Kochia is one of the most problematic weeds in the United States. Field studies were conducted in five states (Wyoming, Colorado, Kansas, Nebraska, and South Dakota) over 2 yr (2010 and 2011) to evaluate kochia control with selected herbicides registered in five common crop scenarios: winter wheat, fallow, corn, soybean, and sugar beet to provide insight for diversifying kochia management in crop rotations. Kochia control varied by experimental site such that more variation in kochia control and biomass production was explained by experimental site than herbicide choice within a crop. Kochia control with herbicides currently labeled for use in sugar beet averaged 32% across locations. Kochia control was greatest and most consistent from corn herbicide programs (99%), followed by soybean (96%) and fallow (97%) herbicide programs. Kochia control from wheat herbicide programs was 93%. With respect to the availability of effective herbicide options, glyphosate-resistant kochia control was easiest in corn, soybean, and fallow, followed by wheat; and difficult to manage with herbicides in sugar beet.

A segment of the debate surrounding the commercialization of genetically engineered (GE) crops, such as glyphosate-resistant (GR) crops, focuses on the theory that implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was conducted in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR corn, cotton, or soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on emerged weed density and diversity was analyzed. Species richness, evenness, Shannon's H′, proportion of forbs, erect growth habit, and C3 species diversity were all greater in agricultural sites that lacked crop rotation or were in a continuous GR crop system. Rotating between two GR crops (e.g., corn and soybean) or rotating to a non-GR crop resulted in less weed diversity than a continuous GR crop. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. The finding that fields with continuous GR crops demonstrated greater weed diversity is contrary to arguments opposing the use of GE crops. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits, or more broadly, GE crops, to provide a more complete characterization of their culture and local adaptation.

Glyphosate-resistant (GR) cropping systems are popular and used extensively by producers. However, the long-term impacts of heavy reliance of this technology on weed community structure are not known. Five fully phased field experiments (two no-tillage and three conventional tillage) were established at four locations in southwestern Ontario where the effects of herbicide system (glyphosate or conventional) in corn and soybean and crop rotation (corn–soybean or corn–soybean–winter wheat) on midseason weed communities were examined. Multivariate analysis on data over the last 3 yr of the 6-yr experiment showed that weed communities were distinctly different among the treatments within each experiment. At several locations, midseason weed communities were more similar in corn and soybean treated with glyphosate compared to the same crops treated with conventional herbicides, reflecting the continuous application of the same selection pressure in both crops. Analysis of trait-densities revealed an increase in species with late initiation of seedling recruitment at the expense of weed species with medium time of initiation of seedling recruitment rather than early recruiting species. Increases in perennial species, species with a short interval between recruitment and anthesis, and wind-dispersed species were also observed. Trait-density–based analysis of the weed community was an effective method for reducing the complexity of divergent weed communities that enabled direct quantitative comparison of the herbicide-induced effects on these weed communities.

Field experiments were conducted from 1999 to 2001 to evaluate preemergence (PRE) activity of cloransulam on broadleaf weed species and to determine the effectiveness of cloransulam as a PRE herbicide in glyphosate-resistant soybean weed management systems. Cloransulam PRE controlled prickly sida, velvetleaf, and morningglory species even at reduced rates (recommended rate 36 g ai/ha) but only suppressed growth of Palmer amaranth, hemp sesbania, and sicklepod. Cloransulam applied PRE provided initial control or suppression of most weeds, but late-season control declined appreciably. Adding metribuzin to cloransulam PRE generally improved control of hemp sesbania, Palmer amaranth, annual grasses, and morningglory species, leading to soybean yield increases. Control of weeds was greater on a silt loam soil compared with a silty clay soil. Delayed herbicide activation by rainfall or irrigation reduced control of hemp sesbania and prickly sida and affected efficacy more than soil texture. Single postemergence (POST) applications of glyphosate or fomesafen plus fluazifop-P provided 90% or less control of most weed species. When glyphosate POST or fomesafen plus fluazifop-P POST followed PRE applications of cloransulam or cloransulam plus metribuzin PRE, control of all weeds was generally greater than 85%. The highest soybean yields were recorded from treatments that contained sequential PRE followed by (fb) POST herbicide applications. Composition of weed flora determined the effect of herbicide program on soybean seed yield. No yield benefit was gained from the sequential program when the dominant species was Palmer amaranth, which was controlled by glyphosate. When hemp sesbania was the dominant species, PRE herbicides fb glyphosate POST increased yield compared with total POST glyphosate.