We aimed to evaluate the effects of magnesium-based supplements and NaHCO3 on ruminal fermentation pa- rameters, 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 pro- moting 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.

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