ABSTRACT This study aimed at evaluating the effects of feed intake level on the efficiency of rumen microbial protein synthesis (EMPS), nitrogen (N) excretion, and N balance in twelve 18-months old Boran (Bos indicus) steers with initial average liveweight of 183 kg (standard deviation (SD) 15.2). The experiment followed a 4 × 4 complete Latin Square design with four dietary treatments tested in four periods. Each period ran for 5 weeks with 3 weeks of adaptation and 2 weeks of sample collection; separated by 2 weeks of re-feeding. Steers were fed at 100%, 80%, 60%, and 40% of their metabolisable energy requirement for maintenance (MER, referred to as MER100, MER80, MER60, and MER40, respectively). Steers receiving MER80, MER60, and MER40 were only fed Rhodes grass hay. MER100 steers were offered Rhodes grass hay at 80% of their MER and cottonseed meal and sugarcane molasses at each 10% of MER. Mean daily dry matter intake differed between treatments (p < 0.001) and ranged between 2.1 kg/animal (SD 0.13) in MER40 and 4.5 kg/animal (SD 0.31) in MER100. Urinary N excretion and N balance did not differ between MER80, MER60, and MER40. According to contrast test, declining feed intake level from MER80 to MER40 reduced duodenal microbial crude protein flow (p < 0.001), but did not alter the EMPS (g microbial N/kg digestible organic matter intake). Yet, if scaled to N intake, EMPS increased (p < 0.049), whereas total N and faecal N excretions decreased linearly with declining intake level (p < 0.001 for both variables). At similar grass hay intake, duodenal microbial crude protein flow was 41% higher in MER100 than in MER80 steers (p < 0.001). In cattle offered poor-quality tropical forage below their MER, the very low EMPS and thus microbial protein supply aggravate the negative effects of low dietary nutrient and energy intakes in periods of feed shortage.

中文翻译：

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采食量对食用热带劣质牧草的博兰牛微生物蛋白质合成效率和氮平衡的影响

摘要 本研究旨在评估采食量对 12 头 18 月龄 Boran (Bos indicus) 阉牛的瘤胃微生物蛋白质合成 (EMPS)、氮 (N) 排泄和氮平衡效率的影响，初始平均体重为183 公斤（标准偏差 (SD) 15.2）。实验遵循 4 × 4 完整的拉丁方设计，在四个时期测试四种饮食处理。每个时期运行 5 周，其中 3 周适应和 2 周样本采集；间隔2周重新喂食。以维持代谢能需求（MER，分别称为 MER100、MER80、MER60 和 MER40）的 100%、80%、60% 和 40% 喂养阉牛。接受 MER80、MER60 和 MER40 的阉牛只喂食罗德草干草。向 MER100 公牛提供其 MER 80% 的罗德草干草，以及每 10% MER 的棉籽粕和甘蔗糖浆。处理之间的平均每日干物质摄入量不同 (p < 0.001)，范围在 MER40 中的 2.1 kg/动物 (SD 0.13) 和 MER100 中的 4.5 kg/动物 (SD 0.31) 之间。MER80、MER60 和 MER40 之间的尿 N 排泄和 N 平衡没有差异。根据对比试验，从 MER80 到 MER40 的采食量水平下降会降低十二指肠微生物粗蛋白流量（p < 0.001），但不会改变 EMPS（g 微生物 N/kg 可消化有机物摄入量）。然而，如果按 N 摄入量计算，EMPS 增加（p < 0.049），而总 N 和粪便 N 排泄量随着摄入水平的下降呈线性下降（两个变量的 p < 0.001）。在类似的草干草摄入量下，MER100 公牛十二指肠微生物粗蛋白流量比 MER80 公牛高 41%（p < 0.001）。在提供低于 MER 的劣质热带草料的牛中，非常低的 EMPS 和微生物蛋白质供应加剧了饲料短缺期间日粮营养和能量摄入量低的负面影响。