Effect of chemical weed control on crop yields in different crop rotations in a long-term field trial

The study is based on data from a long-term field experiment of extent and duration seldom found in weed science. Yield losses from weed infestation of individual crops and crop rotations were studied in a field trial conducted since 1972 at two sites in the Czech Republic. The ongoing trial comprises multi-crop and simple crop rotations with 50 and 75% cereals, respectively. Three herbicide treatments were used: (1) untreated; (2) synthetic auxins (MCPA; 2,4-D; only in simple crop rotation) and (3) targeted herbicide combinations, including sulfonylureas, triazines, ureas, and synthetic auxins. Crop yields and weather data were recorded during the trial period and the effects of herbicide application on yield were determined for crop rotation. Yield losses in untreated controls increased in the following order: spring barley < winter wheat < pea < oilseed rape < potato. Because of the limited weed control spectrum, the use of synthetic auxins had lower yield effect than targeted herbicide combinations. Yields tended to increase over time in treated plots for almost all crops at the more productive Hněvčeves site, while stagnation or even decrease were observed in untreated plots. The mean treatment effect steadily increased as the experiment progressed, raising from 4.9% in 1972-1985 to 76.9% in 2006-2016. Dependent on individual crops and crop rotation, herbicide treatment ensured 13-50% higher crop yields than untreated plots. Treatment with targeted herbicide combinations reduced the negative impact of simple crop rotation on weed infestation and ensured sufficient crop yields.