Improving the ability of animals to convert feed resources into food for humans is needed for more sustainable livestock systems. Genetic selection for animals eating less while maintaining their performance (i.e., low residual feed intake [RFI]) appears a smart strategy but its effectiveness relies on high-throughput animal phenotyping. Here, we explored plasma nitrogen (N) isotope ratios in an attempt to identify easily superior young bulls in terms of RFI. For this, 48 Charolais young bulls fed two contrasting diets (corn vs. grass silage diets) were selected from a larger population as extreme RFI animals (24 low-RFI vs. 24 high-RFI) and their plasma analyzed for natural ¹⁵N abundance (δ¹⁵N) in the whole protein (bulk protein) and in the individual protein-bound amino acids (PbAA). For the first time, we showed that the δ ¹⁵N in plasma bulk protein differed (P = 0.007) between efficient (low-RFI) and inefficient (high-RFI) cattle regardless of diet. Furthermore, most analyzed PbAA followed the same trend as the bulk protein, with lower (P < 0.05) δ ¹⁵N values in more efficient (low-RFI) compared with less efficient (high-RFI) cattle, again regardless of diet. The only three exceptions were Phe, Met, and Lys (P > 0.05) for which the first metabolic reaction before being catabolized does not involve transamination, a pathway known naturally to enrich AAs in ¹⁵N. The contrasted isotopic signatures across RFI groups only in those PbAA undergoing transamination are interpreted as differences in transamination rates and N-use efficiency between low- and high-RFI phenotypes. Natural isotopic N signatures in bulk proteins and specific PbAA can be proposed as biomarkers of RFI in growing beef cattle fed different diets. However, the current study cannot delineate whether this effect only occurs post-absorption or to some extent also in the rumen. Our data support the conclusion that most efficient cattle in terms of RFI upregulate N conservation mechanisms compared with less efficient cattle and justify future research on this topic.

中文翻译：

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特定氨基酸中的天然15N丰度表明转氨率与肉牛剩余饲料摄入量之间存在关联。

为了更可持续的牲畜系统，需要提高动物将饲料资源转化为人类食物的能力。为保持其性能（即低残留饲料摄入量[ RFI ]）而少食的动物进行遗传选择似乎是一个明智的策略，但其有效性取决于高通量动物的表型。在这里，我们探索了血浆氮（N）同位素比率，以试图根据RFI轻松识别出较优的幼小公牛。为此，从较大的种群中选出48种饲喂两种不同日粮（玉米和青贮饲料）的夏洛来牛作为极端RFI动物（24只低RFI与24只高RFI），并分析其血浆中自然¹⁵ N的丰度（δ ¹⁵ ñ）（完整蛋白（散装蛋白）和单个蛋白结合的氨基酸（PbAA））中）。对于第一次，我们表明，δ  ¹⁵ N的不同血浆蛋白散装（P有效（低RFI）之间= 0.007）和低效率的（高RFI）牛无论饮食。此外，大多数分析PBAA遵循同样的趋势作为体蛋白，具有较低（P <0.05），δ  ¹⁵与低效率（高RFI）牛，再次无论饮食相比，更有效的（低RFI）的N个值。仅有的三个例外是Phe，Met和Lys（P > 0.05），在这些代谢中，分解代谢之前的第一个代谢反应不涉及转氨作用，这是一种天然富集AA的途径。¹⁵ N.仅在那些进行氨基转移的PbAA中，跨RFI组的对比同位素特征被解释为低RFI和高RFI表型之间的氨基转移率和氮利用效率的差异。可以提出散装蛋白质和特定PbAA中的天然同位素N标记作为饲喂不同日粮的生长牛的RFI的生物标记。但是，目前的研究不能确定这种作用是仅在吸收后发生还是在瘤胃中某种程度上发生。我们的数据支持这样的结论：就RFI而言，最高效的牛与效率较低的牛相比会上调N养护机制，并为该主题的未来研究辩护。