Weeds in Cover Crops: Context and Management Considerations

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.This work was supported by the USDA National Institute of Food and Agriculture, Organic Research and Extension Initiative under Project PENW-2015-07433 (Grant No. 2015-51300-24156, Accession No. 1007156) and the National Science Foundation (Grant No. DGE1255832)

Cover crop mixture expression is influenced by nitrogen availability and growing degree days.

Cover crop mixtures can provide multiple ecosystem services but provisioning of these services is contingent upon the expression of component species in the mixture. From the same seed mixture, cover crop mixture expression varied greatly across farms and we hypothesized that this variation was correlated with soil inorganic nitrogen (N) concentrations and growing degree days. We measured fall and spring biomass of a standard five-species mixture of canola (Brassica napus L.), Austrian winter pea (Pisum sativum L), triticale (x Triticosecale Wittm.), red clover (Trifolium pratense L.) and crimson clover (Trifolium incarnatum L.) seeded at a research station and on 8 farms across Pennsylvania and New York in two consecutive years. At the research station, soil inorganic N (soil iN) availablity and cumulative fall growing degree days (GDD) were experimentally manipulated through fertilizer additions and planting date. Farmers seeded the standard mixture and a "farm-tuned" mixture of the same five species with component seeding rates adjusted to achieve farmer-desired services. We used Structural Equation Modeling to parse out the effects of soil iN and GDD on cover crop mixture expression. When soil iN and fall GDD were high, canola dominated the mixture, especially in the fall. Low soil iN favored legume species while a shorter growing season favored triticale. Changes in seeding rates influenced mixture composition in fall and spring but interacted with GDD to determine the final expression of the mixture. Our results show that when soil iN availability is high at the time of cover crop planting, highly competitive species can dominate mixtures which could potentially decrease services provided by other species, especially legumes. Early planting dates can exacerbate the dominance of aggressive species. Managers should choose cover crop species and seeding rates according to their soil iN and GDD to ensure the provision of desired services

Multivariate Relationships Influencing Crop Yields during the Transition to Organic Management

Crop yields are influenced by multiple, interacting factors, making it challenging to determine how specific management practices and crop rotations affect agroecosystem productivity. This is especially true in cropping systems experiments in which multiple management practices differ between experimental cropping system treatments. We conducted a cropping systems experiment in central Pennsylvania, USA, to analyze the effects of initial cover crop and tillage intensity on feed grain and forage crop productivity during the transition to organic production. We hypothesized that treatment effects of (1) tillage intensity (full or reduced); and (2) initial cover crops (annual rye (Secale cereale) or timothy/clover (Phleum pratense/Trifolium pratense)) on grain crop yield in a 3-year cover crop/soybean (Glycine max)/corn (Zea mays) rotation would be mediated by key agroecosystem function indicators (soil quality, weed pressure, and predatory arthropod activity). We used structural equation modeling (SEM) to attribute yield variation to treatment effects and abiotic factors as mediated by these ecosystem functions. We found that tillage intensity had both direct and indirect effects on corn yields. Full tillage had a direct, positive effect on corn yields, a negative effect on perennial weed density, and negative effect on a soil quality indicator (labile soil carbon). Full tillage also had an indirect effect on corn yields as mediated by perennial weed density. The initial cover crop influenced predatory arthropod activity-density and perennial weeds in year 2 (soybean phase), but had no effects in year 3 (corn phase). Abiotic and site factors influenced crop yields and other ecosystem functions in both rotation years. Our results highlight the utility of analytical approaches that consider the relationships among agroecosystem components. Through the analysis of management effects on multiple ecosystem functions, our results indicate that managing weed populations through tillage in organic systems can have the strongest effect on crop yields, although short-term profit gains may be at the expense of long-term loss in soil quality and beneficial insect conservation

Ecosystem Services and Disservices Are Bundled in Simple and Diverse Cover Cropping Systems

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 trade-offs: as some services increased, other disservices arose, limiting multifunctionality. However, functionally diverse mixtures ameliorated disservices associated with certain monocultures, thereby increasing cover crop multifunctionality

Paucity of CD4(+)CCR5(+) T cells is a typical feature of natural SIV hosts

In contrast to lentiviral infections of humans and macaques, simian immunodeficiency virus (SIV) infection of natural hosts is nonpathogenic despite high levels of viral replication. However, the mechanisms underlying this absence of disease are unknown. Here we report that natural hosts for SIV infection express remarkably low levels of CCR5 on CD4(+) T cells isolated from blood, lymph nodes, and mucosal tissues. Given that this immunologic feature is found in 5 different species of natural SIV hosts (sooty mangabeys, African green monkeys, mandrills, sun-tailed monkeys, and chimpanzees) but is absent in 5 nonnatural/recent hosts (humans, rhesus, pigtail, cynomolgus macaques, and baboons), it may represent a key feature of the coevolution between the virus and its natural hosts that led to a nonpathogenic infection. Beneficial effects of low CCR5 expression on CD4(+) T cells may include the reduction of target cells for viral replication and a decreased homing of activated CD4(+) T cells to inflamed tissue