normal temperature,heat stress extremely significantly reduced ADG and ADFI（P<0.01）,and significantly increased F/G（P<0.05）,and significantly reduced slaughter rate（SR）of broilers（P<0.05）.Selenium source and B.subtilis had no significant influence on growth and slaughter performance of broilers（P>0.05）.Compared with NC group,PF group significantly reduced ADG and ADFI of broilers（P<0.05）.（3）Compared with normal temperature,heat stress extremely significantly decreased p H,a_24h^* and b_24h^* values in the breast muscle（P<0.01）,and extremely significantly decreased p H_45min,a_45min^*,b_45min^* and b_24h^* values in the thigh muscle（P<0.01）,and significantly increased L^*value,drip loss,cooking loss,and shear force in breast and thigh muscles（P<0.05）.Compared with inorganic selenium,organic selenium extremely significantly increased p H,a_24h^* and b_24h^* values in the breast muscle（P<0.01）,and extremely significantly increased p H,a_45min^*,a_24h^*,b_45min^*,b_24h^* values in the thigh muscle,and significantly reduced L^*value,drip loss,cooking loss and shear force in breast and thigh muscles（P<0.05）.Compared with without B.subtilis,B.subtilis extremely significantly increased p H,a^*,b_45min^*,b_24h^* values in the breast muscle,and extremely significantly increased p H_45min,a_45min^*,b_45min^* and b_24h^*values in the thigh muscle,and extremely significantly reduced L^*value,cooking loss and shear force in breast and thigh muscles（P<0.01）.Feed intake had no significant effect on the above meat quality indexes（P>0.05）.2.Mechanism of heat stress reduced meat qualityWe selected NC,HS and PF groups,and the environmental temperature of NC and PF groups was 22±2℃,while the temperature of HS group was 32±1℃.We studied the mechanism of heat stress reduced meat quality from the aspects of muscle fiber characteristic,intramuscular fat deposition and intestinal microflora.The results as follow:（1）Compared with NC and PF groups,HS group extremely significantly increased the cross-sectional area and diameter of muscle fiber,and extremely significantly decreased the density of muscle fiber in breast and thigh muscles（P<0.01）;In the breast muscle,the expression levels of Myf5,MEF2A,MEF2D,CAPN2 and SM genes were significantly decreased,and the expression levels of Myostatin and FM genes were significantly increased in the HS group（P<0.05）;In the thigh muscle,the expression levels of Myf5,Myo G,Myo D,MRF4,MEF2A,MEF2D,CAPN2,SM,IGF-Ⅰ,IGF-Ⅱand CAPN3 genes were significantly decreased（P<0.05）,and the expression levels of FM and Myostatin genes were extremely significantly increased in the HS group（P<0.01）.（2）Compared with NC and PF groups,the concentration of intramuscular fat of breast and thigh muscles and fat apparent metabolic rate extremely significantly reduced in the HS group（P<0.01）;In the HS group,the expression levels of SLC27A1,H-FABP,A-FABP,APOB-100 and ADP genes in the breast muscle were significantly decreased（P<0.05）,and the expression levels of CPT1,Adipo R1 and MGLL genes in the breast muscle were extremely significantly increased（P<0.01）;In the HS group,the expression levels of SLC27A1,FAT,A-FABP,H-FABP,APOB-100,APOA1,FAS,ACC,ADP,PPARγ,C/EBPα,PPARα,SREBP-1 and SREBP-2 genes in the thigh muscle were extremely significantly decreased（P<0.01）,and the expression level of MGLL gene in the thigh muscle was significantly increased（P<0.05）;In the HS group,the expression levels of SLC27A1,FAT,A-FABP and H-FABP genes in the duodenum were extremely significantly decreased,and the expression levels of SLC27A1 and H-FABP genes in the jejunum were extremely significantly decreased（P<0.01）.Feed intake had no significant effect on the concentration of intramuscular fat,fat apparent metabolic rate and the expression levels of adipose deposition-related genes（P>0.05）.（3）Compared with the NC group,HS group significantly decreased the shannon index（P<0.05）,and decreased the abundance of Firmicutes,Lactobacillus and Bacteroides in the jejunum.（4）The results of correlation analysis showed that there was an extremely significant correlation between meat quality and muscle fiber characteristic,and between the meat quality and concentration of intramuscular fat（P<0.01）.3.Mechanism of selenomethionine and B.subtilis improved meat quality under heat stressWe selected HS,HSS,HSB and HSBS groups（the temperature was 32±1℃）,and designed a 2×2 factorial arrangement.Selenium source included inorganic selenium（sodium selenite）and organic selenium（selenomethionine）;B.subtilis included addition（1×10⁹cfu/kg）and no addition,and to study the mechanism of selenomethionine and B.subtilis improved meat quality under heat stress.The results as follow:（1）Under heat stress,compared with inorganic selenium or without B.subtilis,organic selenium or B.subtilis extremely significantly reduced the diameter of muscle fiber in the breast muscle（P<0.01）.Organic selenium（without bacteria,compared with inorganic selenium）and B.subtilis（fed inorganic selenium,compared with without bacteria）extremely significantly reduced the cross-sectional area of muscle fibers in breast and thigh muscles,and extremely significantly reduced muscle fiber diameter of the thigh muscle,and extremely significantly increased the muscle fiber density in breast and thigh muscles（P<0.01）.Compared with inorganic selenium,organic selenium extremely significantly increased the expression levels of MEF2D and SM genes in the breast muscle（P<0.01）,and extremely significantly reduced the expression level of Myostatin gene in the breast muscle,and significantly increased the expression levels of MRF4,Myf5,Myo D,MEF2A,MEF2D and CAPN3 genes in the thigh muscle（P<0.05）,and significantly reduced the expression level of FM gene in the thigh muscle（P<0.05）.Compared with without B.subtilis,B.subtilis significantly increased the expression levels of MEF2D,SM and Myf5 genes in the breast muscle（P<0.05）,and significantly reduced the expression level of Myostatin gene in the breast muscle（P<0.05）,and significantly increased the expression levels of Myf5,Myo D,MEF2A,MEF2D,CAPN2,CAPN3 and MRF4 genes in the thigh muscles（P<0.05）,and extremely significantly reduced the expression levels of Calpastatin and FM genes in the thigh muscle（P<0.01）.（2）Under heat stress,compared with inorganic selenium or without B.subtilis,organic selenium or B.subtilis extremely significantly increased the concentration of intramuscular fat in the thigh muscle and fat apparent metabolic rate（P<0.01）.Organic selenium（without bacteria,compared with inorganic selenium）and B.subtilis（fed inorganic selenium,compared with without bacteria）extremely significantly increased the concentration of intramuscular fat in the breast muscle（P<0.01）.Compared with inorganic selenium,organic selenium significantly reduced the expression levels of adiponectin,Adipo R1 and CPT1 genes in the breast muscle（P<0.05）,and significantly increased the expression levels of SLC27A1,FAT,H-FABP,ACC and PPARαgenes in the thigh muscle（P<0.05）,and significantly increased the expression levels of SLC27A1 and FAT genes in the duodenum（P<0.05）,and extremely significantly increased the expression level of H-FABP gene in the jejunum（P<0.01）.Compared with without B.subtilis,B.subtilis significantly increased the gene expression levels of ACC,GOS2,SQLE and SCD in the breast muscle（P<0.05）,and extremely significantly reduced the expression level of CPT1 gene in the breast muscle（P<0.01）,and significantly increased the expression levels of SLC27A1,FAT,H-FABP,ACC,PPARα,APOB-100 and FAS genes in the thigh muscle（P<0.05）,and significantly increased the expression levels of SLC27A1 and FAT genes in the duodenum（P<0.05）.In conclusion,heat stress reduced the growth performance,slaughter performance and gut microflora diversity of broilers,and reduced the meat quality through changing muscle fiber characteristics and reducing intramuscular fat deposition.Under heat stress,selenomethionine（without bacteria）,B.subtilis（fed inorganic selenium）changed the muscle fiber characteristics by regulating the expression levels of muscle fiber related genes;additionally,promoted fatty acid absorption and improved fat apparent metabolic rate by increasing the expression of fatty acid transport related genes in duodenum and jejunum,and increased intramuscular fat deposition by regulating the expression levels of fat deposition related genes in breast and thigh muscles;and improved meat quality by regulating muscle fiber characteristics and increasing intramuscular fat deposition.