Institute of Nutritional Physiology 'Oskar Kellner', Leibniz Institute for Farm Animal Biology (FBN), Wilhelm-Stahl-Allee 1, 18196 Dummerstorf, Germany.
Institute of Genetics and Biometry, Leibniz Institute for Farm Animal Biology (FBN), Wilhelm-Stahl-Allee 1, 18196 Dummerstorf, Germany.
Sci Total Environ. 2021 Feb 10;755(Pt 2):143198. doi: 10.1016/j.scitotenv.2020.143198. Epub 2020 Oct 27.
Urine and fecal excretions from cattle contribute to global nitrogen (N) emissions. The milk urea nitrogen (MUN) concentration in dairy cows is positively correlated with urinary urea N (UUN) emissions, and both decline with the reduction in crude protein intake. However, MUN concentration may differ between individual cows despite feeding the same ration. Thus, we hypothesized that due to differences in endogenous N utilization cows with high MUN concentration excrete more UUN than cows with a low MUN concentration. The objective of the present study was to elucidate N partitioning and urea metabolism in dairy cows with divergent MUN concentrations fed two planes of crude protein. Twenty Holstein dairy cows with high (HMU; n = 10) and low (LMU; n = 10) milk urea concentrations were fed two isocaloric diets with a low (LP) and normal (NP) crude protein level. Methane and ammonia emissions were recorded in respiration chambers. Feed intake, feces and urine excretions and milk yield were recorded for four days and subsamples were analyzed for total N and N-metabolites. A carbon-13 labeled urea bolus was administered intravenously followed by a series of plasma samplings. Total N and UUN excretions and ammonia emissions from excreta were lower on the LP diet, however, methane emissions, urinary N excretions and ammonia emissions were comparable between groups. Although plasma and salivary urea concentrations, urea pool size and urea turnover were higher, HMU cows had lower renal urea clearance rates. Additionally, HMU cows had lower renal clearance rates for creatinine, uric acid and creatine and excreted less uric acid (on the LP diet only) and creatine with urine. In conclusion, contrary to our hypothesis, HMU cows did not excrete more UUN than LMU cows. The lower urinary creatine excretion of HMU cows suggests that these animals have a lower environmental nitrogen footprint.
牛的尿液和粪便排泄物会导致全球氮（N）排放。奶牛乳尿素氮（MUN）浓度与尿尿素 N（UUN）排放量呈正相关，且两者均随粗蛋白摄入量的减少而降低。然而，尽管给奶牛喂食相同的饲粮，其 MUN 浓度也可能存在差异。因此，我们假设，由于内源性氮利用的差异，MUN 浓度高的奶牛比 MUN 浓度低的奶牛排泄更多的 UUN。本研究的目的是阐明在饲喂两种粗蛋白水平饲粮时，MUN 浓度不同的奶牛的氮分配和尿素代谢。将 20 头具有高（HMU；n=10）和低（LMU；n=10）乳尿素浓度的荷斯坦奶牛饲喂两种等热量饲粮，其粗蛋白水平分别较低（LP）和正常（NP）。在呼吸室中记录甲烷和氨排放。记录了四天的采食量、粪便和尿液排泄量以及牛奶产量，并对总氮和氮代谢物进行了亚组分析。静脉内给予 13C 标记的尿素 bolus，随后进行一系列血浆取样。LP 饲粮时，总氮和 UUN 排泄量以及排泄物中的氨排放量较低，但两组间的甲烷排放量、尿氮排泄量和氨排放量无差异。尽管 HMU 奶牛的血浆和唾液尿素浓度、尿素池大小和尿素周转率较高，但肾脏尿素清除率较低。此外，HMU 奶牛的肾脏肌酐、尿酸和肌酸清除率较低，且仅在 LP 饲粮时尿液中尿酸和肌酸排泄量较少。总之，与我们的假设相反，HMU 奶牛并未比 LMU 奶牛排泄更多的 UUN。HMU 奶牛的尿肌酐排泄量较低表明，这些动物的环境氮足迹较低。
