Glyphosate contamination in manure-based fertilizers threatens tomato yields, study reveals

Recent research from Finland has found that glyphosate, a widely used herbicide, is contaminating manure-based fertilizers, posing risks to both organic and conventional tomato growers. The study was prompted by reports from a commercial organic tomato grower who suspected that poultry manure fertilizer was damaging their crops. Subsequent testing confirmed elevated glyphosate levels in the fertilizer used.    

To investigate the impact of glyphosate on crop yields, researchers compared two manure-based fertilizers marketed for professional horticulture. The first fertilizer, which had a glyphosate residue of 0.94 mg/kg, was the one used by the affected grower, while a control fertilizer contained a lower level of 0.23 mg/kg. Both products were marketed as suitable for certified organic farming, although the manure source was not necessarily from organic livestock.    

In a controlled greenhouse experiment, the scientists grew 72 Encore variety tomato plants using these fertilizers. The results were striking: the harvest from plants treated with the higher glyphosate fertilizer was 35% smaller, and the yield of first-class tomatoes was 37% lower compared to the control group.  The authors emphasize that the significant difference in tomato production can likely be attributed to the glyphosate present in the commercial fertilizer.    

To explore awareness of this issue, the researchers reached out to several fertilizer companies, farming organizations, and government agencies focused on agricultural sustainability. Notably, two fertilizer producers acknowledged poultry manure as a potential source of glyphosate contamination.    

These findings raise important questions about the origins of glyphosate in poultry manure. Possible sources include the application of glyphosate-based herbicides to desiccate feed grains before harvest, as well as the use of genetically modified crops that are tolerant to glyphosate.  

Another concerning finding from the research was that one fertilizer producer identified high herbicide residues in bakery waste, indicating that regulatory limits on pesticide residues in food do not necessarily ensure safety when recycling these products into fertilizers. This raises alarms about the safety of such products not only for crops but also for human consumption.    

The researchers conclude that the extent of glyphosate contamination in recycled fertilizers remains largely unknown, but their findings reveal negative impacts on crop production. The lack of comprehensive testing and regulation to ensure that these fertilizers are free from harmful pesticide levels poses risks, particularly for organic growers who rely on safe inputs.  

This issue adds to ongoing challenges in agriculture related to pesticide residues. Past incidents involving aminopyralid, another herbicide found in animal manure, have already caused significant damage to crops. Additionally, the problem of dicamba herbicide drifting from genetically modified crop fields continues to affect non-tolerant crops and wild plants across the U.S.    

The findings of this study highlight the urgent need for a shift towards certified pesticide-residue-free fertilizers. There is a clear market demand for products that guarantee low or no pesticide residues, ideally without imposing additional costs on organic farmers. Addressing this issue may require implementing a system where those responsible for pollution contribute through a tax on pesticide products, ensuring a fairer agricultural landscape for all producers.      

 

Source:  

Environmental Sciences Europe