Increasing the dietary fat content of ruminant diets decreases methane (CH₄) production. This effect is caused by the toxic properties of fatty acids on rumen microbial populations, coating of feed particles diminishing the accessibility for microbes, and a reduction in dry matter intake (DMI). The latter effect is caused by postabsorptive long-chain fatty acids eliciting anorexic signaling; however, whether circulating long-chain fatty acids affect rumen CH₄ production alike is unknown. To approach this question, 5 rumen-cannulated Holstein cows in late lactation received 2 jugular catheters and were kept in respiration chambers to measure CH₄ production and DMI for 48 h. In a crossover design, cows were intravenously infused with a 20% lipid emulsion (LIPO) or 0.9% NaCl (CON). The LIPO cows received 2.1 kg of triglycerides/d [0.152 ± 0.007 g of triglycerides/(kg of BW × h)⁻¹] consisting of 12.1% palmitic acid, 4.2% stearic acid, 31.1% oleic acid, and 52.7% linoleic acid. Blood and rumen fluid samples were taken hourly during the day. Results showed that LIPO compared with CON infusion increased plasma triglyceride as well as free fatty acid and serotonin concentrations but reduced the proportion of de novo synthesized milk fatty acids (sum of C6 to C16). Daily CH₄ production and DMI were lower, whereas daily CH₄ yield (CH₄/DMI) was greater in LIPO than CON cows, although CH₄ yield decreased from d 1 to d 2 by 2 to 14% in LIPO-infused cows only. This effect was associated with a higher (acetate + butyrate)/propionate ratio, tending lower propionate concentrations between 24 and 34 h of infusion, reduced relative abundances of genera belonging to Succinivibrio, Ruminococcaceae, and Ruminiclostridium, and greater relative Bacteroidetes genus abundances in the rumen.

反刍动物日粮中脂肪含量的增加会降低甲烷（CH ₄）的产生。这种作用是由于脂肪酸对瘤胃微生物种群的毒性，饲料颗粒的涂层减少了微生物的可及性以及减少了干物质摄入量（DMI）所引起的。后一种效应是由吸收性长链脂肪酸引发厌食信号引起的。然而，循环长链脂肪酸是否会影响瘤胃CH _4的产生尚不清楚。为了解决这个问题，在泌乳后期，对5只瘤胃空心的荷斯坦奶牛接受了2个颈导管，并放置在呼吸室中以测量CH ₄生产和DMI持续48小时。在交叉设计中，将20％脂质乳状液（LIPO）或0.9％NaCl（CON）静脉输注给奶牛。LIPO奶牛接受2.1千克甘油三酯/天[0.152±0.007克甘油三酯/（千克BW×h）^-1 ]，其中包括12.1％的棕榈酸，4.2％的硬脂酸，31.1％的油酸和52.7％的亚油酸。白天每小时采集一次血液和瘤胃液样品。结果表明，与CON输注相比，LIPO增加了血浆甘油三酸酯以及游离脂肪酸和血清素的浓度，但降低了从头合成的牛奶脂肪酸的比例（C6至C16之和）。每天的CH ₄产量和DMI较低，而每天的CH ₄产量（CH ₄/ DMI）在LIPO中比CON母牛更大，尽管仅注入LIPO的母牛的CH ₄产量从d 1降低到d 2降低2％至14％。这种作用与更高（乙酸+基丁酸酯）/丙酸之比相关联的，图24和输注34 H之间趋于较低丙酸浓度，降低的属于属的相对丰度Succinivibrio，Ruminococcaceae，和Ruminiclostridium，和更大的相对拟杆菌属丰度在瘤胃。