目的 评价饲粮中添加过瘤胃叶酸对荷斯坦奶牛妊娠率、血浆生殖激素水平和叶酸代谢指标的影响，为荷斯坦奶牛高效利用叶酸提供科学依据。 方法 试验选择体况、胎次与泌乳天数相近的经产荷斯坦奶牛160头并随机分为4组，每组40头，在人工授精前17 d至授精后35 d期间，分别饲喂补充了0、3、6和9 g过瘤胃叶酸的全混合日粮，并在人工授精第35天对试验奶牛进行B超检查，统计奶牛首次配种妊娠率、二次配种妊娠率，并采集血液测定雌二醇（E₂）、孕酮（P₄）、叶酸和维生素B₁₂的浓度。 结果 饲喂3 g过瘤胃叶酸组的首次配种妊娠率、E₂和叶酸浓度显著高于对照组（0 g）；各组的第2次配种妊娠率以及血浆中P₄、维生素B₁₂的浓度无显著差异。 结论 在本试验条件下，饲粮中添加3 g过瘤胃叶酸可提高荷斯坦奶牛首次配种妊娠率以及血浆中E₂和叶酸浓度。

Objectives The effects of adding rumen-protected folic acid to dietary on the pregnancy rates, levels of reproductive hormones including estrogen and progesterone in plasma, and indexes of folic acid metabolism in Holstein dairy cows were studied to provide the scientific basis for efficiently utilizing folic acid in Holstein dairy cows. Methods 160 multiparous Holstein dairy cows with similar physical condition, parity, and lactation days were randomly divided into 4 groups, with 40 dairy cows in each group. The dairy cows were fed with a total mixed diet supplemented with 0 g, 3 g, 6 g, and 9 g rumen-protected folic acid during the period from 17 days before artificial insemination to 35 days after insemination. The ultrasound examination was conducted on the experimental dairy cows on the 35th day of artificial insemination to count the conception rate of the first and second mating. The blood was collected to measure the content of estrogen (E₂), progesterone (P₄), folic acid, and vitamin B₁₂ in plasma. Results The conception rate of the first mating, the content of E₂ and folic acid in plasma in the group fed with 3 g rumen-protected folic acid was significantly higher than that in the control group fed with 0 g rumen-protected folic acid(P<0.05). There was no significant difference in the conception rate of the second mating, and the content of P₄ and vitamin B₁₂ in plasma among the groups(P>0.05). Conclusions It is indicated that adding 3 g rumen-protected folic acid to dietary can increase the conception rate of the first mating and the content of E₂ and folic acid in plasma in Holstein dairy cows under the conditions of this experiment.