Herbicides with soil-residual activity have the potential for carryover into subsequent crops, resulting in injury to sensitive crops and limiting productivity if severe. The increased use of soil-residual herbicides in the United States for management of troublesome weeds in corn- and soybean-cropping systems has potential to result in more cases of carryover. Soil management practices have different effects on the soil environment, potentially influencing herbicide degradation and likelihood of carryover. Field experiments were conducted at three sites in 2019 and 2020 to determine the effects of corn (clopyralid and mesotrione) and soybean (fomesafen and imazethapyr) herbicides applied in the fall at reduced rates (25% and 50% of labeled rates) and three soil management practices (tillage, no-tillage, and a fall-established cereal rye cover crop) on subsequent growth and productivity of the cereal rye cover crop and the soybean and corn crops, respectively. Most response variables (cereal rye biomass and crop canopy cover at cover crop termination in the spring, early-season crop stand and herbicide injury ratings, and crop yield) were not affected by herbicide carryover. Corn yield was lower when soil was managed with a cereal rye cover crop compared with tillage at all three sites, while yield was lower for no-till compared with tillage at two sites. Soybean yield was lower when managed with a cereal rye cover crop compared with tillage and no-till at one site. Findings from this research indicate a low carryover risk for these herbicides across site-years when label rotational restrictions are followed and environmental conditions favorable for herbicide degradation exist, regardless of soil management practice on silt loam or silty clay loam soil types in the U.S. Midwest region.

Concern raised against using highly competitive, exotic, large-statured, perennial grasses with fast growth rates as bioenergy crops has led to calls for risk assessment before widespread cultivation. Weed risk assessments (WRAs) are decision support tools commonly used throughout the world to determine the invasion risk of new plant taxa—primarily used as a pre-entry screen. Here, we compare the common Australian (A-WRA) and newer U.S. (US-WRA) models to evaluate the invasion risk of 16 candidate bioenergy crops and to compare their WRA scores to 14 important agronomic crops and 10 invasive species with an agronomic origin. Of the 40 species assessed, the A-WRA and US-WRA ranked 34 and 28 species, respectively, as high risk, including the major crops alfalfa, rice, canola, and barley. Surprisingly, in several cases, both models failed to effectively parse weeds from crops. For example, cereal rye received scores above (US-WRA) or comparable to (A-WRA) kudzu, a widespread damaging invader of the Southeastern United States introduced as forage. Our results indicate that these models are unable to accurately address broad, intraspecific variation and that species introduced for agronomic purposes pose special limitations to WRAs. This further supports other calls for postborder evaluation (e.g., field testing) following WRA screening. We should be cautious of the role of WRAs in setting policy, as illustrated by this relative evaluation of novel crops.

The objective of this study was to evaluate weed control, crop yields, potential soil loss, and net returns to management of an integrated weed management system in no-till corn and soybean compared to an herbicide-based strategy. The integrated weed management system reduced herbicide inputs by delayed cover crop termination, herbicide banding, and high-residue cultivation (reduced herbicide [RH]), while the other system used continuous no-tillage and herbicides to control weeds (standard herbicide [SH]). Research was conducted within the Penn State Sustainable Dairy Cropping Systems Experiment, where corn and soybean are each planted once in a 6-yr crop rotation. In this 3-yr study, weed density and biomass were often greater under RH management, but weed biomass never exceeded 19 g m–2 in corn and 21 g m–2 in soybean. Corn yield and population did not differ in any year, and net returns to management were $33.65 ha–1 higher in RH corn due to lower herbicide costs and slightly, though not significantly, higher yields. Soybean yield was lower in RH compared to SH in 2 of 3 yr, and was correlated with soybean population and cover crop residue. Net financial returns were $43.69 ha–1 higher in SH soybean compared to RH. Predicted soil loss never exceeded T (maximum allowable soil loss) for any treatment and slope combination, though soil loss was 100% greater on a 10% slope under RH management (vs. SH) due to cultivation.