Understanding how reduced-tillage practices influence weed community assembly is critical for designing ecologically sustainable organic cropping systems. We evaluated the effects on emerged weed biomass and seedbank dynamics of three cropping systems combining contrasting tillage regimes and cover crop strategies within an organic corn (Zea mays L.)-soybean [Glycine max (L.) Merr.]-spelt (Triticum spelta L.) rotation. Drawing from community assembly theory, we tested the roles of abiotic (soil disturbance), biotic (crop competition), and legacy filters (crop entry and cover crop history) across crop phases and spatial positions (interrow vs. intrarow). Our results show that weed community composition was shaped more by crop identity, spatial heterogeneity, and legacy effects than by tillage intensity alone. In soybean, the system with the lowest disturbance and a 14-mo undisturbed red clover (Trifolium pratense L.) cover crop legacy (IT/C-NT/S) selected for low-diversity communities dominated by giant foxtail (Setaria faberi Herrm.), particularly in intrarow zones. In contrast, in corn, spatial location explained more variation than cropping system, with intrarow communities again consistently dominated by S. faberi. Seedbank composition did not differ among systems but was significantly more diverse in the entry that had spelt in the last year of the rotation compared with the entry that had it in the first year, suggesting a strong filtering effect of the winter crop. Indicator species analysis further confirmed system-level filtering, with S. faberi strongly associated with low-disturbance soybean systems. These findings underscore the importance of considering within-field spatial heterogeneity and rotational legacy when designing organic weed management strategies and support the use of ecological filtering frameworks to understand weed community dynamics in complex organic systems.

Abstract Seed retention, and ultimately seed shatter, are extremely important for the efficacy of harvest weed seed control (HWSC) and are likely influenced by various agroecological and environmental factors. Field studies investigated seed-shattering phenology of 22 weed species across three soybean [Glycine max (L.) Merr.]-producing regions in the United States. We further evaluated the potential drivers of seed shatter in terms of weather conditions, growing degree days, and plant biomass. Based on the results, weather conditions had no consistent impact on weed seed shatter. However, there was a positive correlation between individual weed plant biomass and delayed weed seed–shattering rates during harvest. This work demonstrates that HWSC can potentially reduce weed seedbank inputs of plants that have escaped early-season management practices and retained seed through harvest. However, smaller individuals of plants within the same population that shatter seed before harvest pose a risk of escaping early-season management and HWSC.

Reducing the intensity and frequency of tillage in organic grain production systems is likely to produce several agronomic, economic, and environmental tradeoffs. We evaluated four organic corn (Zea mays L.) sequences in a 3-yr, full-entry organic cropping systems experiment. Two sequences included a hairy vetch (Vicia villosa Roth)/triticale (Triticale hexaploide Lart.) mixture sown after spelt harvest that differed in tillage practices (no-till [NT], conventional) before corn and corn harvest management (silage, grain). Two sequences included a red clover (Trifolium pretense L.)/timothy (Phleum pretense L.) mixture frost-seeded into spelt in late winter followed by conventional tillage preceding corn and either silage or grain harvest. Our results showed that late-season weed biomass did not differ across alternative sequences, although both tillage and NT corn production resulted in high in-row weed pressure in different years. Cover crop management prior to tillage-based corn production did not affect grain yields, but NT silage production resulted in lower yields compared with the tilled sequence. Corn silage yields were positively correlated with corn populations in both tilled and NT systems. The NT sequence lowered tillage frequency and intensity by 39 and 52%, respectively, compared with tillage-based corn production as measured using established metrics, including the soil disturbance rating, but did not significantly influence labile carbon pools. Sequences using underseeded red clover resulted in higher net returns compared with hairy vetch sequences due to additional income from fall forage harvests.

Abstract Organic grain producers are interested in reducing tillage to conserve soil and decrease labor and fuel costs. We examined agronomic and economic tradeoffs associated with alternative strategies for reducing tillage frequency and intensity in a cover crop–soybean (Glycine max L. Merr.) sequence within a corn (Zea mays L.)–soybean–spelt (Triticum spelta L.) organic cropping system experiment in Pennsylvania. Tillage-based soybean production preceded by a cover crop mixture of annual ryegrass (Lolium perenne L. ssp. multiflorum), orchardgrass (Dactylis glomerata L.) and forage radish (Raphanus sativus L.) interseeded into corn grain (Z. mays L.) was compared with reduced-tillage soybean production preceded by roller-crimped cereal rye (Secale cereale L.) that was sown after corn silage. Total aboveground weed biomass did not differ between soybean production strategies. Each strategy, however, was characterized by high inter-annual variability in weed abundance. Tillage-based soybean production marginally increased grain yield by 0.28 Mg ha−1 compared with reduced-tillage soybean. A path model of soybean yield indicated that soybean stand establishment and weed biomass were primary drivers of yield, but soybean production strategy had a measurable effect on yields due to factors other than within-season weed–crop competition. Cumulative tillage frequency and intensity were quantified for each cover crop—sequence using the Soil Tillage Intensity Rating (STIR) index. The reduced-tillage soybean sequence resulted in 50% less soil disturbance compared to tillage-based soybean sequence across study years. Finally, enterprise budget comparisons showed that the reduced-tillage soybean sequence resulted in lower input costs than the tillage-based soybean sequence but was approximately $114 ha−1 less profitable because of lower average yields.

With more than 65% of agronomic crops under no-till in Pennsylvania, herbicides are relied on for weed management. To lessen the environmental impact and selection pressure for herbicide resistance, we conducted a 9-yr experiment to test herbicide reduction practices in a dairy crop rotation at Rock Springs, PA. The rotation included soybean [Glycine max (L.) Merr.]-corn (Zea mays L.)-3-yr alfalfa (Medicago sativa L.)-canola (Brassica napus L.). The following practices were used to reduce herbicide inputs: (a) banding residual herbicides over corn and soybean rows and using high-residue inter-row cultivation; (b) seeding a small grain companion crop with alfalfa; (c) plowing once in 6 yr to terminate the perennial forage. These practices were compared with standard herbicide-based weed management (SH) in continuous no-till. We hypothesized: (a) There would be more weed biomass in the reduced herbicide treatment (RH), (b) leading to more weeds in RH over time, but (c) the added weed pressure would not affect yield (d) or differences in net return. We sampled weed biomass in soybean, corn, and the first two forage years. In corn and soybean, weed biomass was often greater in RH than SH and increased over the years in the RH treatments. In the forage, weed biomass did not always differ between treatments. Yield and differences in net return were similar in most crops and years. Results suggest that weed management with reduced herbicide inputs supplemented with an integrated approach can be effective but may lead to more weeds over time.

Abstract Potential effectiveness of harvest weed seed control (HWSC) systems depends upon seed shatter of the target weed species at crop maturity, enabling its collection and processing at crop harvest. However, seed retention likely is influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed-shatter phenology in 13 economically important broadleaf weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after physiological maturity at multiple sites spread across 14 states in the southern, northern, and mid-Atlantic United States. Greater proportions of seeds were retained by weeds in southern latitudes and shatter rate increased at northern latitudes. Amaranthus spp. seed shatter was low (0% to 2%), whereas shatter varied widely in common ragweed (Ambrosia artemisiifolia L.) (2% to 90%) over the weeks following soybean physiological maturity. Overall, the broadleaf species studied shattered less than 10% of their seeds by soybean harvest. Our results suggest that some of the broadleaf species with greater seed retention rates in the weeks following soybean physiological maturity may be good candidates for HWSC.

Seed shatter is an important weediness trait on which the efficacy of harvest weed seed control (HWSC) depends. The level of seed shatter in a species is likely influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed shatter of eight economically important grass weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after maturity at multiple sites spread across 11 states in the southern, northern, and mid-Atlantic United States. From soybean maturity to 4 wk after maturity, cumulative percent seed shatter was lowest in the southern U.S. regions and increased moving north through the states. At soybean maturity, the percent of seed shatter ranged from 1% to 70%. That range had shifted to 5% to 100% (mean: 42%) by 25 d after soybean maturity. There were considerable differences in seed-shatter onset and rate of progression between sites and years in some species that could impact their susceptibility to HWSC. Our results suggest that many summer annual grass species are likely not ideal candidates for HWSC, although HWSC could substantially reduce their seed output during certain years.

Cover crops are increasingly being adopted to provide multiple ecosystem services such as improving soil health, managing nutrients, and decreasing soil erosion. It is not uncommon for weeds to emerge in and become a part of a cover crop plant community. Since the role of cover cropping is to supplement ecosystem service provisioning, we were interested in assessing the impacts of weeds on such provisioning. To our knowledge, no research has examined how weeds in cover crops may impact the provision of ecosystem services and disservices. Here, we review services and disservices associated with weeds in annual agroecosystems and present two case studies from the United States to illustrate how weeds growing in fall-planted cover crops can provide ground cover, decrease potential soil losses, and effectively manage nitrogen. We argue that in certain circumstances, weeds in cover crops can enhance ecosystem service provisioning. In other circumstances, such as in the case of herbicide-resistant weeds, cover crops should be managed to limit weed biomass and fecundity. Based on our case studies and review of the current literature, we conclude that the extent to which weeds should be allowed to grow in a cover crop is largely context-dependent.

Organic grain producers are interested in interseeding cover crops into corn (Zea mays L.) in regions that have a narrow growing season window for post-harvest establishment of cover crops. A field experiment was replicated across 2 years on three commercial organic farms in Pennsylvania to compare the effects of drill- and broadcast-interseeding to standard grower practices, which included post-harvest seeding cereal rye (Secale cereale L.) at the more southern location and winter fallow at the more northern locations. Drill- and broadcast-interseeding treatments occurred just after last cultivation and used a cover crop mixture of annual ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] + orchardgrass (Dactylis glomerata L.) + forage radish (Raphanus sativus L. ssp. longipinnatus). Higher mean fall cover crop biomass and forage radish abundance (% of total) was observed in drill-interseeding treatments compared with broadcast-interseeding. However, corn grain yield and weed suppression and N retention in late-fall and spring were similar among interseeding treatments, which suggests that broadcast-interseeding at last cultivation has the potential to produce similar production and conservation benefits at lower labor and equipment costs in organic systems. Post-harvest seeding cereal rye resulted in greater spring biomass production and N retention compared with interseeded cover crops at the southern location, whereas variable interseeding establishment success and dominance of winter-killed forage radish produced conditions that increased the likelihood of N loss at more northern locations. Additional research is needed to contrast conservation benefits and management tradeoffs between interseeding and post-harvest establishment methods.

Reducing the intensityand frequency of tillage in an organic grain system requires an emphasis on utilizing ecological processes to manage pests and fertility. Cover crop-based, organic rotational no-till (CCORNT) corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production systems utilize cover crop surface mulch as the primary within-season weed control tactic. Winter-sown cereal rye (Secale cereale L.) was grown preceding soybean and hairy vetch (Vicia villosa Roth) mixtures before corn. We conducted a 3-yr cropping systems experiment in Delaware, Maryland, and Pennsylvania to evaluate crop management (planting date) and integrated weed management (IWM) tactics for CCORNT corn and soybean production in a corn-soybean-winter wheat (Triticum aestivum L.) rotation during the transition to organic. Corn and soybean yields were sensitive to planting date, but optimal planting dates differed among locations. Delayed cover crop termination did not consistently increase total cover crop biomass production or reduce weed biomass levels. High-residue cultivation reduced total weed biomass across locations, but this IWM tactic produced variable results on cash crop yields. Total weed biomass was negatively correlated to soybean yields but did not influence corn yields. At the Pennsylvania location, delaying corn planting dates was positively correlated with predatory athropod activity-density, which was positively correlated with corn populations. Assessment of CCORNT practices on short-term soil health indicators (labile carbon, aggregate stability, entomopathogenic fungi) at the Pennsylvania location produced variable results. Adaptive pest management strategies will need to be used in CCORNT systems within the Mid-Atlantic United States.

The hairy vetch (Vicia villosa Roth) cultivars 'Purple Bounty' (Reg. no. CV-12, PI 648342) and 'Purple Prosperity' (Reg. no. CV-11, PI 654047) were released in 2007 and 2008, respectively, by the USDA-ARS in collaboration with the Rodale Institute and the agricultural experiment stations of Pennsylvania State University and Cornell University. Hairy vetch is a commonly used annual legume cover crop grown for its cold tolerance, fast growth, large biomass production, and ability to fix N-2. However, this species has not been selected for the traits needed to optimize its use as a cover crop. Our breeding program focused on developing a cultivar that was both early flowering and had adequate winter survival and therefore adapted to mechanical termination in organic no-till production in the U.S. Northeast and Mid-Atlantic. Purple Bounty and Purple Prosperity were developed between 1998 and 2005 using recurrent selection at nurseries in Beltsville and Keedysville, MD. In 2005-2006, selections were evaluated against commercial checks for flowering time in Maryland and Pennsylvania, and in the 2006-2007 and 2007-2008 seasons they were evaluated in 10 locations (12 total site-years) across the United States for winter survival. Purple Bounty and Purple Prosperity both flowered earlier than the commercial material against which they were tested (significance depended on the date and site); Purple Bounty was the earlier flowering of the two cultivars. Purple Bounty and Purple Prosperity also had equivalent or improved winter survival compared with 'AU Early Cover', an early-maturing cultivar developed in the southern United States, at all test locations. Purple Prosperity is no longer commercially available, but Purple Bounty is currently licensed and distributed by Allied Seed (Nampa, ID).

Abstract Intensified cover-cropping practices are increasingly viewed as a herbicide-resistance management tool but clear distinction between reactive and proactive resistance management performance targets is needed. We evaluated two proactive performance targets for integrating cover-cropping tactics, including (1) facilitation of reduced herbicide inputs and (2) reduced herbicide selection pressure. We conducted corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] field experiments in Pennsylvania and Delaware using synthetic weed seedbanks of horseweed [Conyza canadensis (L.) Cronquist] and smooth pigweed (Amaranthus hybridus L.) to assess winter and summer annual population dynamics, respectively. The effect of alternative cover crops was evaluated across a range of herbicide inputs. Cover crop biomass production ranged from 2,000 to 8,500 kg ha-1 in corn and 3,000 to 5,500 kg ha-1 in soybean. Experimental results demonstrated that herbicide-based tactics were the primary drivers of total weed biomass production, with cover-cropping tactics providing an additive weed-suppression benefit. Substitution of cover crops for PRE or POST herbicide programs did not reduce total weed control levels or cash crop yields but did result in lower net returns due to higher input costs. Cover-cropping tactics significantly reduced C. canadensis populations in three of four cover crop treatments and decreased the number of large rosettes (>7.6-cm diameter) at the time of preplant herbicide exposure. Substitution of cover crops for PRE herbicides resulted in increased selection pressure on POST herbicides, but reduced the number of large individuals (>10 cm) at POST applications. Collectively, our findings suggest that cover crops can reduce the intensity of selection pressure on POST herbicides, but the magnitude of the effect varies based on weed life-history traits. Additional work is needed to describe proactive resistance management concepts and performance targets for integrating cover crops so producers can apply these concepts in site-specific, within-field management practices.

No‐till farmers who want more from their cover crops (CCs) are delaying CC termination until the main crop is planted. Delaying termination can help dry wet soils and reduce erosion. This process is referred to as planting green (PG). We hypothesized that PG would (i) dry soil at main crop planting, but conserve soil moisture later in the growing season; (ii) reduce soil temperature; (iii) reduce slug damage on main crops; and (iv) not reduce main crop yield. This experiment was conducted in Pennsylvania between 2015 and 2017 to compare two CC termination dates: preplant killed (PK) and planting green (PG) in corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.]. Planting green increased CC biomass an average of 94% and 94 to 181% compared to PK preceding corn and soybean, respectively. Soil was 7 to 24% drier and 0.9°C cooler at corn planting, and 8% drier and 0.7 to 2.4°C cooler at soybean planting in PG compared to PK. Slug damage was not different, lower, or higher in PG corn, and not different or lower in PG soybean compared to PK. Corn yield was reduced and not impacted by PG in higher and lower yielding environments, respectively. Soybean yield was stable across locations, and not affected by cover crop termination date. We concluded that corn was more vulnerable to yield losses from conditions created by PG than soybean; therefore, growers who desire potential benefits and lower risk from PG should first consider soybean. Core Ideas Planting green refers to planting the main crop into a living cover crop. Planting green increased cover crop biomass by 94% in corn and by 94 to 181% in soybean. Planting green dried the soil at main crop planting. Planting green cooled soil 0.7 to 2.4°C at planting. Soybean yield was not influenced by planting green; corn yield was reduced.

Abstract Proactive integrated weedmanagement (IWM) is critically needed in no-till production to reduce the intensity of selection pressure for herbicide-resistant weeds. Reducing the density of emerged weed populations and the number of larger individuals within the population at the time of herbicide application are two practical management objectives when integrating cover crops as a complementary tactic in herbicide-based production systems. We examined the following demographic questions related to the effects of alternative cover-cropping tactics following small grain harvest on preplant, burndownmanagement of horseweed (Erigeron canadensis L.) in no-till commodity-grain production: (1) Do cover crops differentially affect E. canadensis density and size inequality at the time of herbicide exposure? (2)Which cover crop response traits are drivers of E. canadensis suppression at time of herbicide exposure? Interannual variation in growing conditions (study year) and intra-annual variation in soil fertility (low vs. high nitrogen) were the primary drivers of cover crop response traits and significantly affected E. canadensis density at the time of herbicide exposure. In comparison to the fallow control, cover crop treatments reduced E. canadensis density 52% to 86% at the time of a preplant, burndown application. Cereal rye (Secale cereale L.) alone or in combination with forage radish (Raphanus sativus L.) provided the most consistent E. canadensis suppression. Fall and spring cover crop biomass production was negatively correlated with E. canadensis density at the preplant burndown application timing. Our results also show that winter-hardy cover crops reduce the size inequality of E. canadensis populations at the time of herbicide exposure by reducing the number of large individuals within the population. Finally, we advocate for advancement in our understanding of complementarity between cover crop– and herbicide-based management tactics in no-till systems to facilitate development of proactive, herbicide-resistant management strategies.

Core Ideas Annual ryegrass and orchardgrass performed similarly in field corn. Medium red clover was the most consistent legume species in field corn. No effect of annual ryegrass cultivar was observed in field corn. Annual ryegrass was the most consistent species in soybean. ABSTRACTDrill‐interseeding is becoming a viable method for integrating cover crops in no‐till corn (Zea mays L.) production in the Mid‐Atlantic region. Development of best management practices for drill‐interseeding cover crops into no‐till grain crops requires greater understanding of cover crop performance at the species and cultivar level. Experiments were conducted at multiple Mid‐Atlantic locations (Maryland, Pennsylvania, New York) in two consecutive growing seasons (2013–2014, 2014–2015) to evaluate establishment and performance of drill‐interseeded: (i) grass and legume cover crop species (n = 8) and annual ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] cultivars (n = 10) in field corn, and (ii) grass and legume cover crop species (n = 6) in soybean [Glycine max (L.) Merr]. Fall biomass production of drill‐interseeded cover crops was higher and less variable among locations in field corn than soybean. Annual ryegrass, orchardgrass (Dactylis glomerata L.), medium red clover (Trifolium pratense L.), and crimson clover (Trifolium incarntum L.) produced greater mean fall biomass than other species in field corn, but variable winter hardiness of crimson clover resulted in less spring biomass than alternative species. No differences were observed among annual ryegrass cultivars. Annual ryegrass, medium red clover, and crimson clover produced greater mean fall biomass than other species in soybean across locations. Our results highlight the viability of a narrow suite of cover crop species for interseeding in Mid‐Atlantic no‐till grain systems and point to the need for development of agronomic practices that facilitate greater niche complementarity between cover and cash crops.

Abstract Herbicide resistance is ‘wicked’ in nature; therefore, results of the many educational efforts to encourage diversification of weed control practices in the United States have been mixed. It is clear that we do not sufficiently understand the totality of the grassroots obstacles, concerns, challenges, and specific solutions needed for varied crop production systems. Weed management issues and solutions vary with such variables as management styles, regions, cropping systems, and available or affordable technologies. Therefore, to help the weed science community better understand the needs and ideas of those directly dealing with herbicide resistance, seven half-day regional listening sessions were held across the United States between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide resistance management. The major goals of the sessions were to gain an understanding of stakeholders and their goals and concerns related to herbicide resistance management, to become familiar with regional differences, and to identify decision maker needs to address herbicide resistance. The messages shared by listening-session participants could be summarized by six themes: we need new herbicides; there is no need for more regulation; there is a need for more education, especially for others who were not present; diversity is hard; the agricultural economy makes it difficult to make changes; and we are aware of herbicide resistance but are managing it. The authors concluded that more work is needed to bring a community-wide, interdisciplinary approach to understanding the complexity of managing weeds within the context of the whole farm operation and for communicating the need to address herbicide resistance.

AbstractSeven half-day regional listening sessions were held between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide-resistance management. The objective of the listening sessions was to connect with stakeholders and hear their challenges and recommendations for addressing herbicide resistance. The coordinating team hired Strategic Conservation Solutions, LLC, to facilitate all the sessions. They and the coordinating team used in-person meetings, teleconferences, and email to communicate and coordinate the activities leading up to each regional listening session. The agenda was the same across all sessions and included small-group discussions followed by reporting to the full group for discussion. The planning process was the same across all the sessions, although the selection of venue, time of day, and stakeholder participants differed to accommodate the differences among regions. The listening-session format required a great deal of work and flexibility on the part of the coordinating team and regional coordinators. Overall, the participant evaluations from the sessions were positive, with participants expressing appreciation that they were asked for their thoughts on the subject of herbicide resistance. This paper details the methods and processes used to conduct these regional listening sessions and provides an assessment of the strengths and limitations of those processes.

Core Ideas Observed trade‐off between corn planting density and drill interseeded cover crop biomass. Effect of corn density on cover crop biomass was mediated by light and weed biomass. Interseeded cover crops suppressed weeds in the fall and did not affect corn grain yield. A field experiment was conducted at three sites (New York, Pennsylvania, and Maryland) in 2016 to test the effects of drill interseeding a cover crop mixture consisting of cereal rye ( Secale cereale L.), annual ryegrass ( Lolium multiflorum Lam.), hairy vetch ( Vicia villosa Roth), and red clover ( Trifolium pratense L.) into organically managed corn ( Zea mays L.). We quantified the effects of corn density on weed biomass, cover crop biomass, and corn grain yield. Increasing corn density had a direct negative effect on interseeded cover crop biomass as well as indirect effects that were mediated by light transmission and weeds. At two sites, corn grain yield at the low corn density (3.71 plants m −2 ) did not differ from corn grain yield at the standard density (7.41 plants m −2 ). We also compared plots with and without interseeded cover crops at the same standard corn planting density. Corn grain yield did not differ, but weed biomass at the October sample date was 31% lower in plots with interseeded cover crops compared to plots without. Our results suggest that organic farmers may be able to (i) improve weed suppression in corn by interseeding cover crops and (ii) optimize cropping system performance by planting corn at a slightly lower rate (e.g., 5–10%) than what is typically used when interseeding cover crops. Additional research should be conducted across a wider range of environments to determine corn planting rate recommendations that optimize corn yield, cover crop growth, weed suppression, and profitability in organic cropping systems.