Guanidinoacetic acid (GAA) can improve the growth performance of bulls. This study investigated the influences of GAA addition on growth, nutrient digestion, ruminal fermentation and serum metabolites in bulls. Forty-eight Angus bulls were randomly allocated to experimental treatments, that is, control, low-GAA (LGAA), medium-GAA (MGAA) and high-GAA (HGAA), with GAA supplementation at 0, 0.3, 0.6 and 0.9 g/kg DM, respectively. Bulls were fed a basal diet containing 500 g/kg DM concentrate and 500 g/kg DM roughage. The experimental period was 104 days, with 14 days for adaptation and 90 days for data collection. Bulls in the MGAA and HGAA groups had higher DM intake and average daily gain than bulls in the LGAA and control groups. The feed conversion ratio was lowest in MGAA and highest in the control. Bulls receiving 0.9 g/kg DM GAA addition had higher digestibility of DM, organic matter, NDF and ADF than bulls in other groups. The digestibility of CP was higher for HGAA than for LGAA and control. The ruminal pH was lower for MGAA, and the total volatile fatty acid concentration was greater for MGAA and HGAA than for the control. The acetate proportion and acetate-to-propionate ratio were lower for MGAA than for LGAA and control. The propionate proportion was higher for MGAA than for control. Bulls receiving GAA addition showed decreased ruminal ammonia N. Bulls in MGAA and HGAA had higher cellobiase, pectinase and protease activities and Butyrivibrio fibrisolvens, Prevotella ruminicola and Ruminobacter amylophilus populations than bulls in LGAA and control. However, the total protozoan population was lower for MGAA and HGAA than for LGAA and control. The total bacterial and Ruminococcus flavefaciens populations increased with GAA addition. The blood level of creatine was higher for HGAA, and the activity of l-arginine glycine amidine transferase was lower for MGAA and HGAA, than for control. The blood activity of guanidine acetate N-methyltransferase and the level of folate decreased in the GAA addition groups. The results indicated that dietary addition of 0.6 or 0.9 g/kg DM GAA improved growth performance, nutrient digestion and ruminal fermentation in bulls.

胍基乙酸（GAA）可以提高公牛的生长性能。本研究调查了添加 GAA 对公牛生长、养分消化、瘤胃发酵和血清代谢物的影响。将 48 头安格斯公牛随机分配到实验处理组，即对照、低 GAA（LGAA）、中 GAA（MGAA）和高 GAA（HGAA ）)，GAA 补充量分别为 0、0.3、0.6 和 0.9 g/kg DM。给公牛喂食含有 500 g/kg DM 浓缩物和 500 g/kg DM 粗饲料的基础日粮。实验期为104天，其中适应期14天，数据收集期90天。MGAA 和 HGAA 组的公牛比 LGAA 和对照组的公牛具有更高的 DM 摄入量和平均日增重。饲料转化率在 MGAA 中最低，而在对照中最高。添加 0.9 g/kg DM GAA 的公牛对 DM、有机物、NDF 和 ADF 的消化率高于其他组的公牛。HGAA 的 CP 消化率高于 LGAA 和对照。MGAA 的瘤胃 pH 值较低，MGAA 和 HGAA 的总挥发性脂肪酸浓度高于对照。MGAA 的乙酸比例和乙酸与丙酸的比例低于 LGAA 和对照。MGAA 的丙酸盐比例高于对照。接受 GAA 添加的公牛显示瘤胃氨氮减少。 MGAA 和 HGAA 的公牛具有较高的纤维二糖酶、果胶酶和蛋白酶活性，并且Butyrivibrio fibrisolvens、Prevotella ruminicola和Ruminobacter amylophilus种群在 LGAA 和对照中高于公牛。然而，MGAA 和 HGAA 的原生动物总数低于 LGAA 和对照。总细菌和黄瘤瘤胃球菌种群随着 GAA 的添加而增加。HGAA 的血肌酸水平较高，l的活性较高。MGAA和HGAA的-精氨酸甘氨酸脒转移酶低于对照。GAA添加组乙酸胍N-甲基转移酶的血液活性和叶酸水平降低。结果表明，日粮中添加 0.6 或 0.9 g/kg DM GAA 可改善公牛的生长性能、养分消化和瘤胃发酵。