The Contribution of Factors to Weed Infestation of Grain–Grass–Row Crops in the Northwest of the Russian Federation

The aim of this study was to evaluate the effect of natural and anthropogenic factors on weed infestation of fields in the crop rotation, including cereals, row crops, and grasses, to understand the possibilities to manage the species composition and population of weed plants in the northwest of the Russian Federation. The experiments were performed in 2012–2018 in Leningrad oblast during the fifth rotation with the following crops: blue lupine, winter rye, spring barley with sowing of perennial grasses (red clover and timothy-grass), perennial grasses of the first and second year, potato, and spring rape. Weed infestation of fields in the crop rotation depended more on biological features of cultivated crops (species abundance of 76.1%, initial density of 29.6%, and phytomass after harvest of 21.0%) than on individual parameters of weed infestation (3.5, 8.3, and 17.7%, respectively). Weather conditions exerted the decisive effect on initial weed infestation (49.7%) and smaller effect on the species abundance (30.7%) and aboveground biomass of weeds (20.6%). Weather conditions influenced the effects of applied mineral fertilizers (interaction of 4.0–11.9%) and integrated plant-protection system (interaction of 2.9–12.0%). Among the studied factors of crop production intensification, application of herbicides in the variant with integrated plant-protection system exerted the greatest impact on the species abundance (26%) and final phytomass of weeds (26.2%), whereas long term application of mineral fertilizers affected the initial weed infestation of agrocenoses (10.2%), which was statistically significant for every year of the research (23.0–67.8%). The combined effect of mineral fertilizers and herbicides determined to the greatest rate the formation of the aboveground mass of weeds (interaction of 0.9–5.0%), in years with excessive humidity in particular.