Collaborative study to evaluate of the crude protein content in feeds using the “Kjeldahl” method

SUMMARY A collaborative study was conducted to evaluate the crude protein (CP) content in feeds in seven feed analysis laboratories from institutions linked to the National Institute of Science and Technology in Animal Science (INCT-CA). Were evaluated four samples of different feeds (sugarcane, corn silage, soybean meal and corn grain) and four nitrogen compounds assumed as standards (urea, monobasic ammonium phosphate, ethylene diamine tetraacetic acid and ammonium sulfate) to measure the accuracy of the estimates. Each laboratory analyzed the samples for six days, consecutive or not, with three replicates per sample per day of evaluation. The variances among laboratories and within of laboratories (repeatability) represented 34.5 and 10.3% of the total variation, respectively. The evaluation of nitrogen recovered from the standards showed that 25% of observations presented a significant bias (P<0.001). The overall assessment of the samples demonstrated significant variations (P<0.001) in function of the day of analysis, interaction of material with laboratory, and among laboratories. Additionally, it was found that the variation among laboratories corresponded of 43.6 to 68.2% of the total of random variation, while the repeatability (intra-laboratory variation) represented of 11.9 to 42.8% of reproducibility. The CP contents estimated are dependent of the interaction effect of the evaluated material and laboratory, which reflect the differences between the procedures utilized by each laboratory.

本协作研究旨在评估隶属于国家动物科学与技术研究所（National Institute of Science and Technology in Animal Science, INCT-CA）的7家饲料分析实验室所测饲料中的粗蛋白质（crude protein, CP）含量。研究共纳入4种不同饲料样品（甘蔗、玉米青贮、豆粕及玉米籽粒），以及4种作为标准物质的含氮化合物（尿素、磷酸二氢铵、乙二胺四乙酸（ethylene diamine tetraacetic acid, EDTA）、硫酸铵），用于测定估算结果的准确性。每家实验室以连续或非连续的方式开展为期6天的分析，每份样品每日设置3次重复测定。实验室间方差与实验室内方差（重复性）分别占总变异的34.5%与10.3%。对标准物质回收氮的评估显示，25%的观测值存在显著偏倚（P<0.001）。对所有样品的整体评估表明，分析日期、样品与实验室的交互作用以及实验室间均存在显著差异（P<0.001）。此外，研究发现实验室间变异占总随机变异的43.6%至68.2%，而重复性（实验室内变异）占再现性的11.9%至42.8%。粗蛋白质（CP）的估算值依赖于待测样品与实验室的交互效应，这反映了各实验室所采用的操作流程存在差异。