Manures allow for nitrogen (N) and phosphorus (P) recycling but low N:P ratios result in P over-fertilization when applying manures to meet crop N demand. Such stoichiometric imbalances can result in inefficient use of non-renewable P resources as well as aquatic eutrophication. On average, P surpluses in wheat (Triticum aestivum L.) and corn (Zea mays L.) systems of the United States vary between 1.4 and 3.1 kg P per kg of plant-available nitrogen (PAN) applied. Hybrid systems combining manures and biological N₂-fixation from cover crops can limit P over-fertilization and reduce synthetic N fertilizer inputs. On average, about 40–48% of corn and 69–75% of wheat crop N demand could be met by manures and legume cover crops in the United States. However, the range of their possible N contribution to a specific system is wide given the variability in nutrient concentrations/availability and transfer to a subsequent crop. The adoption of hybrid systems is also limited by other constraints, most notably the spatial decoupling between animal and crop production.

粪肥允许氮（N）和磷（P）的循环利用，但是低的N：P比率会导致在施肥以满足作物对氮的需求时磷肥过多。这种化学计量的不平衡会导致无效利用不可再生P资源以及富营养化。平均而言，美国的小麦（Triticum aestivum L.）和玉米（Zea mays L.）系统中的磷过剩量在每千克施用的植物可利用的氮（PAN）之间为1.4至3.1千克磷。结合肥料和生物氮_2的混合系统覆盖作物的固氮可以限制磷的过度施肥并减少合成氮肥的投入。在美国，平均而言，大约40-48％的玉米和69-75％的小麦对作物的氮需求可以通过肥料和豆类覆盖作物来满足。然而，考虑到养分浓度/有效性和转移至后续作物的可变性，它们对特定系统的可能氮贡献范围很广。混合系统的采用还受到其他限制的限制，最显着的是动植物生产之间的空间解耦。