We aimed to evaluate the effects of magnesium-based supplements and NaHCO₃ on ruminal fermentation parameters, milk fatty acid (FA) profile, and milk fat synthesis in dairy cows fed high-starch diets. Eight second-lactation Holstein dairy cows (4 ruminally-cannulated) averaging 97 ± 12 days in milk and 33 ± 2.5 kg/d of milk were assigned to a replicated 4 × 4 Latin square design with 21-d periods. Cows were fed the following isoenergetic diets (1.64 Mcal/kg DM): (1) the control [342 g/kg starch on dry matter (DM) basis] without any buffering or alkalizing agent, (2) the control plus 8 g/kg DM NaHCO₃, (3) the control plus 4 g/kg DM MgO, and (4) the control plus 8 g/kg DM CaMg(CO₃)₂. Ruminal pH was continuously measured for 72-h in each period using indwelling pH electrodes. Mean ruminal pH was not affected by treatments, but the duration of ruminal pH below 5.8 was shorter (P < 0.01) in cows fed NaHCO₃, MgO, or CaMg(CO₃)₂ diets compared with those fed the control whereas NaHCO₃ was more effective than magnesium-based supplements. Similar ability observed for CaMg(CO₃)₂ and MgO with respect to the duration of ruminal pH below 5.8 responses. The ruminal molar proportion of acetate and acetate-to-propionate ratio was greater (P < 0.05) with magnesium-based supplements and NaHCO₃ compared with the control diet. However, the ruminal propionate concentration decreased (P = 0.02) with magnesium-based supplements and NaHCO₃ versus the control. The milk proportion and yield of trans-10 C18:1 and total trans-FA dropped and the trans-10/trans-11 C18:1 ratio decreased further, when cows were fed magnesium-based supplements and NaHCO₃ compared with the control diet (P < 0.01). Also, adding NaHCO₃, MgO, or CaMg(CO₃)₂ significantly increased yields of trans-11 C18:1 (P < 0.01), de novo (P < 0.01), mixed (P = 0.04), and also preformed FA (P < 0.01) compared with the control diet. The yields of milk, 3.5% fat-corrected, and energy-corrected milk increased (P < 0.01) in cows fed magnesium-based supplements and NaHCO₃ due to the improved concentration of milk fat. Overall, supplementing NaHCO₃, MgO, and CaMg(CO₃)₂ in a high-starch diet appears to promote milk fat output by modulating diurnal ruminal pH and ruminal volatile fatty acids profile in dairy cows. Also, it can be concluded that CaMg(CO₃)₂ has similar ability of MgO in promoting milk fat synthesis and alleviating the time ruminal pH remains below 5.8. Therefore, CaMg (CO₃)₂ can be used effectively as an alkalizing agent in dairy cow diets containing high-starch and helps to reduce the costs of consumption of the most popular dietary buffers or alkalizers such as NaHCO₃ and MgO.

我们旨在评估以镁为基础的补充剂和NaHCO ₃对饲喂高淀粉饮食的奶牛瘤胃发酵参数，牛奶脂肪酸（FA）谱和牛奶脂肪合成的影响。将八头平均泌乳时间为97±12天的荷斯坦奶牛（4头经瘤胃插管）和33±2.5 kg / d的牛奶分配给重复的4×4拉丁方设计，周期为21天。奶牛接受以下同等能量日粮（1.64 Mcal / kg DM）：（1）对照[342 g / kg淀粉，以干物质（DM）为基础]，没有任何缓冲剂或碱化剂，（2）对照加8 g / kg DM NaHCO ₃，（3）对照加4 g / kg DM MgO，（4）对照加8 g / kg DM CaMg（CO ₃）₂。使用留置的pH电极在每个周期中连续测量72小时的瘤胃pH值。平均数瘤胃液pH不影响治疗方法，但瘤胃pH低于5.8的持续时间较短（P在馈送的NaHCO牛<0.01）₃，MgO或为CaMg（CO ₃）_2周的饮食与馈送的控制而碳酸氢钠相比₃是比以镁为基础的补品更有效。CaMg（CO ₃）₂和MgO相对于瘤胃pH值低于5.8的持续时间观察到相似的能力。用镁基补充剂和NaHCO _3时，瘤胃中乙酸盐和乙酸盐/丙酸的比例的瘤胃摩尔比例更大（P <0.05）与对照饮食相比。然而，与对照组相比 ，镁基补充剂和NaHCO ₃可使瘤胃丙酸浓度降低（P = 0.02）。与对照日粮相比，当饲喂以镁为基础的补充剂和NaHCO ₃时，反式-10 C18：1和总反式FA的牛奶比例和产量下降，反式-10 /反式-11 C18：1的比例进一步降低（P <0.01）。另外，添加NaHCO ₃，MgO或CaMg（CO ₃）_2可以显着提高反式-11 C18：1的产率（P <0.01），从头开始（与对照饮食相比，P <0.01），混合食品（P  = 0.04）和预先形成的FA（P <0.01）。饲喂镁基补充剂和NaHCO _3的奶牛的牛奶，3.5％脂肪校正的和能量校正的牛奶的产量增加（P <0.01），这是因为牛奶脂肪的浓度提高了。总体而言，在高淀粉饮食中补充NaHCO ₃，MgO和CaMg（CO ₃）₂似乎可以通过调节奶牛的昼夜瘤胃pH值和瘤胃挥发性脂肪酸谱来促进乳脂输出。此外，可以得出结论，CaMg（CO ₃）₂MgO具有类似的促进乳脂合成和缓解瘤胃pH值保持在5.8以下的能力。因此，CaMg（CO ₃）₂可以有效地用作含高淀粉奶牛日粮中的碱化剂，并有助于降低最流行的饮食缓冲剂或碱化剂（如NaHCO ₃和MgO）的食用成本。