The recent interest in cover crops as component of Midwest corn and soybean production systems has led to the need for additional research, including the effects of residual corn and soybean herbicide treatments on fall cover crop establishment. Field studies were conducted in 2013, 2014, and 2015 in Columbia, Missouri to investigate the effects of common residual herbicides applied in corn and soybean on establishment of winter wheat, tillage radish, cereal rye, crimson clover, winter oat, Austrian winter pea, Italian ryegrass, and hairy vetch. Cover crops were evaluated for stand and biomass reduction 28 d after emergence (DAE). Rainfall from herbicide application to cover crop seeding date was much greater in 2014 and 2015, which resulted in less carryover in these years compared to 2013. When averaged across all herbicides evaluated in these experiments, the general order of sensitivity of cover crops to herbicide carryover, from greatest to least was Austrian winter pea=crimson clover>oilseed radish>Italian ryegrass>hairy vetch>wheat >winter oat>cereal rye. Cereal rye had the fewest instances of biomass or stand reduction with only four out of the 27 herbicides adversely effecting establishment. Pyroxasulfone consistently reduced Italian ryegrass and winter oat biomass at least 67% in both the corn and soybean experiments. In the soybean experiment, imazethapyr- and fomesafen-containing products resulted in severe stand and biomass reduction in both years while flumetsulam-containing products resulted in the greatest carryover symptoms in the corn experiment. Results from these experiments suggest that several commonly used corn and soybean herbicides have the potential to hinder cover crop establishment, but the severity of damage will depend on weather, cover crop species, and the specific herbicide combination.

Although water is crucial to the performance of preemergence herbicides, pesticide performance has rarely been related to irrigation management. This 2-yr study investigated the effect that amount of irrigation water applied had on activity of simazine. Three rates of simazine at 0, 1.12, and 2.24 kg/ha were applied to a 3-yr-old nectarine orchard that was irrigated with microsprinklers. The performance of simazine was compared between irrigation treatments initially targeted to provide water at 110 (efficient) and 175% (overwatered) of crop water requirements. Simazine effectiveness was based on the survival of oat and cucumber plants that were seeded at 0, 14, 28, 56, and 84 d after herbicide application. A longer time interval to 50% survival indicated prolonged herbicidal activity. Results were consistent between years in that simazine's performance was consistently greater in efficient irrigation treatments. The greatest increases were measured at the higher simazine application rate (2.24 kg/ha); overall averages for the length of time to reach 50% survival for cucumber were 50 and 23 d and for oats were 55 and 15 d for efficient and overwatered irrigation treatments, respectively. Use of an efficient irrigation management technique could have enhanced simazine's performance through a decreased leaching of residues from the weed root zone or less chemical or biological degradation (or both). Adoption of efficient irrigation management has been identified as a best management practice to mitigate leaching of pesticide residues to groundwater in coarse soils in California. This study indicates that efficient irrigation improves simazine performance and that both factors, pesticide application and irrigation management, should be considered when developing a weed management system.

Field studies were conducted to examine the dissipation of three soil-applied residual herbicides for bare soil compared with soil under low-density polyethylene (LDPE) mulch in 2003 and 2004. Studies indicated that halosulfuron and S-metolachlor dissipation was more rapid for bare soil than soil under LDPE mulch. Sulfentrazone dissipation from bare soil was equal to soil under LDPE mulch in 2003. However, sulfentrazone dissipation in 2004 was more rapid for soil under LDPE mulch than for bare soil. The order for half-life, defined as time for 50% dissipation (DT50), varied by herbicide and soil exposure. Averaged across 2003 and 2004, metolachlor DT50 was 2 d, halosulfuron 7 d, and sulfentrazone 16 d for bare soil. S-metolachlor DT50 was 4 d, halosulfuron 10 d, and sulfentrazone 13 d for soil under LDPE mulch. Correlation between quantified herbicide dissipation and bioassay for bare soil compared with soil under LDPE mulch in 2004 indicated that assay species root dry weights were negatively correlated with herbicide concentration. Data indicated that S-metolachlor and sulfentrazone bioassays, with oat and cotton, respectively, could be used to assess the level of dissipation for each herbicide.

Growing crops that exhibit a high level of competition with weeds increases opportunities to practice integrated weed management and reduce herbicide inputs. The recent development and market dominance of hybrid canola cultivars provides an opportunity to reassess the relative competitive ability of canola cultivars with small-grain cereals. Direct-seeded (no-till) experiments were conducted at five western Canada locations from 2006 to 2008 to compare the competitive ability of canola cultivars vs. small-grain cereals. The relative competitive ability of the species and cultivars was determined by assessing monocot and dicot weed biomass at different times throughout the growing season as well as oat (simulated weed) seed production. Under most conditions, but especially under warm and relatively dry environments, barley cultivars had the greatest relative competitive ability. Rye and triticale were also highly competitive species under most environmental conditions. Canada Prairie Spring Red wheat and Canada Western Red Spring wheat cultivars usually were the least competitive cereal crops, but there were exceptions in some environments. Canola hybrids were more competitive than open-pollinated canola cultivars. More importantly, under cool, low growing degree day conditions, canola hybrids were as competitive as barley, especially with dicot weeds. Under most conditions, hybrid canola growers on the Canadian Prairies are well advised to avoid the additional selection pressure inherent with a second in-crop herbicide application. Combining competitive cultivars of any species with optimal agronomic practices that facilitate crop health will enhance cropping system sustainability and allow growers to extend the life of their valuable herbicide tools.

Canada thistle poses a particular threat to organic producers in temperate agriculture due to its ability to reproduce through an extensive system of underground roots. The Canada thistle life cycle, growth, and development are seasonally affected, and exploiting this biology may be useful for weed management. The objective of this study was to evaluate smother crop mixtures, seeded at different times, for Canada thistle control. Field trials were established in 2009 and 2010 to evaluate the ability of smother crop mixtures to suppress Canada thistle growth and development. Canada thistle aboveground biomass was suppressed 50% in 2009 and 87% in 2010 by the sorghum–sudangrass mixture, averaged over planting times. The oat mixture suppressed annual weed biomass more than 58% in 2009 and 67% in 2010 in all planting dates. Percent cover of Canada thistle was affected by crop mixture in 2009 and 2010, with sorghum–sudangrass being the most suppressive. The sorghum–sudangrass mixture was more suppressive of Canada thistle, probably because it included soybean and sunflower, all high-biomass, competitive crops. Planting date affected smother crop suppression of Canada thistle growth, but the effect was not consistent between 2009 and 2010 due to differences in weather conditions.

The activity-density of Amara aenea (DeGeer) and Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) was monitored in an experiment that compared five management treatments representing a range of disturbance frequencies, crops, and aboveground biomass production. In 2004 and 2005, three treatments comprised of multiple summer cover crops were compared to bare fallow and soybean, the latter of which used mechanical cultivation to manage weeds. In 2005 weed seed predation was assessed from June to September in two of the treatments (bare fallow and oat–pea/rye–hairy vetch). Beetle activity-density varied with treatment, time of sampling, and year. In 2004 peak activity-density of A. aenea was highest in the mustard/buckwheat/canola, but there was no difference in H. pensylvanicus activity-density. In 2005 activity-density of H. pensylvanicus was higher in oat–pea/rye–hairy vetch than in soybean treatment. Seed predation rates were relatively consistent across treatments, averaging between 38 and 63%. In fallow and oat–pea/rye–hairy vetch, H. pensylvanicus activity-density accounted for 29 and 33% of the variation in seed predation, respectively. Our findings suggest cover crops have a positive effect on the activity-density of A. aenea and H. pensylvanicus and that disturbance negatively influences their activity-density in the absence of cover crops.