A fall-established cereal rye (Secale cereale L.) cover crop has potential to provide weed suppression in fields that will be planted with corn (Zea mays L.) and/or soybean [Glycine max (L.) Merr.], reducing the selection pressure for herbicide resistance as part of an integrated weed management approach. However, biomass from an overwintering cover crop can be limited in areas with prolonged cool-spring growing conditions, like in the Upper Midwest region of the United States. This study was conducted to determine if a cereal rye cover crop grown in Wisconsin and terminated at the time of crop planting could provide effective levels of early-season weed suppression in corn-soybean systems practicing no-till soil management. Cover crop biomass, weed biomass, and weed density were assessed at two Wisconsin locations (eight site-crop-years total; four site-years of each corn and soybean) in 2019 and 2020. Cereal rye biomass production at the time of crop planting ranged from 290-1,480 kg ha -1 . Overall, no-till management with a cover crop reduced weed density by 31% and reduced weed biomass by 61% compared with no-till without a cover crop. These results indicate that the use of a cereal rye cover crop in the Upper Midwest has potential to reduce the exclusive reliance on spring burndown herbicides in no-till, but limited spring cover crop biomass in this region may diminish the weedsuppressive effects from this management practice compared with other parts of the United States where higher amounts of cover crop biomass can be achieved at crop planting time.

Adoption of a fall established, high biomass cereal rye cover crop has potential to diversify weed management in corn and soybean production systems, reducing the selection pressure for resistance to postemergence herbicides. However, farmers and crop consultants express concern about limited weed suppression from an overwintering cover crop in areas where high biomass production is limited by cooler spring temperatures, such as in the Upper-Midwest U.S. Use of a preemergence herbicide, regardless of cover crop adoption, is a standard recommendation for improving early season weed control in corn and soybean. Field experiments were conducted at two sites in Wisconsin to assess the effects of six soil management practices (tillage, no-till, and four cereal rye cover crop termination timings/methods) with or without the use of a preemergence herbicide on weed suppression at the time of postemergence herbicide application and crop productivity. Results showed that cereal rye biomass increased > 6x between termination at the time of cash crop planting versus termination two weeks later. In corn and soybean, weed ground cover was lower for soil management with cereal rye cover crop terminated two weeks after cash crop planting (≤ 7% weed cover) compared to all other soil management practices (≥ 23% weed cover) when a preemergence herbicide was not used. Use of a preemergence herbicide resulted in low weed ground cover across treatments in corn (≤ 7% weed cover) and soybean (≤ 13% weed cover). Corn and soybean yield was not affected by preemergence herbicide treatments. Corn yield was lower at the south-central Wisconsin location for the soil management with a cereal rye cover crop terminated two weeks after cash crop planting (9.82 Mg ha-1) compared to all other soil management practices (≥ 12.07 Mg ha-1); at the southwest Wisconsin location, corn yield was greater for the conventional tillage treatment (14.28 Mg ha-1) compared to all other soil management treatments (≤ 10.89 Mg ha-1). Soil management did not affect soybean yield, although yields were different between locations with 3.44 Mg ha-1 at the south-central Wisconsin compared to 4.77 Mg ha-1 at the southwest Wisconsin location. These results indicate that in the absence of a high biomass cereal rye cover crop, preemergence herbicides are important for in-season weed control. Also, the inclusion of a late-terminated cereal rye cover crop in soybean should be considered as an effective management practice for reducing weed ground cover without affecting crop yield.

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 to tillage at all three sites while yield was lower for no-till compared to tillage at two sites. Soybean yield was lower when managed with a cereal rye cover crop compared to 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 Midwest U.S. region.