Milk urea nitrogen (MUN) content is closely related to blood urea nitrogen (BUN) concentration and reflects the balance of dietary crude protein and energy supply for ruminal microbial metabolism. The present study investigated whether the higher MUN content in Brown Swiss (BSW) compared with Holstein (HOL) cows is related to milk production and feeding conditions. Therefore, we evaluated test‐day records of three consecutive official milk recordings from 86 BSW and 200 HOL cows kept on the same farm under identical feeding and management conditions. In addition, the relationships between BUN and MUN assessed either by an enzymatic method or by mid‐infrared spectroscopy were analysed. For the second objective, milk and blood samples (1,112 samples each) were collected in parallel from day 5 until day 150 of lactation from 141 dairy cows housed at different farms. Concentrations of MUN determined either enzymatically (MUN_ENZ) or by Fourier transform infrared spectroscopy (FTIR; MUN_IR) were regressed on the BUN concentrations of the concomitantly obtained blood samples. Both MUN_IR and MUN_ENZ were well correlated with BUN (r = .93 and .89 respectively). Concentration of MUN was higher in BSW compared with HOL independently of lactational stage or parity number (p < .0001). Protein and fat content were higher in BSW than in HOL (p < .05). Primiparous cows had lower milk yield and ECM (p < .001 for BSW, p < .0001 for HOL) than multiparous in both breeds. Multiparous BSW had a similar milk production than primiparous HOL (p = .13). In conclusion, BSW cows have higher MUN than HOL when kept under identical feeding and management conditions. The higher MUN in BSW compared with HOL cows is not related to milk production and therefore rather genetically determined.

中文翻译：

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尽管喂养相同，但布朗瑞士奶牛的牛奶尿素氮浓度高于荷斯坦奶牛。

牛奶尿素氮（MUN）含量与血尿素氮（BUN）浓度密切相关，反映了日粮粗蛋白和瘤胃微生物代谢能量供应的平衡。本研究调查了与荷斯坦奶牛 (HOL) 奶牛相比，Brown Swiss (BSW) 奶牛较高的 MUN 含量是否与产奶量和饲养条件有关。因此，我们评估了同一农场在相同饲养和管理条件下饲养的 86 头 BSW 和 200 头 HOL 奶牛的连续三个官方牛奶记录的测试日记录。此外，还分析了通过酶法或中红外光谱评估的 BUN 和 MUN 之间的关系。对于第二个目标，牛奶和血液样本 (1, 从饲养在不同农场的 141 头奶牛的泌乳第 5 天到第 150 天，每个样本 112 个平行收集。酶法测定的 MUN 浓度 (MUN_ENZ ) 或傅里叶变换红外光谱 (FTIR; MUN _IR ) 对伴随获得的血样的 BUN 浓度进行回归。MUN _IR和 MUN _ENZ都与 BUN 密切相关（ 分别为r = .93 和 .89）。与 HOL 相比，BSW 中 MUN 的浓度更高，与泌乳阶段或胎次数无关 ( p  < .0001)。BSW 中的蛋白质和脂肪含量高于 HOL ( p  < .05)。初产奶牛的产奶量和 ECM（ BSW p < .001， HOL p < .0001）低于两个品种的经产奶牛。经产 BSW 与初产 HOL 的产奶量相似（p = .13）。总之，当饲养和管理条件相同时，BSW 奶牛的 MUN 高于 HOL。与 HOL 奶牛相比，BSW 中较高的 MUN 与产奶量无关，因此是由基因决定的。