Richard G. Smith

Navigating a Critical Juncture for Sustainable Weed Management

Agricultural weed management has become entrenched in a single tactic—herbicide—resistant crops—and needs greater emphasis on integrated practices that are sustainable over the long term. In response to the outbreak of glyphosate-resistant weeds, the seed and agrichemical industries are developing crops that are genetically modified to have combined resistance to glyphosate and synthetic auxin herbicides. This technology will allow these herbicides to be used over vastly expanded areas and will likely create three interrelated challenges for sustainable weed management. First, crops with stacked herbicide resistance are likely to increase the severity of resistant weeds. Second, these crops will facilitate a significant increase in herbicide use, with potential negative consequences for environmental quality. Finally, the short-term fix provided by the new traits will encourage continued neglect of public research and extension in integrated weed management. Here, we discuss the risks to sustainable agriculture from the new resistant crops and present alternatives for research and policy.

Effects of crop diversity on agroecosystem function: crop yield response.

Understanding the role of diversity in the functioning of ecosystems has important implications for agriculture. Previous agricultural research has shown that crop rotation and the use of cover crops can lead to increases in yield relative to monoculture; however, few studies have been performed within the broader context of diversity–ecosystem function theory and in the absence of chemical inputs. We performed a field experiment in SW Michigan, USA, in which we manipulated the number of crop species grown in rotation and as winter cover crops over a 3-year period to test if varying the number of species in a rotation affected grain yield, a critical metric of ecosystem function in row-crops. The experimental design was unique in that no fertilizer or pesticides were used, and the only management variable manipulated was number of species in the rotation, thus providing a strong comparison to grassland diversity–ecosystem function experiments. Treatments included continuous monocultures of three row-crops, corn Zea mays L., soybean Glycine max (L.) Merr., and winter wheat Triticum aestivum L., and 2- and 3-year annual rotations with and without cover crops (zero, one, or two legume/small grain species), encompassing a range of crop diversity from one to six species. Crop yields and weed biomass were measured annually for 3 years and plant available soil nitrogen was measured over the course of the growing season in the final year of the study. In all 3 years, corn grain yield increased linearly in response to the number of crops in the rotation. Corn yields in the highest diversity treatment (three crops, plus three cover crops) were over 100% higher than in continuous monoculture and were not significantly different from the county average for each of the 3 years despite the absence of chemical inputs. Corn yields in the diversity treatments were strongly correlated with the availability of inorganic soil nitrogen, which was likely influenced by the number of different legume species (crops and cover crops) present in the rotation. In soybean and winter wheat, yield differences among crop diversity treatments were also significant, but of lower magnitude (32 and 53%, respectively), and showed little direct relationship to the number of crop species grown in a rotation. Results demonstrate that agricultural research motivated by ecological theory can provide important insights into the functioning of agroecosystems and enhance our understating of the linkages between diversity and ecosystem function. Importantly, these results suggest that reduced chemical inputs do not necessarily result in yield penalties and provide support for incorporation of crop or species diversity when determining how ecosystem services can be included in food, fiber, and biofuel production.

Lessons from agriculture may improve the management of invasive plants in wildland systems

The current motivating concept underlying our view of invasive plant species is that they are dangerous and destructive invaders that drive native plant communities to extinction and must be eradicated at almost any cost. This negative view of non-native plants has resulted in the adoption and implementation of control efforts that may not be entirely successful and have the potential to result in severe non-target effects. The history of weed control in agroecosystems may provide insights on how to deal with non-native plant species in non-crop-lands, and shed light on some of the potential consequences of current control programs for the environment and for non-target organisms. Alternative research and management foci, based on knowledge gleaned from agricultural systems, may improve the effectiveness and sustainability of non-native plant species management in range- and wildlands.

Management Filters and Species Traits: Weed Community Assembly in Long-Term Organic and Conventional Systems

Abstract Community assembly theory provides a useful framework to assess the response of weed communities to agricultural management systems and to improve the predictive power of weed science. Under this framework, weed community assembly is constrained by abiotic and biotic “filters” that act on species traits to determine community composition. We used an assembly approach to investigate the response of weed seed banks to 25 yr of management-related filtering in three different row-crop management systems in southeastern Pennsylvania: organic manure-based, organic legume-based, and conventional. Weed seed banks were sampled in April of 2005 and 2006 and quantified by direct germination in a greenhouse. We also assessed the filtering effects of weed management practices and relationships between assembled seed bank and emergent weed communities by allowing or excluding weed control practices within each management system and measuring emergent weed community response. Germinable weed seed bank densities and species richness in the final year of the study were over 40% and 15% higher, respectively, in the organic systems relative to the conventional system. Seed bank community structure in the organic systems was different from the conventional system, and the relationships between assembled seed banks and the emergent flora varied. Primary tillage, weed control, timing of planting, and fertility management appeared to be the main filters that differentiated weed seed banks in the three systems. Weed life history, emergence periodicity, seed size, and responsiveness to soil fertility and hydrology appeared to be the most important functional traits determining how weed species responded to management-related filters. Our results suggest that management systems can exert strong filtering effects that can persist over relatively long (greater than one growing season) time scales. Legacy effects of community-level filtering might be more important than previously assumed, and should be incorporated into predictive models of weed community assembly.

Agriculture in 2050: Recalibrating Targets for Sustainable Intensification

http://bioscience.oxfordjournals.org XXXX XXXX / Vol. XX No. X BioScience 1 BioScience XX: 1–6.

Weed community and corn yield variability in diverse management systems

Abstract The effects of crop rotation and management system on annual variability in weed communities and crop yields were assessed in a 4-yr study in Michigan. Variability of the weed community and corn yields were assessed using the coefficient of variation (CV) and a multivariate dissimilarity index (Bray-Curtis) that accounted for changes in both weed species abundance and composition. The treatments included two rotations: continuous corn and a corn–corn–soybean–wheat rotation, and two management systems: conventional (CONV) and organic-based (ORG). Weed biomass was significantly higher in the ORG system; however, there was no effect of crop rotation on weed biomass or number of weed species in a treatment (species richness). Annual variability in weed community composition and structure was affected by both crop rotation and management system and was highest in the ORG rotation. In contrast to the weed community, variability in corn yield was highest in the least-diverse cropping system (CONV monoculture), despite that system having a more constant weed community. Corn yield in the ORG rotation was not significantly different from that in the CONV monoculture. Results of this study suggest that management aimed at increasing cropping system diversity may have additional effects on weed communities and crop yields beyond those commonly reported, and these may have important implications for the development of more efficient and sustainable weed and crop management practices. Nomenclature: Corn, Zea mays L.; soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.

Increased productivity of a cover crop mixture is not associated with enhanced agroecosystem services.

Cover crops provide a variety of important agroecological services within cropping systems. Typically these crops are grown as monocultures or simple graminoid-legume bicultures; however, ecological theory and empirical evidence suggest that agroecosystem services could be enhanced by growing cover crops in species-rich mixtures. We examined cover crop productivity, weed suppression, stability, and carryover effects to a subsequent cash crop in an experiment involving a five-species annual cover crop mixture and the component species grown as monocultures in SE New Hampshire, USA in 2011 and 2012. The mean land equivalent ratio (LER) for the mixture exceeded 1.0 in both years, indicating that the mixture over-yielded relative to the monocultures. Despite the apparent over-yielding in the mixture, we observed no enhancement in weed suppression, biomass stability, or productivity of a subsequent oat (Avena sativa L.) cash crop when compared to the best monoculture component crop. These data are some of the first to include application of the LER to an analysis of a cover crop mixture and contribute to the growing literature on the agroecological effects of cover crop diversity in cropping systems.

Rapid change in the germinable fraction of the weed seed bank in crop rotations

Abstract The dynamics of soil seed banks in crop rotations of corn, soybean, and winter wheat were investigated to determine whether weed seed inputs associated with the winter wheat phases of the rotation were present in the readily germinable fraction of the seed bank in subsequent phases. Two studies were conducted, each in chisel-plowed systems. In one study, we compared seed banks in plots after 8 yr of corn grown continuously and with rotation that included winter wheat and soybean. A second study followed seed bank composition and abundance for 3 yr in plots that were planted to corn and soybean in successive years after planting to winter wheat. Seed banks were measured by direct germination in a heated greenhouse. In the first study, seed banks in plots planted to continuous corn (never planted to winter wheat) and the corn phase of the rotation (winter wheat planted 3 yr earlier) did not differ in species composition or abundance despite significant differences in seed banks in plots that had been planted to winter wheat the previous season. In the second study, seed bank abundance and composition in plots planted to winter wheat in 2001 rapidly changed after planting of corn and soybean in 2002 and 2003, respectively. Data from the two experiments suggest that seed banks in annual row crops experience rapid change in composition and abundance and can be strongly influenced by the most recent crop. This could limit our ability to infer longer term trends associated with changes in management practices from studies of soil seed banks. Nomenclature: Corn, Zea mays L.; soybean, Glycine max (L.) Merr.; winter wheat, Triticum aestivum L.

Weed science research and funding: A call to action

Abstract Weed science has contributed much to agriculture, forestry and natural resource management during its history. However, if it is to remain relevant as a scientific discipline, it is long past time for weed scientists to move beyond a dominating focus on herbicide efficacy testing and address the basic science underlying complex issues in vegetation management at many levels of biological organization currently being solved by others, such as invasion ecologists and molecular biologists. Weed science must not be circumscribed by a narrowly-defined set of tools but rather be seen as an integrating discipline. As a means of assessing current and future research interests and funding trends among weed scientists, the Weed Science Society of America conducted an online survey of its members in summer of 2007. There were 304 respondents out of a membership of 1330 at the time of the survey, a response rate of 23%. The largest group of respondents (41%) reported working on research problems primarily focused on herbicide efficacy and maintenance, funded mainly by private industry sources. Another smaller group of respondents (22%) reported focusing on research topics with a complex systems focus (such as invasion biology, ecosystem restoration, ecological weed management, and the genetics, molecular biology, and physiology of weedy traits), funded primarily by public sources. Increased cooperation between these complementary groups of scientists will be an essential step in making weed science increasingly relevant to the complex vegetation management issues of the 21st century.

2,4-Dichlorophenoxyacetic acid (2,4-D)–resistant crops and the potential for evolution of 2,4-D–resistant weeds

In their recent article, we feel that Wright et al. (1) misrepresented the potential for 2,4-dichlorophenoxyacetic acid (2,4-D)–resistant weeds in 2,4-D–resistant cropping systems and exaggerated the sustainability of their approach to addressing glyphosate-resistant weed problems in agriculture.