Long-term effects of spring wheat-soybean cropping systems on weed populations

A long-term cropping systems experiment was conducted near Fargo, North Dakota, from 1994 to 2001, to examine the effect of tillage systems and crop rotations on weed populations over time. The cropping systems were a conventional spring wheat-soybean rotation, a no-till spring wheat-soybean rotation, and a mulch-till system with a spring wheat-yellow sweetclover-winter-annual rye-soybean rotation. Emerged weeds were sampled three times each year in wheat and soybean at pre-plant, before post-emergence herbicide treatment, and post-harvest. Soil seed banks were determined at initiation and termination of the experiment. Thirty-nine weed species were identified during the experiment and annual broadleaf weeds were most prevalent. Canada thistle, common lambsquarters, dandelion, green and yellow foxtail, kochia, redroot pigweed, wild buckwheat, and wild mustard were dominant weeds in each cropping system over the 8-year experiment. Between 1994 and 2001, total weed density decreased in the mulch-till system based on weeds sampled in wheat (r = −0.93) and soybean (r = −0.68), primarily due to a decrease in annual broadleaf weeds. Total density of weeds sampled in wheat increased in the no-till system (r = 0.63), primarily due to an increase in common lambsquarters. The mulch-till system in combination with reduced amounts of herbicide, which were used to control Canada thistle, kept weed densities similar to conventional and no-tillage systems. Final soil seed bank densities were lower in the conventional system than either the no-till or mulch-till systems. Common lambsquarters and redroot pigweed seed banks were greater in no-till than the conventional or mulch-till systems in 2001, whereas green and yellow foxtail seed banks were greatest in the mulch-till system. In 2001, no-till and conventional system soil seed banks consisted of about 87% broadleaf and 13% grass species, whereas the mulch-till system soil seed bank consisted of 68% broadleaf and 32% grass species.