牦牛作为多经济用途的高原特种牛种，其养殖业是青海省的特色优势产 业，舍饲育肥是青海省牦牛产业化养殖发展大环境下的积极探索，但由于牦牛 特殊的日粮结构，目前舍饲育肥牦牛尚无专门化日粮饲喂体系。同时，舍饲条 件下如何提高育肥牦牛日粮能量利用效率、优化瘤胃发酵和加快脂肪沉积的分 子机理尚不明确。基于此，本试验以舍饲育肥牦牛为研究对象，随机分为 3 组，通过饲喂不同精粗比日粮（A 组：精:粗=3:7、B 组：精:粗=5:5、C 组：精: 粗=7:3），探究日粮精粗比对舍饲育肥牦牛机体代谢的影响，期望为青海牦牛规 模化、产业化和现代化养殖过程中日粮精粗比调控提供参考资料。试验分为 3 部分，具体结果如下： 1、日粮精粗比对牦牛生产性能和血液指标的影响 日粮不同精粗比对舍饲育肥牦牛生产性能和血液指标影响显著。HE 切片显 示，本试验日粮精粗比未对牦牛肝脏组织造成损伤。随着日粮精料占比上升， 出栏体重、平均日增重（ADG）、胴体重以及屠宰率均显著增加（P﹤0.05）；日 干物质采食量（ADFI）和料重比显著下降（P﹤0.05）。当饲喂日粮精粗比为 7:3 时，舍饲育肥牦牛产生的利润最多。随着日粮精料占比上升，牦牛血常规指 标中白细胞水平显著下降（P﹤0.05），红细胞和血小板水平显著升高（P﹤ 0.05）。随着日粮精料占比增加，牦牛血清中甘油三酯（TG）、高密度脂蛋白胆 固醇（HDL-C）、低密度脂蛋白胆固醇（LDL-C）和游离脂肪酸（NEFA）浓度 显著升高（P﹤0.05），C 组牦牛血清中葡萄糖（GLU）、总胆固醇（T-CHO）显 著高于 A 组（P﹤0.05），生长激素（GH）浓度显著降低（P﹤0.05），胰岛素 （INS）和胰岛素样生长因子 1（IGF-1）显著升高（P﹤0.05）。 2、日粮精粗比对牦牛瘤胃代谢的影响 日粮精粗比显著影响瘤胃细菌菌群结构，试验表明，舍饲育肥牦牛瘤胃门 水平上，日粮精粗比对牦牛瘤胃细菌中厚壁菌门（Firmicutes）、拟杆菌门 （Bacteroidetes）、螺旋体门（Spirochaetes）、黏胶球形菌门（Lentisphaerae）和 软壁菌门（Tenericutes）影响显著（P﹤0.05）。随着精料占比升高， Bacteroidetes、Spirochaetes 和 Lentisphaerae 相对丰度显著上升（P﹤0.05）， Firmicutes 和 Tenericutes 相对丰度显著下降（P﹤0.05）。就属水平而言，随着精料占比升高，理研菌科 RC9 肠道群（Rikenellaceae RC9 gut group）、克里斯腾森 菌科 R7 群（Christensenellaceae R7 group）、Saccharofermentans、产琥珀酸菌属 （Succiniclasticum）和 Lachnoclostridium 相对丰度显著升高（P﹤0.05），C 组 牦牛瘤胃球菌属（Ruminococcus）和拟杆菌属（Bacteroides）相对丰度显著低于 A 组（P﹤0.05），B 组和 C 组牦牛普氏菌属（Prevotella）、假丁酸弧菌 （Pseudobutyrivibrio）和密螺旋体属（Treponema）相对丰度显著高于 A 组（P ﹤0.05）。日粮精粗比对瘤胃 pH 及挥发性脂肪酸（VFA）产量影响显著，精粗 比上升显著下调瘤胃内容物 pH(P﹤0.05)，乙酸含量、乙酸/丙酸显著降低(P﹤ 0.05)，丙酸、戊酸和总 VFA 含量显著升高(P﹤0.05)，与 A、B 组相比，C 组丁 酸和异丁酸含量显著升高(P﹤0.05)。日粮精粗比对瘤胃上皮 VFA 转运载体表达 量影响显著。随着日粮精料占比升高，腺瘤下调蛋白（DRA）、假定阴离子转 运载体 1（PAT1）、单羧酸转运载体 1（MCT1）和单羧酸转运载体 4（MCT4） 在 mRNA 水平和蛋白水平的相对表达量显著升高（P﹤0.05），而阴离子交换蛋 白 2（AE2）的含量在二者水平上则显著下降（P﹤0.05）。 3、日粮精粗比对牦牛肝脏、小肠及皮下脂肪中脂肪代谢的影响 饲喂不同精粗比日粮显著影响舍饲育肥牦牛肝脏、小肠及皮下脂肪中脂肪 合成和脂肪酸氧化基因 mRNA 和蛋白水平的表达。肝脏、小肠及皮下脂肪中脂 肪合成的基因固醇调节元件结合蛋白 1c（SREBP-1c）、乙酰辅酶 A 羧化酶 （ACC）、酯酰辅酶 A 去饱和酶 1（SCD1）和脂肪酸合成酶（FASN）的 mRNA 和蛋白表达水平随日粮精料占比升高而上升，脂肪分解的基因过氧化物酶体增 殖物激活受体 α（PPARα）、酯酰辅酶 A 氧化酶（ACO）、肉碱酯酰辅酶 A 转移 酶 1（CPT1）和激素敏感性脂解酶（HSL）mRNA 和蛋白表达量随日粮精料占 比升高而降低。 综上所述，在舍饲育肥模式下提高日粮精料水平可有效改善牦牛生产性 能、瘤胃发酵和脂肪代谢水平。饲喂高精料日粮可提升牦牛瘤胃中非纤维素降 解菌的相对丰度，使大量能量饲料和蛋白饲料快速转化为 VFA，为机体供能。 激活瘤胃上皮细胞中 VFA 转运的载体，提高其表达量，加快 VFA 转运入血的 速度，以提高脂肪代谢底物。随着日粮精料占比升高，肝脏、小肠和皮下脂肪 中参与脂肪合成的基因表达量上升，机体利用底物提高脂肪合成速率，同时脂 肪分解速率降低，脂肪沉积加快。本试验全面解析了舍饲育肥条件下，牦牛饲 喂不同精粗比日粮，其生产性能、瘤胃代谢水平以及脂肪代谢的分子机理，为 青海省舍饲育肥牦牛的规模化、产业化养殖提供了重要参考资料。

Yak, a plateau special cow, which is multi-economic use. Yak breeding industry is a characteristic and advantageous industry in Qinghai Province. House-fed and fattening is an active exploration under the environment of the development of yak industrialized breeding in Qinghai Province. However, due to the special diet structure of yaks, there is no specialized diet feeding system for house feeding and fattening yaks at present. Meanwhile, the mechanism of how to lift the feed utilization rate of fattening yak diet, optimize rumen fermentation and accelerate fat deposition under house feeding conditions is unclear. Therefore, this experiment is based on the fertilizer yak to the research object. It is randomly divided into 3 groups. They are fed with diets with different concentrate-to-roughage ratios (group A: concentrate: roughage = 3:7, group B: concentrate: roughage = 5:5, group C: concentrate: roughage = 7:3), to explore its effect on the body metabolism of fattening yaks. It is expected to provide reference for the regulation and control of diet in Qinghai yaks in the process of large-scale, industrial and modern breeding. The experiment is divided into three parts and the specific results are as follows:

1. Effects of dietary concentrate-to-forage ratio on production performance and blood indexes of yaks

Different levels of concentrate-to-forage ratio in diet had significant effects on the production performance, blood routine and serum biochemical indexes of house-fed yaks. HE slice shows that the ratio of concentrate to roughage in diet did not cause damage to yak liver tissue. With the increase of dietary concentrate level, final body weight, average daily gain , carcass weight and slaughter rate increased significantly (P < 0.05); The daily dry matter intake and material to weight ratio decreased significantly (P < 0.05). When the ratio of concentrate to roughage was 7:3, the maximum profit of fattening yaks was obtained. With the increase of dietary concentrate level, the level of white blood cells in yak blood routine indexes decreased significantly (P < 0.05), and the levels of red blood cells and platelets increased significantly (P < 0.05). With the increase of dietary concentrate level, the concentrations of triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and free fatty acid (NEFA) in yak serum increased significantly (P < 0.05). Group C yak serum glucose (Glu), total cholesterol (T-CHO) is significantly higher than that in group A (P < 0.05). The concentration of growth hormone (GH) decreased significantly (P < 0.05), and insulin-like growth factor-1 (IGF-1) increased significantly (P < 0.05). The content of insulin (INS) in yak serum in group C was significantly higher than that in group A and group B (P < 0.05).

2. Effects of dietary concentrate-to-forage ratio on rumen bacterial flora structure, fermentation parameters and VFA transporter expression in rumen epithelium of yaks

The ratio of concentrate to roughage significantly affected the structure of rumen bacterial flora. The results showed that the ratio of concentrate to roughage had significant effects on Firmicutes, Bacteroidetes, Spirochaetes, Lentisphaerae and Tenericutes in rumen bacteria of yak. The relative abundances of Bacteroidetes, Spirochaetes and Lentisphaerae increased significantly with the increase of the proportion of concentrate (P < 0.05). The relative abundances of Firmicutes and Tenericutes decreased significantly (P < 0.05). On the level of the genus, With the increase of concentrate proportion, the relative abundance of Rikenellaceae RC9 gut group, Christensenellaceae R7 group, Saccharofermentans, Succiniclasticum and Lachnoclostridium increased significantly (P < 0.05). Ruminococcus and Bacteroides in Group C have a relative abundance significantly lower than that of Group A (P < 0.05), Prevotella, Pseudobutyrivibrio and Treponema in Group B and Group C have a relative abundance significantly higher than that of Group A (P < 0.05). The rumen pH and VFA were significantly affected by different concentrate-to-forage ratio diets. With the increase of concentrate-to-forage ratio, the rumen pH of yak decreased significantly (P < 0.05), The contents of propionic acid, valeric acid and total VFA increased significantly (P < 0.05), while the contents of acetic acid and acetic acid / propionic acid decreased significantly (P < 0.05), The contents of butyric acid and isobutyric acid in group C were significantly higher than those in group A and group B (P < 0.05). The results showed that Downregulated in adenoma（DRA）, Putative anion transporter 1（PAT1）, Monocarboxylate transporter 1（MCT1）and Monocarboxylate transporter 4（MCT4）, which are involved in the absorption and transport of VFA in the rumen epithelium, increased significantly in mRNA level and protein levels with the increase of the proportion of dietary concentrate(P < 0.05), while Anion exchanger 2（AE2）decreased significantly(P < 0.05).

3. Effect of dietary concentrate-to-forage ratio on fat metabolism in liver, small intestine and subcutaneous fat of yaks

The expression of genes mRNA and protein of fat synthesis and lipolysis in liver, small intestine and subcutaneous fat was significantly affected by the increase of dietary concentrate-to-crude ratio. The expression of gene Sterol regulatory element-binding transcription factor 1c(SREBP-1c), Acetyl-CoA carboxylase(ACC), Stearoyl-CoA desaturase 1(SCD1) and Fatty acid synthase(FASN) mRNA and protein involved in fat synthesis in liver, small intestine and subcutaneous fat increased with the increase of the proportion of dietary concentrate, while the expression of gene Peroxisome proliferator- activated receptor α(PPARα), Acyl-CoA Oxidase Carnitine palmitoyl transferase 1(CPT1) and Hormone-sensitive lipase(HSL) mRNA and protein involved in fat decomposition are decreased.

In conclusion, increasing dietary concentrate level in fattening yak can effectively improve produotion performance, rumen fermentation and lipid metabolism levels of yaks. Feeding a high concentrate diet can increase the relative abundance of non-cellulose-degrading bacteria in the rumen of yaks, so that a large amount of energy feed and protein feed can be rapidly converted into VFA, providing energy for the body. The vectors involved in the absorption and transport of VFA in rumen epithelial cells were activated to increase their expression and speed up the transport of VFA into blood, so as to improve fat metabolism substrates. The genes involved in fat synthesis in liver, small intestine and subcutaneous fat were upregulated with the increase of dietary concentrate ratio, and the body used substrates to increase fat synthesis rate, inhibit fat decomposition rate and accelerate fat deposition. This study comprehensively analyzed the molecular mechanism of production performance, rumen metabolism level and fat metabolism of yaks fed diets with different concentrate to forage ratio under the condition of fattening in house, providing important reference for large-scale and industrial breeding of house-fed yaks in Qinghai Province.