Abstract：
Few studies have examined the N kinetics of individual feeds with stable isotope tracing. We hypothesized that N partitioning to milk and excreta pools as well as the rates of the processes that drive this partitioning would differ for alfalfa silage, corn silage, corn grain, and soybean meal. Feed ingredients were endogenously labeled with 15N and included in 4 diets to create treatments with the same dietary composition and different labeled feed. Diets were fed to 12 late-lactation dairy cows for 4 d (96 h) and feces, urine, and milk collection proceeded during the 4 d of 15N enrichment and for 3 d (80 h) after cessation of label feeding. Nonlinear models of 15N enrichment and decay were fit to milk (MN), urine (UN), and fecal N (FN) in R with the nlme package and feed-specific parameter estimates were compared. The estimated proportions of feed N that were excreted in feces supported our understanding that N from soybean meal and corn grain is more digestible than N from alfalfa and corn silage. Estimates for the N partitioning between milk (MN) and urine (UN) from the 2 concentrate feeds (soybean meal and corn grain) indicated that UN:MN ratios were less than or equal to 1:1 indicating either more or equal nitrogen partitioning to milk compared with urine. It is important to maintain factual accuracy in representing the results rather than implying a desired outcome unsupported by the data. In contrast, UN:MN ratios for forage feeds (corn and alfalfa silage) were > 1:1, indicating more N partitioning to urine than milk. The modeled proportion of total FN that originated from feed N was 82.2% which is in line with previous research using a similar 15N measurement timeframe. However, the proportion of urinary and MN originating from feed N was much lower (60.5% for urine, 57.9% for milk), suggesting that approximately 40% of urinary and MN directly originate from body N sources related to protein turnover.
摘要：
很少有研究通过稳定的同位素示踪检查单个饲料的N动力学。我们假设苜蓿青贮的N分配到牛奶和排泄物池以及驱动这种分配的过程的速率会有所不同，玉米青贮,玉米粒,和豆粕。饲料成分用15N内源性标记并包括在4种饮食中以产生具有相同饮食组成和不同标记饲料的处理。饲喂12头晚期泌乳奶牛4天(96小时)，粪便，尿液,在15N富集4d和停止标签喂养后3d（80h）进行牛奶收集。15N富集和衰变的非线性模型适用于牛奶（MN），尿液（联合国），比较了R中的粪便N（FN）与nlme包装和饲料特异性参数估计。粪便中排出的饲料N的估计比例支持了我们的理解，即豆粕和玉米粒中的N比苜蓿和玉米青贮中的N更易消化。对来自2种浓缩饲料(大豆粉和玉米粒)的奶(MN)和尿(UN)之间的N分配的估计表明,UN:MN比率小于或等于1:1,表明与尿相比,奶的氮分配更多或相等。重要的是在表示结果时保持事实准确性，而不是暗示数据不支持的期望结果。相比之下,饲用饲料（玉米和苜蓿青贮）的UN：MN比率>1：1，表明与牛奶相比，尿液中的氮分配更多。源自进料N的总FN的建模比例为82.2%,这与使用类似15N测量时间帧的先前研究一致。然而,来自饲料N的尿和MN的比例要低得多(尿占60.5%，57.9％的牛奶），表明大约40％的尿液和MN直接来自与蛋白质周转有关的体内N源。
