Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.

Cover crop mixtures retain higher diversity when allowed sufficient growth in fall. Cereal rye dominates mixtures in spring, particularly when fall planting is delayed. Grasses overperform in cover crop mixtures compared to their growth in monoculture. Brassicas underperform in cover crop mixtures compared to their growth in monoculture. Legumes’ growth in cover crop mixtures varies depending on species and planting time. Cover crop mixtures may provide greater diversity of benefits than monocultures. To develop management principles to establish diverse cover crop mixtures, we conducted a 3‐yr study in which monocultures and mixtures of six cover crop species (cereal rye [Secale cereale L.], oat [Avena sativa L.], common medium red clover [Trifolium pratense L.], Austrian winter pea [Pisum sativum L.], forage radish [Raphanus sativus L.], and winter canola [Brassica napus L.]) were planted in a wheat (Triticum aestivum L.)–maize (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation after wheat (AW) and after maize (AM). Post‐emergence stand counts and aboveground biomass in fall and spring were measured by species for all cover crop treatments. All species planted manifested in monocultures and mixtures in fall, though oat dominated and red clover, canola, and radish underperformed in mixtures. Cereal rye had the highest spring biomass in all mixtures, especially AM. Pea spring biomass was disproportionally greater in relation to seeding rate in the six‐species mixture (6 Spp.) than in monoculture when planted AW. A four‐species mixture (4 Spp.) planted AW retained the highest diversity after overwintering in two of the three planting years. Our study demonstrated that (i) cover crop mixtures retain higher diversity when allowed sufficient growth in fall; (ii) cereal rye dominates mixtures in spring, particularly when fall planting is delayed; (iii) grasses overperform in mixtures compared to their growth in monocultures; (iv) brassicas underperform in mixtures vs. monocultures; and (v) legume growth in mixtures depends on species and planting time.

Summary 1.Agricultural intensification can cause a huge increase in productivity. However, associated costs in terms of reduced, self-regulation and increased reliance on external inputs for the control of pests, diseases and weeds are seldom taken into account or acknowledged. A pro-active approach in which ecosystems services are documented and potential effects of changes in agricultural practices evaluated may lead to more informed decisions prior to implementation. 2. We investigated the effects of management of cereal production in a semi-arid region on weed seed mortality caused by predators. Seed losses have a greater impact on weed population size than any other life cycle process and should therefore be of significance for natural weed control. We hypothesized that the conversion from rain-fed to irrigated production should lead to reduced and the adoption of no-till techniques to increased seed predation. 3. Seed removal and seed predator populations were monitored in irrigated ( N = 3) and rain-fed cereal fields ( N = 6) and field margins. Of the dryland fields half was conventionally tilled and the other half no-till. Seed removal (g g − 1 2-days − 1 ) was followed from April 2007 until June 2008, using Petri-dishes and exclosure cages. Populations of harvester ants were estimated by direct nest counts; rodent populations by Sherman live traps. 4. Seed removal in dryland cereals, mainly by harvester ants Messor barbarus was high from mid April to mid October, and should cause a strong weed suppressive effect. Seed removal in irrigated cereals, mainly by granivorous rodents Mus spretus , was low. 5. Seed removal was higher in no-till than in conventional fields and corresponded to differences in harvester ant nest densities. 6. Synthesis and applications . Our results show that tillage and irrigation in a semi-arid cereal production system results in a reduction and total annihilation of granivorous harvester ants, respectively. The concurrent decline in weed seed mortality could lead to increased herbicide use and dependency. In particular, in areas where economic margins are small or the environmental costs of tillage and irrigation high, the increased costs of chemical weed control may exceed the benefits. Here, preserving biodiversity to enhance natural weed control is a viable alternative to agricultural intensification.

Agroecosystems are increasingly expected to provide multiple ecosystem services. We tested whether and how cover crop selection (identity and number of species) affects provisioning of multiple services (multifunctionality). In a 3-yr study of 10 cover crop treatments and eight ecosystem services, certain services consistently co-occurred. One such service "bundle" included cover crop biomass production, weed suppression, and nitrogen retention. Another set of bundled services included cash crop production, nitrogen supply, and profitability. We also identified tradeoffs: as some services increased, other disservices arose, limiting multifunctionality. However, functionally diverse mixtures ameliorated disservices associated with certain monocultures, thereby increasing cover crop multifunctionality. Core Ideas• Cover crop monocultures and mixtures support multiple ecosystem services.• Service interactions can lead to bundling, or co-occurrence, of certain services.• Service interactions also create trade-offs among services and disservices.• Cover crop mixtures can mitigate disservices to increase multifunctionality.

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.