Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061.
Department of Animal Science, Cornell University, Ithaca, NY 14853.
J Dairy Sci. 2021 Jun;104(6):6715-6726. doi: 10.3168/jds.2020-19787. Epub 2021 Mar 25.
Milk urea nitrogen (MUN) and blood urea nitrogen are correlated with nitrogen balance and nitrogen excretion; however, there is also a genetic component to MUN concentrations that could be associated with differences in urea transport. It was hypothesized that a portion of the variation in MUN concentrations among cows is caused by variation in gastrointestinal and kidney urea clearance rates. Eight lactating cows with varying MUN concentrations while fed a common diet were infused with [NN]urea to determine urea N entry rate (UER), gastrointestinal entry rate, returned to ornithine cycle, urea N used for anabolism, urea N excretion in feces and urine. Urea clearance rates by the kidneys and gastrointestinal tract were calculated from isotopic enrichment of urea excretion in urine and gut entry rate, respectively, and plasma urea N concentrations (PUN). Over the course of the experiment, animals weighed an average of 506 ± 62 kg and produced 26.3 ± 4.39 kg of milk/d, with MUN concentrations ranging from 11.6 to 17.3 mg/dL (average of 14.9 ± 2.1 mg/dL). Plasma urea N was positively correlated with UER, urea N excretion in urine, and urea N used for anabolism. Plasma urea N and MUN were negatively correlated with gut clearance rates and ratio of gastrointestinal entry rate to UER. This relationship supports the hypothesis that differences in gut urea transport activity among animals causes variation in PUN and MUN concentrations, and that cows with high PUN and MUN are less efficient at recycling PUN to the gastrointestinal tract and thus may be more susceptible to ruminal N deficiencies when fed low RDP diets. Such biological variation in urea metabolism necessitates an adequate safety margin when setting regulations for maximal MUN levels as an indicator of herd N efficiency.
牛奶尿素氮（MUN）和血液尿素氮与氮平衡和氮排泄有关；然而，MUN 浓度也存在遗传成分，这可能与尿素转运的差异有关。据推测，奶牛之间 MUN 浓度的变化部分是由于胃肠道和肾脏尿素清除率的变化造成的。对 8 头在饲喂相同日粮时具有不同 MUN 浓度的泌乳奶牛进行了[NN]尿素输注，以确定尿素氮进入率（UER）、胃肠道进入率、返回鸟氨酸循环、用于合成代谢的尿素氮、粪便和尿液中尿素氮排泄。通过同位素示踪法分别从尿液中尿素排泄的同位素丰度和肠道进入率计算肾脏和胃肠道的尿素清除率，并计算血浆尿素氮浓度（PUN）。在实验过程中，动物平均体重为 506±62kg，产奶量为 26.3±4.39kg/d，MUN 浓度范围为 11.6-17.3mg/dL（平均 14.9±2.1mg/dL）。血浆尿素氮与 UER、尿液中尿素氮排泄和用于合成代谢的尿素氮呈正相关。血浆尿素氮和 MUN 与肠道清除率和胃肠道进入率与 UER 的比值呈负相关。这种关系支持这样一种假设，即动物之间肠道尿素转运活性的差异导致 PUN 和 MUN 浓度的变化，并且 PUN 和 MUN 浓度高的奶牛将 PUN 再循环到胃肠道的效率较低，因此当饲喂低 RDP 日粮时，可能更容易受到瘤胃氮缺乏的影响。尿素代谢的这种生物学变化需要在制定最大 MUN 水平的规定时，为了作为牛群氮效率的指标，需要有足够的安全裕度。
