Supplementary file 1_The optimal dietary crude protein level improves goat production performance by enhancing the body’s antioxidant function and energy metabolism.docx

The current research was conducted to evaluate the impact of various crude protein (CP) concentrations in diets on growth performance, nutrient digestibility, nitrogen deposition, rumen fermentation, microbial community, and serum metabolomics in growing goats. Fifty healthy 4-month-old Chuannan black goats (Capra hircus) with similar body weight (13.75 ± 0.27 kg) were randomly distributed into 5 groups. Goats were fed diets with five different levels of CP: 8.12% (T8), 10.15% (T10), 12.17% (T12), 14.13% (T14), and 16.18% (T16), respectively. The total duration of the trial was 70 d, including a 14-day adaptation period. The average daily gain and feed conversion ratio displayed a quadratic upsurge and reduce respectively, with the rise of CP content in the diet. The group T14 exhibited the highest average daily gain and demonstrated the best feed conversion efficiency. A linearly (p < 0.05) increase of the digestibility of dry matter, neutral detergent fiber, and acid detergent fiber was observed, whereas a quadratic effect (p < 0.001) on nitrogen intake, fecal nitrogen, and urinary nitrogen was obtained with the increase of dietary CP. Moreover, dietary CP levels had a quadratic effect on the concentration of ruminal ammonia nitrogen (p = 0.021), rumen microbial protein (p = 0.042), total volatile fatty acid (p = 0.012), acetate (p = 0.040), isobutyrate (p = 0.024), and isovalerate (p < 0.001). Microbial metagenomics results showed that the relative abundance of Burkholderia and Bacillus increased (p < 0.05), while the relative abundance of Pseudomonas and Salmonella decreased (p < 0.05) when comparing group T14 to group T8. Metabolomic results showed that differently expressed metabolites were found to enrich the proline, glutathione and arginine metabolism, and citric acid cycle metabolic pathway. The concentration of serum genistein was positively correlated (p < 0.05, r = 0.665) with the abundance of Bacillus and negatively correlated (p < 0.05, r = −0.734) with the abundance of Pseudomonas. It is concluded that a dietary CP level of 14% enhances the antioxidant function and energy metabolism of the goats by altering the composition of rumen microorganisms, thereby improving production performance.

本研究旨在评估日粮中不同粗蛋白质（crude protein, CP）水平对生长山羊生长性能、养分消化率、氮沉积、瘤胃发酵、微生物群落及血清代谢组学（serum metabolomics）的影响。选取50只健康的4月龄川南黑山羊（Capra hircus），初始体重为13.75±0.27 kg，将其随机分为5组。试验山羊分别饲喂粗蛋白质水平为8.12%（T8）、10.15%（T10）、12.17%（T12）、14.13%（T14）及16.18%（T16）的日粮。试验总时长为70 d，其中包含14 d的适应期。随着日粮粗蛋白质水平升高，山羊的平均日增重呈二次升高趋势，饲料转化率则呈二次降低趋势。其中T14组的平均日增重最高，饲料转化效率最佳。日粮干物质、中性洗涤纤维（neutral detergent fiber）与酸性洗涤纤维（acid detergent fiber）的消化率呈线性升高（p<0.05）；而日粮粗蛋白质水平对氮摄入量、粪氮及尿氮排泄量存在显著的二次效应（p<0.001）。此外，日粮粗蛋白质水平对瘤胃氨氮（ruminal ammonia nitrogen）浓度（p=0.021）、瘤胃微生物蛋白（rumen microbial protein）（p=0.042）、总挥发性脂肪酸（total volatile fatty acid）（p=0.012）、乙酸（acetate）（p=0.040）、异丁酸（isobutyrate）（p=0.024）及异戊酸（isovalerate）浓度（p<0.001）均存在显著的二次影响。微生物宏基因组学（microbial metagenomics）结果显示，与T8组相比，T14组瘤胃微生物中伯克霍尔德菌属（Burkholderia）与芽孢杆菌属（Bacillus）的相对丰度显著升高（p<0.05），而假单胞菌属（Pseudomonas）与沙门氏菌属（Salmonella）的相对丰度显著降低（p<0.05）。代谢组学结果表明，差异代谢物（differently expressed metabolites）主要富集于脯氨酸（proline）代谢、谷胱甘肽（glutathione）代谢、精氨酸（arginine）代谢及三羧酸循环（citric acid cycle）代谢通路。血清中染料木素（genistein）的浓度与芽孢杆菌属的丰度呈显著正相关（p<0.05，r=0.665），与假单胞菌属的丰度呈显著负相关（p<0.05，r=-0.734）。综上可知，当日粮粗蛋白质水平为14%时，可通过改变瘤胃微生物组成增强山羊的抗氧化功能与能量代谢，进而改善其生产性能。