Farmland management practices, including manuring, are crucial for the evolution and sustainable development of early dryland agriculture in China. Previous studies have demonstrated that S15N values serve as reliable indicators of manuring effects on foxtail and broomcorn millet. However, a comprehensive indicator system for millets remains undeveloped, hindering the understanding of the underlying causes, development processes, and impacts of early farmland management. This study investigated three sites on the Loess Plateau-wasteland in Yulin, farmland in Changzhi (Shanxi), and in Xi'an (Shaanxi)-through multi-site comparative experiments to explore the relationship between land management and the agronomic traits, yields, and isotopic compositions of millet. Results from Yulin and Xi'an show that, compared to foxtail millet without soil management, the ear length, diameter and weight under field management increased severalfold, or even more than tenfold. Key indicators, such as yield, thousand-grain weight and grain size, also show significant improvements, with pig manure having the most pronounced effect, where a relatively low amount sufficed to achieve remarkable traits and yields. This undoubtedly facilitates the emergence of early farmland management. The comprehensive analysis of crops and soil in Xi 'an and Changzhi reveals that soil characteristics are key factors influencing crop S15N values. Minor but systematic soil S15N differences between regions significantly influence crop values. Based on existing manuring experiments in millet fields along the Yellow River, this study proposes that S15N values of foxtail and broomcorn millet below 2 %o are largely unaffected by animal manure, values above 3 %o indicate a higher likelihood of manure input, and values between 2 %o and 3 %o suggest low intensity or residual manuring effects. This study provides a scientific foundation for exploring the impact of farmland management, particularly pig manure application, on millet agriculture, and offers valuable insights for comparative isotopic analysis across regions and the study of past field management strategies.