Reduced herbicide use does not increase crop yield loss if it is compensated by alternative preventive and curative measures
Résumé
Herbicide use must be reduced because of environmental and health issues. This raises the question of whether weeds and the resulting crop yield loss will increase. Previous studies analysing relationships between herbicide use intensity, weeds and yield loss suffer from methodological shortcomings in terms of weed flora and farm diversity as well as temporal scales. Here, we collected data on 272 arable cropping systems from one Spanish and six French regions, from farm surveys, the Biovigilance-Flore network, expert opinion, cropping system trials, crop advisors and scientists. Each system was simulated over 27 years and with 10 weather repetitions, using the virtual-field model FlorSys. This process-based model simulates multi-species weed floras and crop canopies as a function of cropping systems and pedoclimate at a daily time-step over the years. Two series of simulations were run, starting with a typical regional weed flora, 1) simulating the recorded practices, and 2) eliminating all herbicides without any other change in management practices. These two series were run again, starting without weed seeds in the soil (series 3 and 4). Comparing series 1 and 2 to respectively 3 and 4 led to calculating a crop yield loss due to weeds in series 1 and 2. Comparing series 1 and 2 quantified the herbicide impact on weeds, crop production and, particularly, additional yield loss due to suppressing herbicides. The simulations showed that (1) crop yield loss increased with increasing weed biomass, and that the weed/crop biomass ratio at crop flowering was the best indicator of the year’s yield loss, (2) herbicide use intensity was not correlated to either weed variables or yield loss, because herbicide use intensity greatly depended on other management practices (e.g. tillage, mechanical weeding); it was also higher for farmers than for trial managers or in cropping systems designed with simulations, (3) average weed biomass during crop growth and yield loss respectively increased by +116% and +62% (averaged over rotation) when herbicides were eliminated without any other change in management practices, and these variations increased with increasing herbicide use intensity in the original cropping systems, (4) effects were more visible at the multi-annual (rotation) than the annual scales. The systems whose yield loss increased the most when herbicides were suppressed were characterized by monotonous rotations with short crop cover, high herbicide use, no plough or winter ploughing and frequent rolling operations. Finally, a decision tree predicting yield loss as a function of management practices was proposed to support farmers and crop advisors when designing innovative cropping systems reconciling low herbicide use and low yield loss.