Sub-acute ruminal acidosis (SARA) is sometimes observed along with reduced milk fat synthesis. Inconsistent responses may be explained by dietary fat levels. Twelve ruminally cannulated cows were used in a Latin square design investigating the timing of metabolic and milk fat changes during Induction and Recovery from SARA by altering starch levels in low-fat diets. Treatments were (1) SARA Induction, (2) Recovery and (3) Control. Sub-acute ruminal acidosis was induced by feeding a diet containing 29.4% starch, 24.0% NDF and 2.8% fatty acids (FAs), whereas the Recovery and Control diets contained 19.9% starch, 31.0% NDF and 2.6% FA. Relative to Control, DM intake (DMI) and milk yield were higher in SARA from days 14 to 21 and from days 10 to 21, respectively (P < 0.05). Milk fat content was reduced from days 3 to 14 in SARA (P < 0.05) compared with Control, while greater protein and lactose contents were observed from days 14 to 21 and 3 to 21, respectively (P < 0.05). Milk fat yield was reduced by SARA on day 3 (P < 0.05), whereas both protein and lactose yields were higher on days 14 and 21 (P < 0.05). The ruminal acetate-to-propionate ratio was lower, and the concentrations of propionate and lactate were higher in the SARA treatment compared with Control on day 21 (P < 0.05). Plasma insulin increased during SARA, whereas plasma non-esterified fatty acids and milk β-hydroxybutyrate decreased (P < 0.05). Similarly to fat yield, the yield of milk preformed FA (>16C) was lower on day 3 (P < 0.05) and tended to be lower on day 7 in SARA cows (P < 0.10), whereas yield of de novo FA (<16C) was higher on day 21 (P < 0.01) in the SARA group relative to Control. The t10- to t11-18:1 ratio increased during the SARA Induction period (P < 0.05), but the concentration of t10-18:1 remained below 0.5% of milk fat, and t10,c12 conjugated linoleic acid remained below detection levels. Odd-chain FA increased, whereas branched-chain FA was reduced during SARA Induction from days 3 to 21 (P < 0.05). Sub-acute ruminal acidosis reduced milk fat synthesis transiently. Such reduction was not associated with ruminal biohydrogenation intermediates but rather with a transient reduction in supply of preformed FA. Subsequent rescue of milk fat synthesis may be associated with higher availability of substrates due to increased DMI during SARA.

有时会观察到亚急性瘤胃酸中毒（SARA）以及牛奶脂肪合成的减少。饮食中的脂肪含量可能解释了反应不一致。在拉丁方形设计中使用了十二只反刍动物插管奶牛，通过改变低脂饮食中的淀粉含量，研究了从SARA诱导和恢复过程中代谢和乳脂变化的时机。处理方法是（1）SARA诱导，（2）恢复和（3）对照。饲喂含29.4％淀粉，24.0％NDF和2.8％脂肪酸（FAs）的饮食可诱发亚急性瘤胃酸中毒，而恢复和对照饮食则含19.9％淀粉，31.0％NDF和2.6％FA。相对于控制的DM摄入量（DMI）和SARA的牛奶产量分别从第14天到21天和第10天到21天更高（P <0.05）。与对照组相比，SARA中的乳脂含量从3天减少到14天（P <0.05），而从14天到21天和3-21天分别观察到更高的蛋白质和乳糖含量（P <0.05）。第3天，SARA降低了乳脂产量（P <0.05），而第14天和第21天，蛋白质和乳糖的产量均较高（P <0.05）。与第21天的对照组相比，在SARA处理中瘤胃中乙酸与丙酸的比率较低，丙酸和乳酸的浓度较高（P<0.05）。SARA期间血浆胰岛素增加，而血浆非酯化脂肪酸和牛奶β-羟基丁酸酯减少（P <0.05）。与脂肪产量相似，在SARA奶牛中，牛奶预制FA的产量（> 16C）在第3天较低（P <0.05），在第7天往往较低（P <0.10），而从头FA的产量（<与对照组相比，SARA组第21天的16C）值更高（P <0.01）。在SARA诱导期间，t 10至t 11-18：1的比率增加（P <0.05），但t 10-18：1的浓度仍低于乳脂的0.5％，而t 10，c12种共轭亚油酸仍低于检测水平。从3天到21天，在SARA诱导过程中，奇数链FA升高，而支链FA降低（P <0.05）。亚急性瘤胃酸中毒会暂时减少乳脂的合成。这种减少与瘤胃生物氢化中间体无关，而是与预制FA的供应暂时减少有关。由于SARA期间DMI增加，随后的乳脂合成的挽救可能与底物的更高可用性有关。