Exploring the option space for phosphorus reduction in livestock production: modelling results from a Swiss watershed

Summary

Livestock production is a major driver of phosphorus (P) surpluses, threatening nutrient-sensitive regions, such as the watershed of Lake Sempach in Switzerland. While reducing livestock density may alleviate these impacts, it risks undermining food production and rural livelihoods. This study investigates regionally adapted strategies to reduce livestock-related P excretion while maintaining animal-source protein (ASP) production. Using the dynamic LEAF.livestock model, we simulated nearly five million combinations of herd structures and management practices. Each scenario was assessed across five key indicators: P excretion, ASP output, land use, food-feed competition, and manure fertilizing potential. Region-specific ecological and production constraints were applied, including limits on P excretion, maintenance of ASP output, and adherence to available land. No scenario fulfilled all constraints simultaneously. When the land-use constraint was relaxed, over 514’000 viable and 7’174 Pareto-optimal scenarios emerged, showing clear trade-offs between livestock density, herd composition, feed sourcing, and nutrient outputs. Pareto-optimal scenarios consistently required reduced livestock density (65%–90% of business-as-usual (BAU)), pig shares ⩾20%, and fewer suckler cows. Pig-dominated scenarios performed best on P excretion and ASP output but required more arable land and intensified feed-food competition. Dairy-dominated scenarios used less arable land and had better N:P ratios, but produced less ASP, though equal or greater than BAU. Similar results emerged therefore from different strategies, highlighting the importance to identify local optima rather than rely on a single global solution. In all scenarios, more plant-source protein than ASP could be produced if the same land were used directly for food production. To maintain the current ASP production, many scenarios relied on external land for feed production, manure export, or off-site rearing, shifting environmental burdens beyond the catchment. This study provides a model-based foundation for designing ecologically sound and socially acceptable livestock transition pathways, highlighting region-specific trade-offs and actionable strategies to reduce P surpluses while maintaining food production within environmental limits.