No-tillage field studies were conducted in 1999 and 2000 at Columbia, MO, to determine the interaction of grass weed interference and side-dressed N fertilization on corn and weed growth, corn yield, and the N content of the soil and plant biomass at various intervals early in the growing season and at harvest. Ammonium nitrate (112 kg N ha−1) was applied before planting. A herbicide was applied to the entire experimental area before planting to control winter vegetation and to reduce broadleaf weed emergence. A mixture of large crabgrass, giant foxtail, and barnyardgrass was allowed to reinfest the experiment after corn planting and was sprayed with glyphosate when the weeds were 8, 15, 23, or 31 cm tall. Each removal date treatment was duplicated; one series received 45 kg N ha−1 side-dressed when the corn was approximately 60 cm tall, and the other series received no additional N. Corn dry weight and the N content of the corn biomass early in the growing season were similar between the weed-free control and treatments with grass interference up to 23-cm height. Side-dressed N applications before grass weeds were controlled did not increase the early-season N content of the corn biomass in 1999, but the N content of the grass weeds increased by 11 kg ha−1. In 2000, side-dressed N increased the N content of the corn and grass biomass by 13 and 18 kg ha−1, respectively. This suggests that grass weeds should be controlled before the side-dressed N applications to ensure that the N applied can be used by the corn rather than by the grass weeds. Grass weed interference beyond 15-cm height reduced corn yield by at least 1.13 Mg ha −1 and N content in the corn biomass at corn harvest by at least 30 kg ha−1.

Certain sulfonylurea (SU) herbicides are used to remove overseeded cool-season species from bermudagrass. The effects of nitrogen (N) on the efficacy of a new SU herbicide, flazasulfuron, have not been determined. Field and laboratory studies were conducted in 2008 and 2009 evaluating the efficacy of flazasulfuron for control of overseeded perennial ryegrass contaminated with annual bluegrass. Flazasulfuron was applied at rates of 4.4, 8.8, and 17.5 g ha−1 alone, and in between sequential applications of N fertilizer at 73 kg N ha−1. N was granularly applied immediately prior to herbicide treatment and 4 wk later. In both years, the level of annual bluegrass control with flazasulfuron and two applications of N at 73 kg N ha−1 was significantly greater than following treatment with flazasulfuron alone. This response was observed for all application rates of flazasulfuron on every rating date. The level of annual bluegrass control with flazasulfuron at 4.4 g ha−1 and two applications of N at 73 kg ha−1 was greater than flazasulfuron at 17.5 g ha−1 alone each year. No significant differences in perennial ryegrass control were observed for flazasulfuron with and without N fertility. In laboratory studies with annual bluegrass, treatment with N fertilizer at 73 kg N ha−1 increased translocation of 14C flazasulfuron (and any potential metabolites) from treated annual bluegrass leaves to other shoot tissues by 18% at 1 h after treatment and 22% at 4 h after treatment compared to plants not treated with N fertilizer. This increase in translocation may explain the increased level of annual bluegrass control observed in the field.

Torpedograss is a serious problem in southern turfgrass, especially along the U.S. gulf coast. Studies were conducted during 1998, 1999, and 2000 to evaluate quinclorac for torpedograss control in bermudagrass turf. Three applications of quinclorac at 0.6 kg/ha spaced 21 d apart provided better torpedograss control (88%) than two applications at 0.8 kg/ha (69%) or one application at 1.7 kg/ha (69%). Two applications of quinclorac (0.8 kg/ha) plus diclofop (0.8 kg/ha) provided better torpedograss control (82%) than either herbicide applied alone when evaluated after a single season of application. Increasing the mowing interval prior to quinclorac application to allow for more foliage to be present did not improve control. Nitrogen application prior to quinclorac treatment did not improve torpedograss control. Long-term control will most likely require quinclorac applications for more than one season.