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Over the past half-century, crop rotations have become increasingly simplified, with whole regions producing only one or two crops in succession. Simplification is problematic from a weed management perspective, because it results in weeds' repeated exposure to the same set of ecological and agronomic conditions. This can exacerbate weed infestations and promote the evolution of herbicide resistance. Diversifying crop rotations through addition of crop species and their associated managements may suppress weeds and reduce selection pressure for herbicide resistance by altering stress and mortality factors affecting weed dynamics. Here we report the results of a meta-analysis using 298 paired observations from 54 studies across six continents to compare weed responses due to simple and more diverse crop rotations. We found diversifying from simple rotations reduced weed density (49%), but did not have a significant effect on weed biomass. We investigated the effect of management practices, environmental factors, and rotation design on this effect. Diversification that increased the variance around crop planting dates was more effective in suppressing weeds than increasing crop species richness alone. Increasing rotational diversity reduced weed density more under zero-tillage conditions (65%) than tilled conditions (41%), and did so regardless of environmental context and auxiliary herbicide use. Our findings highlight the value of diversifying crop rotations to control weed populations, and support its efficacy under varied environmental conditions and management scenarios.

As landscapes have become increasingly dominated by intensive agricultural production, plant diversity has declined steeply along with communities of pollinating insects including bees. Semi-natural habitats, such as field edge meadows and hedgerows, can be maintained to provide a diversity of flowering plants that can increase floral resources required by bees. An additional habitat enhancement practice is that of sowing strips of native prairie vegetation within row-cropped fields. In this study, conducted in Iowa, USA, we found that increases in both the abundance and diversity of floral resources in strips of native prairie vegetation within agricultural production fields greatly and positively influenced the bee community. The benefits to the bee community were important for both common and uncommon species and the effect may be strongest early in the season. Using networks of co-occurrence between plant and bee species, we were able to identify two native prairie plants, Ratibida pinnata and Zizia aurea, as potentially keystone resources that can be used to support native bees. When we evaluated the effect of reconstructed prairie strips on bees in the context of the surrounding landscape, we found that these conservation practices had positive effects on bees in agriculturally-dominated areas and that these effects were detectable in low to high complexity landscapes with 8-69% natural habitat. In landscapes dominated by crops with few pollen and nectar resources the inclusion of native prairie strips can buffer the decline of bees and effectively increase bee abundance and diversity.

Summary Recent studies have revealed many potential benefits of increasing plant diversity in natural ecosystems, as well as in agroecosystems and production forests. Plant diversity potentially provides a partial to complete substitute for many costly agricultural inputs, such as fertilizers, pesticides, imported pollinators and irrigation. Diversification strategies include enhancing crop genetic diversity, mixed plantings, rotating crops, agroforestry and diversifying landscapes surrounding croplands. Here we briefly review studies considering how increasing plant diversity influences the production of crops, forage, and wood, yield stability, and several regulating and supporting agroecosystem services. We also discuss challenges and recommendations for diversifying agroecosystems. There is consistently strong evidence that strategically increasing plant diversity increases crop and forage yield, wood production, yield stability, pollinators, weed suppression and pest suppression, whereas effects of diversification on soil nutrients and carbon remain poorly understood. Synthesis. The benefits of diversifying agroecosystems are expected to be greatest where the aims are to sustainably intensify production while reducing conventional inputs or to optimize both yields and ecosystem services. Over the next few decades, as monoculture yields continue to decelerate or decline for many crops, and as demand for ecosystem services continues to rise, diversification could become an essential tool for sustaining production and ecosystem services in croplands, rangelands and production forests. The benefits of diversifying agroecosystems are expected to be greatest where the aims are to sustainably intensify production while reducing conventional inputs or to optimize both yields and ecosystem services. Over the next few decades, as monoculture yields continue to decelerate or decline for many crops, and as demand for ecosystem services continues to rise, diversification could become an essential tool for sustaining production and ecosystem services in croplands, rangelands and production forests.

Balancing productivity, profitability, and environmental health is a key challenge for agricultural sustainability. Most crop production systems in the United States are characterized by low species and management diversity, high use of fossil energy and agrichemicals, and large negative impacts on the environment. We hypothesized that cropping system diversification would promote ecosystem services that would supplement, and eventually displace, synthetic external inputs used to maintain crop productivity. To test this, we conducted a field study from 2003–2011 in Iowa that included three contrasting systems varying in length of crop sequence and inputs. We compared a conventionally managed 2-yr rotation (maize-soybean) that received fertilizers and herbicides at rates comparable to those used on nearby farms with two more diverse cropping systems: a 3-yr rotation (maize-soybean-small grain + red clover) and a 4-yr rotation (maize-soybean-small grain + alfalfa-alfalfa) managed with lower synthetic N fertilizer and herbicide inputs and periodic applications of cattle manure. Grain yields, mass of harvested products, and profit in the more diverse systems were similar to, or greater than, those in the conventional system, despite reductions of agrichemical inputs. Weeds were suppressed effectively in all systems, but freshwater toxicity of the more diverse systems was two orders of magnitude lower than in the conventional system. Results of our study indicate that more diverse cropping systems can use small amounts of synthetic agrichemical inputs as powerful tools with which to tune, rather than drive, agroecosystem performance, while meeting or exceeding the performance of less diverse systems.

Biofuel production from cellulosic feedstocks may increase during the next century. To be sustainable, this production should protect environmental quality and biodiversity. Fertilized mixed‐species prairie can deliver substantial quantities of cellulosic ethanol per unit land area with minimal losses of NO3‐N in drainage water, but the long‐term maintenance of biodiversity in such systems has been uncertain. We report how nitrogen, phosphorus, and potassium fertilizer application, precipitation, and time affected the species composition and productivity of reconstructed prairie communities harvested annually as biofuel feedstocks over a 9‐year period. Results indicated that both precipitation and fertilizer application drove above‐ground biomass production, with the greatest response to fertilizer occurring in wetter than average years. Fertilization reduced species richness, but increased species evenness. Consequently, Simpson's diversity index did not differ between the fertilized and unfertilized communities, though it declined in both communities over time. A total of 59 plant species was recorded, with eight of them explaining most of the differences in vegetation cover between the fertilized and unfertilized treatments. After 9 years, the high fertility community was dominated by the C4 grass Andropogon gerardii, the C3 grass Elymus canadensis, and the non‐leguminous forbs Heliopsis helianthoides, Helianthus maximiliani, and Monarda fistulosa, whereas the low fertility community was dominated by the C4 grasses A. gerardii and Sorghastrum nutans, the C3 grass E. canadensis, and the non‐leguminous forb M. fistulosa. Fertilization increased the abundance of flowering forbs available to pollinators in the early, middle and late portions of the growing season. Synthesis and applications. Results of our study suggest that maintenance of reasonably high levels of productivity and biodiversity are possible in fertilized prairie communities harvested annually for bioenergy, with plant cover more evenly distributed among different functional groups. In the future, if policy and markets favour biofuels and better delivery of ecosystem services from harvested land, prairie‐based feedstocks could become part of a renewable energy portfolio that fosters biodiversity and contributes to the provision of floral resources for pollinators. Foreign Language RÉSUMÉ La production de biocarburant à partir de matières premières cellulosiques pourrait s'accroître au cours du siècle prochain. Afin d’être durable, cette production doit pouvoir préserver l'environnement et la biodiversité. Les prairies mixtes fertilisées peuvent fournir une quantité importante d’éthanol cellulosique par unité de surface avec des pertes minimales de NO3‐N dans les eaux de drainage, cependant il y a une incertitude du maintien de la biodiversité à long terme. Nous présentons ici l'effet de l'application d'engrais NPK, des précipitations et du temps sur la composition d'espèces et la productivité des communautés de prairies reconstituées récoltées annuellement en tant que matière première pour la production de biocarburant sur une période de neuf ans. Les résultats montrent que les précipitations ainsi que l'application d'engrais ont accentué la production de biomasse aérienne, avec une plus forte réponse à l'application d'engrais durant les années plus humides que les années moyennes. La fertilisation a réduit la richesse d'espèces mais a augmenté leur régularité. Par conséquent, l'indice de diversité de Simpson n'a pas divergé entre les communautés fertilisées et non fertilisées bien qu'il ait diminué dans les deux communautés au cours du temps. Au total 59 espèces de plantes ont été recensées, dont huit expliquant la majorité de la différence du couvert végétal entre les traitements fertilisés et non‐fertilisés. Après neuf ans, la communauté à haut niveau de fertilisant était dominée par les graminées C4 Andropogon gerardii et C3 Elymus canadensis ainsi que les phorbes non‐légumineuses Heliopsis helianthoides, Helianthus maximiliani et Monarda fistulosa. La communauté à bas niveau de fertilisant était dominée par les graminées C4 A. gerardii, Sorghastrum nutans et C3 E. canadensis ainsi que la phorbe non‐légumineuse M. fistulosa. La fertilisation a permis d'augmenter l'abondance de phorbes à floraison disponibles pour les pollinisateurs en début, milieu et fin du cycle de croissance des cultures. Synthèse et applications. Les résultats de notre étude suggèrent que le maintien de la productivité et de la biodiversité à un niveau raisonnablement élevé est possible pour des communautés de prairie fertilisées récoltées annuellement à des fins bioénergétiques, avec un couvert végétal distribué de manière plus égale entre les groupes fonctionnels. À l'avenir, si les politiques et marchés favorisent les biocarburants et un meilleur maintien des services écosystémiques fournis par les champs cultivés, les matières premières fournies par les prairies pourraient rejoindre l’éventail des sources d’énergies renouvelables et contribuer à la production de ressources florales pour les pollinisateurs. Results of our study suggest that maintenance of reasonably high levels of productivity and biodiversity are possible in fertilized prairie communities harvested annually for bioenergy, with plant cover more evenly distributed among different functional groups. In the future, if policy and markets favour biofuels and better delivery of ecosystem services from harvested land, prairie‐based feedstocks could become part of a renewable energy portfolio that fosters biodiversity and contributes to the provision of floral resources for pollinators.

Sudden death syndrome (SDS) is one of the major yield-limiting soybean diseases in the Midwestern United States. Effective management for SDS requires accurate detection in soybean fields. Since traditional scouting methods are time-consuming, labor-intensive, and often destructive, alternative methods to monitor SDS in large soybean fields are needed. This study explores the potential of using high-resolution (3 m) PlanetScope satellite imagery for detection of SDS using the random forest classification algorithm. Image data from blue, green, red, and near-infrared (NIR) spectral bands, the calculated normalized difference vegetation index (NDVI), and crop rotation information were used to detect healthy and SDS-infected quadrats in a soybean field experiment with different rotation treatments, located in Boone County, Iowa. Datasets collected during the 2016, 2017, and 2018 soybean growing seasons were analyzed. The results indicate that spectral features, when combined with ground-based information, can detect areas in soybean plots that are at risk for disease, even before foliar symptoms develop. The classification of healthy and diseased soybean quadrats was >75% accurate and the area under the receiver operating characteristic curve (AUROC) was >70%. Our results indicate that high-resolution satellite imagery and random forest analyses have the potential to detect SDS in soybean fields, and that this approach may facilitate large-scale monitoring of SDS (and possibly other economically important soybean diseases). It may also be useful for guiding recommendations for site-specific management in current and future seasons.

Weed management is a critically important activity on both agricultural and non-agricultural lands, but it is faced with a daunting set of challenges: environmental damage caused by control practices, weed resistance to herbicides, accelerated rates of weed dispersal through global trade, and greater weed impacts due to changes in climate and land use. Broad-scale use of new approaches is needed if weed management is to be successful in the coming era. We examine three approaches likely to prove useful for addressing current and future challenges from weeds: diversifying weed management strategies with multiple complementary tactics, developing crop genotypes for enhanced weed suppression, and tailoring management strategies to better accommodate variability in weed spatial distributions. In all three cases, proof-of-concept has long been demonstrated and considerable scientific innovations have been made, but uptake by farmers and land managers has been extremely limited. Impediments to employing these and other ecologically based approaches include inadequate or inappropriate government policy instruments, a lack of market mechanisms, and a paucity of social infrastructure with which to influence learning, decision-making, and actions by farmers and land managers. We offer examples of how these impediments are being addressed in different parts of the world, but note that there is no clear formula for determining which sets of policies, market mechanisms, and educational activities will be effective in various locations. Implementing new approaches for weed management will require multidisciplinary teams comprised of scientists, engineers, economists, sociologists, educators, farmers, land managers, industry personnel, policy makers, and others willing to focus on weeds within whole farming systems and land management units.

Plant-soil relations may explain why low-external input (LEI) diversified cropping systems are more efficient than their conventional counterparts. This work sought to identify links between management practices, soil quality changes, and root responses in a long-term cropping systems experiment in Iowa where grain yields of 3-year and 4-year LEI rotations have matched or exceeded yield achieved by a 2-year maize (Zea mays L.) and soybean (Glycine max L.) rotation. The 2-year system was conventionally managed and chisel-ploughed, whereas the 3-year and 4-year systems received plant residues and animal manures and were periodically moldboard ploughed. We expected changes in soil quality to be driven by organic matter inputs, and root growth to reflect spatial and temporal fluctuations in soil quality resulting from those additions. We constructed a carbon budget and measured soil quality indicators (SQIs) and rooting characteristics using samples taken from two depths of all crop-phases of each rotation system on multiple dates. Stocks of particulate organic matter carbon (POM-C) and potentially mineralizable nitrogen (PMN) were greater and more evenly distributed in the LEI than conventional systems. Organic C inputs, which were 58% and 36% greater in the 3-year rotation than in the 4-year and 2-year rotations, respectively, did not account for differences in SQI abundance or distribution. Surprisingly, SQIs did not vary with crop-phase or date. All biochemical SQIs were more stratified (p<0.001) in the conventionally-managed soils. While POM-C and PMN in the top 10 cm were similar in all three systems, stocks in the 10-20 cm depth of the conventional system were less than half the size of those found in the LEI systems. This distribution was mirrored by maize root length density, which was also concentrated in the top 10 cm of the conventionally managed plots and evenly distributed between depths in the LEI systems. The plow-down of organic amendments and manures established meaningful differences in SQIs and extended the rhizosphere of the LEI systems. Resulting efficiencies observed in the LEI grain crops indicate that resource distribution as well as abundance is an important component of soil function that helps explain how LEI systems can maintain similar or greater yields with fewer inputs than achieved by their conventional counterparts.

Cool-season cover crops have been shown to reduce soil erosion and nutrient discharge from maize ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] production systems. However, their effects on long-term weed dynamics are not well-understood. We utilized five long-term research trials in Iowa to quantify germinable weed seedbank densities and compositions after 10+ years of cover cropping treatments. All five trials consisted of zero-tillage maize-soybean rotations managed with and without the inclusion of a yearly winter rye ( Secale cereal L.) cover crop. Seedbank sampling was conducted in the early spring before crop planting at all locations, with three of the five trials having grown a soybean crop the preceding year, and two a maize crop. Two of the trials (both previously soybean) showed significant and biologically relevant decreases (4,070 and 927 seeds m −2 , respectively) in seedbank densities in cover crop treatments compared to controls. In another two trials, one previously maize and one previously soybean, no difference was detected in seedbank densities. In the fifth trial (previously maize), there was a significant, but biologically unimportant increase of 349 seeds m −2 . All five trials' weed communities were dominated by common waterhemp [ Amaranthus tuberculatus (Moq.)], and changes in seedbank composition from cover-cropping were driven by changes in this species. Although previous studies have shown that increases in cover crop biomass are strongly correlated with weed suppression, in our study we did not find a relationship between seedbank changes and the mean amount of cover crop biomass produced over a 10-years period (experiment means ranging from 0.5 to 2.0 Mg ha −1 yr −1 ), the stability of the cover crop biomass production, nor the amount produced going into the previous crop's growing season. We conclude that long-term use of a winter rye cover crop in a maize-soybean system has the potential to meaningfully reduce the size of weed seedbanks compared to winter fallows. However, identifying the mechanisms by which this occurs requires further research into processes such as seed predation and seed decay in cover cropped systems.