Feed-feces-fertilizer: Greenhouse study and interviews with fertilizer producers indicate persistence and negative effect of glyphosate residue in manure-based fertilizers

Abstract

BACKGROUND: The herbicide glyphosate is the most widely used active ingredient in pesticides globally. Residues have been found in people, livestock, food and animal feed, and in the environment, but little is known about glyphosate residue in manure-based fertilizer. “Feed-feces-fertilizer” describes how manure fertilizer can be contaminated with glyphosate. This exposure can harm sensitive plants, such as tomato, and pose a risk to effective waste disposal and nutrient cycling along principles of the circular economy. We review the use and history and present a mixed methods research based on a real-world case from Finland where glyphosate residue in poultry manure fertilizer was suspected of inhibiting commercial organic tomato production. To test the fertilizer, we grew 72 ‘Encore’ variety tomatoes for 14 weeks using the same commercial greenhouse methods. To ascertain awareness and potential contamination mitigation measures, we also contacted five fertilizer companies with sales of biogenic fertilizer in Finland, representatives of two farming organizations, and two government organizations working on nutrient cycling and agricultural circular economy. RESULTS: The total harvest of tomatoes grown with fertilizer with glyphosate residue was 35% smaller and the yield of first-class tomatoes 37% lower than that of the control. Two of the five fertilizer companies identified poultry manure as a source of glyphosate contamination. Companies with awareness of pesticide residues reported interest in establishing parameters for pesticide residues. CONCLUSIONS: The extent of glyphosate contamination of recycled fertilizers is unknown, but this study shows that such contamination occurs with negative impacts on crop production. The example from Finland shows that a model of co-production between fertilizer producers and state regulatory agencies to establish safe limits can benefit both fertilizer producers and their customers.