Improving nitrogen use efficiency in crop-livestock systems: A review of mitigation technologies and management strategies, and their potential applicability for egg supply chains

One of the main challenges in crop-livestock systems is an inefficient use of nitrogen, which precipitates a variety of negative economic, human/animal health, and environmental impacts. This review aims to identify and compare potentially efficacious technologies and management strategies that reduce nitrogen emissions and improve nitrogen use efficiency along crop-livestock supply chains, with an emphasis on the egg sector. Studies addressing the mitigation of reactive nitrogen emissions at key supply chain stages, including feed input production, feed formulation, and manure management were reviewed. The assessment was based on a combination of environmental, technical and economic criteria. Several priority mitigation options were identified. This included implementation of “4Rs” practices and biochar addition to cropland at the feed input production stage, and reduced crude protein diets supplemented with synthetic amino acids or enzymes at the feed formulation stage. For manure management, three substages were considered. Promising strategies identified for these substages were: implementation of mechanical ventilation and bedded systems, along with use of manure belts or scrapers and ammonia scrubbers in poultry housing; covering and adding biochar to manure at the manure storage; and incorporating manure at the land application stage. We also underscore that nitrogen use efficiency efforts should consider all major reactive nitrogen species as well as potential interactions between mitigation options across supply chain stages, while simultaneously considering other potential resources and environmental impacts so as to prevent unintended burden shifting. Towards this end, nitrogen use efficiency initiatives should be supported by detailed life cycle assessment research to elucidate potential interactions, minimize burden-shifting, and maximize systems-level efficacy.