Spek J W, Dijkstra J, Bannink A
Department of Animal Nutrition, Wageningen UR Livestock Research, PO Box 338, 6700 AH Wageningen, the Netherlands; Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands.
Animal Nutrition Group, Wageningen University, PO Box 338, 6700 AH Wageningen, the Netherlands.
J Dairy Sci. 2016 May;99(5):3880-3888. doi: 10.3168/jds.2015-9421.
The relationship between milk urea nitrogen (MUN; mg of N/dL) and urinary N excretion is affected, among others, by diurnal dynamics in MUN, which in turn is largely influenced by feed intake pattern and characteristics of urea transfer from blood plasma to milk and vice versa. This study aimed to obtain insight in urea transfer characteristics within the mammary gland and from the mammary gland to blood plasma in dairy cows at various concentrations of plasma urea nitrogen (PUN; mg of N/dL) and MUN. Urea transfer from milk to blood plasma and urea transfer within the mammary gland itself was evaluated in a 4×4 Latin square design using 4 lactating multiparous Holstein-Friesian cows (milk production of 39.8±4.70kg/d and 90±3.9 d in milk). Treatments consisted of 4 primed continuous intravenous urea infusions of 0, 5, 10, and 15g of urea/h. Boluses of [(15)N(15)N]urea were injected in cistern milk at 20, 60, and 100 min before the 1700h milking. Milk was collected in portions of approximately 2 L at the 1700h milking. Milk samples were analyzed for urea and enrichment of (15)N-urea. Results from one cow were discarded because of leakage of milk from the teats after injection of boluses of [(15)N(15)N]urea. Increasing urea infusion rate linearly increased PUN from 11.4 (0g of urea/h) to 25.9mg/dL (15g of urea/h) and MUN from 10.3 (0g of urea/h) to 23.5 (15g of urea/h) mg of N/dL. The percentage of injected [(15)N(15)N]urea recovered from milk at the time of injection was not affected by urea infusion rate and varied between 65.1 and 73.0%, indicating that a substantial portion of injected [(15)N(15)N]urea was not accounted for by collected milk. The estimated fractional disappearance rate of (15)N-urea from milk to blood (Kurea; per hour) linearly increased from 0.429 (0g of urea/h) to 0.641 per hour (15g of urea/h). Cistern injected [(15)N(15)N]urea diffused within 20 min after injection toward alveoli milk. Calculations with the average Kurea estimated in this study show that 89% of an initial difference between PUN and MUN will have disappeared after 4 h. In conclusion, urea disappearance from milk in the mammary gland is substantial, as well as the intramammary urea exchange between cistern, duct, and alveoli milk. However, results have to be interpreted with caution given the lack of full recovery of dosed (15)N urea at time of injection. Information on Kurea is useful to quantify the effects of diurnal variation in PUN on MUN, which enhances the utility of MUN as an indicator for N excretion in urine.
乳尿素氮（MUN；毫克氮/分升）与尿氮排泄之间的关系，除其他因素外，还受MUN的昼夜动态影响，而MUN的昼夜动态又在很大程度上受采食量模式以及尿素在血浆与乳汁之间双向转运特性的影响。本研究旨在深入了解不同血浆尿素氮（PUN；毫克氮/分升）和MUN浓度下，奶牛乳腺内以及从乳腺到血浆的尿素转运特性。采用4×4拉丁方设计，对4头经产泌乳荷斯坦 - 弗里生奶牛（日产奶量39.8±4.70千克，泌乳天数90±3.9天）进行研究，评估从乳汁到血浆的尿素转运以及乳腺自身内部的尿素转运。处理方式为以0、5、10和15克尿素/小时的速率进行4次静脉注射尿素预充量并持续输注。在17:00挤奶前20、60和100分钟，向乳池乳中注射[(15)N(15)N]尿素丸剂。在17:00挤奶时，分多次收集约2升乳汁。对乳汁样本进行尿素和(15)N - 尿素富集分析。由于注射[(15)N(15)N]尿素丸剂后乳头漏奶，一头奶牛的结果被舍弃。随着尿素输注速率增加，PUN从11.4（0克尿素/小时）线性增加至25.9毫克/分升（15克尿素/小时），MUN从10.3（0克尿素/小时）增加至23.5（15克尿素/小时）毫克氮/分升。注射时从乳汁中回收的注射[(15)N(15)N]尿素的百分比不受尿素输注速率影响，在65.1%至73.0%之间变化，这表明注射的[(15)N(15)N]尿素有很大一部分未在收集的乳汁中体现。从乳汁到血液的(15)N - 尿素估计分数消失率（Kurea；每小时）从0.429（0克尿素/小时）线性增加至0.641每小时（15克尿素/小时）。乳池注射的[(15)N(15)N]尿素在注射后20分钟内扩散至肺泡乳。根据本研究估算的平均Kurea进行计算表明，PUN与MUN之间的初始差异在4小时后将有89%消失。总之，乳腺中乳汁尿素的消失量很大，乳池、导管和肺泡乳之间的乳腺内尿素交换量也很大。然而，鉴于注射时给予的(15)N尿素未完全回收，对结果的解释需谨慎。Kurea信息有助于量化PUN昼夜变化对MUN的影响，从而增强MUN作为尿氮排泄指标的实用性。
