Broderick G A, Clayton M K
Agricultural Research Service, USDA, US Dairy Forage Research Center, Madison 53706, USA.
J Dairy Sci. 1997 Nov;80(11):2964-71. doi: 10.3168/jds.S0022-0302(97)76262-3.
Data from 35 trials with 482 lactating cows fed 106 diets were used to study the effects of animal and dietary factors on the relationship between milk and blood urea N and the value of milk urea N in the assessment of protein status. In two trials, urea N in whole blood and in blood plasma were closely related (r2 = 0.952); the slope was not significantly different from 1.0, and the intercept was not significantly different from 0. Regression of milk urea N on blood urea N with a mixed effects model using all 2231 observations yielded the equation: milk urea N (milligrams of N per deciliter) = 0.620 x blood urea N (milligrams of N per deciliter) + 4.75 (r2 = 0.842); this model accounted for a significant interaction of cow and blood urea N. Single factors that yielded significant regressions on milk urea N with the mixed effects models were dietary crude protein (CP) (percentage of dry matter; r2 = 0.839), dietary CP per megacalorie of net energy for lactation (NEL) (r2 = 0.833), excess N intake (r2 = 0.772), N efficiency (r2 = 0.626), and ruminal NH3 (r2 = 0.574). When all factors were analyzed at once, 12 were significant in a mixed effects model. Blood urea N, body weight, yield of fat-corrected milk, dietary CP content, excess N intake, dry matter intake, and days in milk were positively related to milk urea N, and parity, milk and fat yield, dietary CP per unit of NEL content, and NEL intake were negatively related to milk urea N. In one trial, the mean urea concentration was 35 times greater in urine than in milk; lower proportions of total urea excretion in milk were observed in the a.m. sampling (1.8%) than in the p.m. sampling (3.3%). Measuring urea N in a composite milk sample from the whole day substantially improved reliability of data. The number of cows fed a specific diet that must be sampled to determine mean milk urea N within 95% confidence intervals with half widths of 1.0 and 2.0 mg of N/dl was estimated to be 16 and 4, respectively.
来自35项试验的数据，涉及482头泌乳奶牛采食106种日粮，用于研究动物和日粮因素对牛奶与血液尿素氮之间关系以及牛奶尿素氮在评估蛋白质状态时的价值的影响。在两项试验中，全血和血浆中的尿素氮密切相关（r2 = 0.952）；斜率与1.0无显著差异，截距与0无显著差异。使用所有2231个观测值，通过混合效应模型对牛奶尿素氮与血液尿素氮进行回归分析，得到方程：牛奶尿素氮（毫克氮/分升）= 0.620×血液尿素氮（毫克氮/分升）+ 4.75（r2 = 0.842）；该模型考虑了奶牛与血液尿素氮之间的显著交互作用。在混合效应模型中，对牛奶尿素氮产生显著回归的单一因素有日粮粗蛋白（CP）（干物质百分比；r2 = 0.839）、每兆卡泌乳净能（NEL）的日粮CP（r2 = 0.833）、过量氮摄入量（r2 = 0.772）、氮效率（r2 = 0.626）和瘤胃NH3（r2 = 0.574）。当同时分析所有因素时，在混合效应模型中有12个因素显著。血液尿素氮、体重、校正乳产量、日粮CP含量、过量氮摄入量、干物质摄入量和泌乳天数与牛奶尿素氮呈正相关，胎次、牛奶和脂肪产量、每单位NEL含量的日粮CP以及NEL摄入量与牛奶尿素氮呈负相关。在一项试验中，尿液中的平均尿素浓度比牛奶中的高35倍；上午采样时牛奶中尿素排泄总量的比例（1.8%）低于下午采样时（3.3%）。测量一整天的混合牛奶样本中的尿素氮可显著提高数据的可靠性。估计要确定平均牛奶尿素氮在95%置信区间内，半宽为1.0和2.0毫克氮/分升时，采食特定日粮的奶牛样本数量分别为16头和4头。
