Herbicide intensity depends on cropping system and weed control target: Unraveling the effects in field experiments

Herbicide use simplified cropping systems, which lead to short rotations and sowing dates rather focusing on yield than on weed management. Re-diversifying these cropping systems should reduce the vast use of herbicides. We hypothesize that herbicide use decreases with more diverse cropping systems even with few common crops. Cropping system experiments raise methodological questions connected with herbicide use. So far, herbicide use in field experiments is a-priori fixed and not adapted to changing situations. To overcome this, we developed a heuristic decision rule to employ herbicide intensities as response variable in statistical models. Field trials were set up at two locations in Germany as split plot design with cropping systems as main factor and weed control targets as sub-factor. The cropping systems contained maize, winter wheat and winter oilseed rape, and differed in length and sowing date of winter wheat. Weed control targeted a high, medium and low multi-year level. The outcome 'herbicide intensity' was described as treatment frequency index (TFI) and as number of active ingredients (NAI). Finally, crops were harvested and yields measured. The heuristic decision rule enabled reproducible, objective herbicide decisions to adapt herbicide intensities to various weed infestations. Herbicide intensities sorted themselves according to the multi-year control target. All crops required a lower herbicide intensity when grown in longer rotations: on average TFI decreased by 19% and 11% resp. depending on the location, and NMAI decreased by 0.3 on average per year. Highest yields were harvested with a medium to high weed control target. The heuristic approach opens up new opportunities to collect data on herbicide use as a dependent variable. More diversity in cropping systems reduced herbicide intensity, nevertheless, could not completely compensate the use of herbicides. Thus, we consider them as a useful tool in integrated weed management.