Organic cropping systems maintain yields but have lower yield levels and yield stability than conventional systems – Results from the DOK trial in Switzerland

Abstract

Sufficient and stable crop yields are the basis for feeding a growing world population. Limited cropland, climate change, degradation of soil quality and loss of biodiversity coupled with excessive use of non-renewable resources require new solutions for future cropping systems beyond existing management practices. Here we analyzed mean yields, temporal yield trends and the stability of organic and conventional cropping systems from the currently longest-lasting cropping system comparison, the DOK long-term systems comparison trial (DOK) comparing biodynamic, bioorganic and conventional cropping systems, over a period of 40 years. We used yield data of winter wheat, potatoes, grass-clover, maize and soybean in a seven-year rotation, where bioorganic and biodynamic farming systems were compared with conventional mixed and sole mineral fertilized systems. System treatments have been established at a reduced half and a regular fertilization level, which corresponds to standard Swiss farming practices. Yields were significantly lower in organic systems in non-legumes between 13% and 34%, depending on the investigated crop, whereas in legumes, no yield reduction was observed in soybean and only 10% was observed in grass-clover. Half the amount of fertilizer reduced yields by around 10% in all systems and crops. Applied mineral N determined yields mainly in winter wheat and potatoes. Temporal yield trends did not differ between organic and conventional systems, nor between half and regular fertilization over all crops. However, in winter wheat, both conventional and biodynamic management with regular fertilization showed a stronger temporal increase in yield, while yield of grass-clover under biodynamic management with half-fertilization decreased. Increased yield differences between systems in single years were due to poor performance of organic systems rather than better performance of conventional systems. Absolute stability (measured by the variance) did not differ, but conventional systems were more stable than organic ones in relative stability, measured by the coefficient of variation, expressing the stability in relation to the yield level. We found no difference in both absolute and relative stabilities between half and regular fertilization. Long-term organic management results in lower yields than conventional management, but not in a decrease of yields over time. The similarity in both stability measures between half and regular fertilization suggests that the variation in relative stability between organic and conventional management might be more related to plant protection than to fertilization intensity.