Do pasture-based mixed dairy systems with higher milk production have lower environmental impacts? A Uruguayan case study

The increased milk production of pastured-based mixed dairy systems has involved higher use of concentrates and inputs, which led to increased environmental impacts. Usually these impacts are expressed per unit of product, which does not reflect the local impact per unit of land. The objective of this paper was to estimate global and local environmental impacts per kg of milk and per hectare. Fossil energy use, greenhouse gas emissions, water ecotoxicity, nutrient aquatic eutrophication potential, and integrated standardised indices of environmental impacts (IEI) were estimated for 277 pastured-based dairy systems in Uruguay, clustered in seven milk production groups. Dairy systems with higher milk production had lower greenhouse gas emissions but higher ecotoxicity per kg of milk than systems with lower milk production, and the integrated index did not change. In contrast, all the individual indicators and the integrated indices per hectare were higher for the higher milk production systems. The results suggest that increasing milk productivity did not reduce environmental impacts per kg, and increased impacts per ha. Management factors including type of diet (pasture vs concentrate), amount of inputs, and type of grazing system may be more relevant in determining environmental impacts of dairy systems than productivity per se.

基于放牧的混合乳牛生产系统若要提升牛奶产量，往往需要更多使用精饲料与各类生产投入品，由此加剧了环境影响。当前这类环境影响通常以每单位产品为核算单位，但该核算方式无法反映每单位土地对应的本地环境负荷。本研究的目标为核算每千克牛奶与每公顷土地对应的全球尺度与本地尺度环境影响。本研究针对乌拉圭的277个放牧型乳牛生产系统，核算了化石能源消耗、温室气体排放、水生生态毒性、养分水体富营养化潜势，以及环境影响综合标准化指数（Integrated Standardised Indices of Environmental Impacts, IEI）共六项指标，并将所有研究对象聚类为7个牛奶生产组别。相较于低产乳牛系统，高产系统每千克牛奶对应的温室气体排放更低，但水生生态毒性更高，而环境影响综合标准化指数则无显著变化。与之形成对比的是，若以每公顷土地为核算单位，高产系统的所有单项环境指标与综合标准化指数均显著更高。研究结果表明，提升牛奶生产效率并未降低每单位产品的环境影响，反而加剧了每公顷土地的环境负荷。相较于生产效率本身，乳牛系统的环境影响更受日粮类型（放牧饲草 vs 精饲料）、生产投入品用量以及放牧系统类型等管理因素的影响。