Adam S. Davis

Increasing Cropping System Diversity Balances Productivity, Profitability and Environmental Health

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.

Evolutionary limits ameliorate the negative impact of an invasive plant

Invasive species can quickly transform biological communities due to their high abundance and strong impacts on native species, in part because they can be released from the ecological forces that limit native populations. However, little is known about the long-term dynamics of invasions; do invaders maintain their dominant status over long time spans, or do new ecological and evolutionary forces eventually develop to limit their populations? Alliaria petiolata is a Eurasian species that aggressively invades North American forest understories, in part due to the production of toxic phytochemicals. Here we document a marked decline in its phytotoxin production and a consequent decline in their impact on three native species, across a 50+ year chronosequence of Alliaria petiolata invasion. Genetic evidence suggests that these patterns result from natural selection for decreased phytotoxin production rather than founder effects during introduction and spread. These patterns are consistent with the finding of slowing A. petiolata population growth and rebounding native species abundance across a separate chronosequence in Illinois, U.S. These results suggest that this invader is developing evolutionary limits in its introduced range and highlight the importance of understanding the long-term processes that shape species invasions and their impacts.

Weed seedbank and community shifts in a long-term cropping systems experiment

Abstract Characterizing the long-term effect of agricultural management systems on weed communities will aid in developing sustainable weed management practices. Weed seedbanks and aboveground biomass were measured within a corn–soybean–wheat crop sequence from 1990 through 2002 at Hickory Corners, MI. Four management systems were compared: conventional (CONV; full rates of N fertilizer and herbicides, moldboard tillage), no till (NT; same as CONV with no primary tillage), reduced input (RI; reduced rates of N fertilizer and herbicides, moldboard tillage, mechanical weed control, wheat underseeded with red clover), and organic (ORG; same as RI but no synthetic inputs). Multivariate ordinations of weed seedbanks showed a divergence of the CONV and NT systems from the RI and ORG systems. The CONV and NT seedbanks were dominated by grass species (mainly fall panicum and large crabgrass), whereas the RI and ORG systems were dominated by common lambsquarters and common chickweed. Within a single growing season, weed seedbanks in the RI and ORG systems were positively correlated with weed biomass whereas seedbanks in the CONV and NT system had little predictive value. Weed biomass from 1990 through 2002 showed a strong association of grass weed species with the corn phase of the CONV and NT system and common lambsquarters and redroot pigweed with the corn and soybean phases of the RI and ORG systems. Weed biomass diversity measures were negatively correlated with soybean yields in RI and ORG and wheat yields in NT, RI, and ORG. It is not clear whether crops were less competitive in the NT, RI, and ORG treatments, allowing new weed species to enter the plots, or whether less effective weed management in the NT, RI, and ORG treatments resulted in increased species richness, causing reduced crop yields. Mechanistic studies are needed to elucidate the relationship between weed community diversity and crop performance. Nomenclature: Common chickweed, Stellaria media (L.) Vill. STEME; common lambsquarters, Chenopodium album L. CHEAL; fall panicum, Panicum dichotomiflorum Michx. PANDI; large crabgrass, Digitaria sanguinalis L. DIGSA; redroot pigweed, Amaranthus retroflexus L., AMARE; corn, Zea mays L. ‘Pioneer 3573’; red clover, Trifolium pratense L. ‘Michigan Mammoth Red’; soybean, Glycine max (L.) Merr. ‘Pioneer 9172’; wheat, Triticum aestivum L. ‘Pioneer 2552’.

Demographic models inform selection of biocontrol agents for garlic mustard (Alliaria petiolata).

Nonindigenous invasive plants pose a major threat to natural communities worldwide. Biological control of weeds via selected introduction of their natural enemies can affect control over large spatial areas but also risk nontarget effects. To maximize effectiveness while minimizing risk, weed biocontrol programs should introduce the minimum number of host-specific natural enemies necessary to control an invasive nonindigenous plant. We used elasticity analysis of a matrix model to help inform biocontrol agent selection for garlic mustard (Alliaria petiolata (M. Bieb.) Cavara and Grande). The Eurasian biennial A. petiolata is considered one of the most problematic invaders of temperate forests in North America. Four weevil species in the genus Ceutorhynchus (Coleoptera: Curculionidae) are currently considered potential biocontrol agents. These species attack rosettes (C. scrobicollis), stems (C. roberti, C. alliariae), and seeds (C. constrictus) of A. petiolata. Elasticity analyses using A. petiolata demographic parameters from North America indicated that changes in the rosette-to-flowering-plant transition and changes in fecundity consistently had the greatest impact on population growth rate. These results suggest that attack by the rosette-feeder C. scrobicollis, which reduces overwintering survival, and seed or stem feeders that reduce seed output should be particularly effective. Model outcomes differed greatly as A. petiolata demographic parameters were varied within ranges observed in North America, indicating that successful control of A. petiolata populations may occur under some, but not all, conditions. Using these a priori analyses we predict: (1) rosette mortality and reduction of seed output will be the most important factors determining A. petiolata demography; (2) the root-crown feeder C. scrobicollis will have the most significant impact on A. petiolata demography; (3) releases of single control agents are unlikely to control A. petiolata across its full range of demographic variability; (4) combinations of agents that simultaneously reduce rosette survival and seed production will be required to suppress the most vigorous A. petiolata populations. These predictions can be tested using established long-term monitoring sites coupled with a designed release program. If demographic models can successfully predict biocontrol agent impact on invasive plant populations, a continued dialogue and collaboration between empirical and theoretical approaches may be the key to the development of successful biocontrol tactics for plant invaders in the future.

Cover-Crop Roller–Crimper Contributes to Weed Management in No-Till Soybean

Abstract Termination of cover crops prior to no-till planting of soybean is typically accomplished with burndown herbicides. Recent advances in cover-crop roller–crimper design offer the possibility of reliable physical termination of cover crops without tillage. A field study within a no-till soybean production system was conducted in Urbana, IL, from 2004 through 2007 to quantify the effects of cover crop (cereal rye, hairy vetch, or bare soil control), termination method (chemical burndown or roller–crimper), and postemergence glyphosate application rate (0, 1.1, or 2.2 kg ae ha−1) on soybean yield components, weed–crop interference, and soil environmental variables. Biomass of weeds surviving management within a soybean crop following either a vetch or rye cover crop was reduced by 26 and 56%, respectively, in the rolled system compared to the burndown system. Soybean yield loss due to weed interference was unaffected by cover-crop termination method in soybean following a rye cover crop, but was higher in the rolled than burndown treatment in both hairy vetch and bare soil treatments. In soybean following a rye cover crop, regardless of termination method, yield loss to weed interference was unaffected by glyphosate rate, whereas in soybean following a vetch cover crop or bare soil, yield loss decreased with glyphosate rate. Variation in soybean yield among cover crops and cover-crop termination treatments was due largely to differences in soybean establishment, rather than differences in the soil environment. Use of a roller–crimper to terminate a cover crop preceding no-till soybean has the potential to achieve similar yields to those obtained in a chemically terminated cover crop while reducing residual weed biomass. Nomenclature: Common waterhemp, Amaranthus rudis Sauer, AMARU; giant foxtail, Setaria faberi Herrm., SETFA; hairy vetch, Vicia villosa Roth.; cereal rye, Secale cereale L. ‘FS Hi-Rye 500’; soybean, Glycine max (L). Merr.

Cropping system effects on giant foxtail (Setaria faberi) demography: I. Green manure and tillage timing

Abstract Manipulation of cropping systems to improve weed management requires a better understanding of how crop- and soil-related factors affect weed life cycles. Our objective was to assess the impacts of timing of primary tillage and use of legume green manure on giant foxtail demography and soil properties. We measured giant foxtail seed survival and dormancy, seedling emergence and survival, and fecundity, in addition to soil phytotoxicity, chemical properties affecting soil fertility and soil water, in the transition between the wheat and corn phases of a wheat–corn–soybean crop sequence. Postdispersal predation of giant foxtail seeds was measured in all three phases of the crop sequence. Wheat was grown either as a sole crop (W) or underseeded with red clover (R), and residues from this phase were rototilled either in the fall (FT) or in spring (ST). There were strong interactions between Red clover and Tillage timing in their effects on giant foxtail recruitment and fecundity in corn. Giant foxtail seedling emergence was 30% lower, and time to 50% emergence was more than 1 wk later, in the ST/R treatment than in the ST/W, FT/W, and FT/R treatments, which did not differ. However, fecundity of giant foxtail was 200% greater in the ST/R treatment than in the other three treatments because of suppressed early corn growth. The net effect of the ST/R treatment on giant foxtail demography in corn was to greatly increase inputs to the seedbank compared with the ST/W, FT/W, and FT/R treatments. Giant foxtail demography in the wheat phase was also affected by Red clover. There was a 200% increase in daily rates of postdispersal seed predation in the wheat phase of the R treatment compared with the W treatment. High-seed predation in the wheat phase and low fecundity in the corn phase of the FT/R treatment suggest that population growth rate of giant foxtail will be lower in this treatment than in the other treatments. The degree of soil phytotoxicity from red clover residues, the changes in the amount of interference from the corn crop early in the growing season, and the differential suitability of crop residues in the different rotations as habitat for seed predators all contributed to changes in giant foxtail demography. Understanding the effects of cropping system characteristics on entire weed life cycles will facilitate the design of integrated suites of complementary weed management tactics. Nomenclature: Giant foxtail, Setaria faberi Herrm. SETFA; corn, Zea mays L. ‘Pioneer 3512’; red clover, Trifolium pratense L. ‘Cherokee’; soybean, Glycine max (L.) Merr. ‘IA 2039’; spring wheat, Triticum aestivum L. ‘Sharp’.

Screening bioenergy feedstock crops to mitigate invasion risk

The cultivation of plants for the production of hydrocarbon fuels is gaining global momentum. These biofuels offer potential benefits as alternatives to fossil fuels, but mitigating any environmental risks posed by large-scale cultivation of bioenergy feedstock (biofeedstock) species poses new challenges: some proposed biofuel crops have become naturalized, and even invasive; the relative risks and benefits of growing monocultures versus polycultures of biofeedstock species require more thorough examination; and prompt development of a robust ecological risk assessment framework and careful screening are needed before these biofuel species are widely cultivated. Greater collaboration between agronomists developing crops for biofeedstock production and invasion biologists could substantially lower the risk of new plant invasions.

Do microorganisms influence seed-bank dynamics?

Abstract Reduction of seed-bank persistence is an important goal for weed management systems. Recent interest in more biological-based weed management strategies has led to closer examination of the role of soil microorganisms. Incidences of seed decay with certain weed species occur in the laboratory; however, their persistence in soil indicates the presence of yet-unknown factors in natural systems that regulate biological mechanisms of seed antagonism by soil microorganisms. A fundamental understanding of interactions between seeds and microorganisms will have important implications for future weed management systems targeting seed banks. Laboratory studies demonstrate susceptibility to seed decay among weed species, ranging from high (velvetleaf) to very low (giant ragweed). Microscopic examinations revealed dense microbial assemblages formed whenever seeds were exposed to soil microorganisms, regardless of whether the outcome was decay. Microbial communities associated with seeds of four weed species (woolly cupgrass, jimsonweed, Pennsylvania smartweed, and velvetleaf) were distinct from one another. The influence of seeds on microbial growth is hypothesized to be due to nutritional and surface-attachment opportunities. Data from velvetleaf seeds suggests that diverse assemblages of bacteria can mediate decay, whereas fungal associations may be more limited and specific to weed species. Though microbial decay of seeds presents clear opportunities for weed biocontrol, limited success is met when introducing exogenous microorganisms to natural systems. Alternatively, a conservation approach that promotes the function of indigenous natural enemies through habitat or cultural management may be more promising. A comprehensive ecological understanding of the system is needed to identify methods that enhance the activities of microorganisms. Herein, we provide a synthesis of the relevant literature available on seed microbiology; we describe some of the major challenges and opportunities encountered when studying the in situ relationships between seeds and microorganisms, and present examples from studies by the ARS Invasive Weed Management Unit. Nomenclature: Giant ragweed, Ambrosia trifida L.; jimsonsweed, Datura stramonium L.; Pennsylvania smartweed, Polygonum pensylvanicum L.; velvetleaf, Abutilon theophrasti Medic.; woolly cupgrass, Eriochloa gracilis (Fourn) A. S. Hitchc.

Nitrogen source influences wild mustard growth and competitive effect on sweet corn

Abstract Manipulations of the soil environment can affect the growth and competitive ability of annual weeds because of the large influence that soil conditions exert on seedlings early in the growing season. Our objective was to identify soil nitrogen (N) management systems with weed suppression potential. We hypothesized that competition from wild mustard against sweet corn would be weaker when N was supplied by organic sources (organic) or a split application of NH4NO3 fertilizer applied at planting and 4 wk thereafter (split) than when NH4NO3 fertilizer was applied in a single dose at planting (early). This hypothesis was tested in a 2-yr field experiment conducted in central Maine. Wild mustards maximal relative growth rate (RGR) was 12% lower (P < 0.05) in 1997 and 1998, and the amount of time needed to achieve maximal RGR was delayed by 0.8 d (P < 0.05) in 1997 in the organic compared to the early treatment. The competitive effect of wild mustard on sweet corn yield was lower in the organic treatment than in the early and split treatments in 1 of 2 yr. In 1997, competition from wild mustard reduced marketable ear yields of sweet corn by 30%, but the magnitude of yield reduction did not differ between the three N addition treatments. In contrast, in 1998, sweet corn yield in the organic treatment was not reduced by weed competition, whereas yield loss in the early and split treatments was 20 and 35%, respectively. The mechanisms underlying selective suppression of weeds, but not crops, by organic N sources require further attention. Nomenclature: Red clover, Trifolium pratense L., ‘Mammoth’; spring wheat, Triticum aestivum L., ‘Belvedere’; sweet corn, Zea mays L. ‘Clockwork’; wild mustard, Brassica kaber (D.C.) L.C. Wheeler SINAR.

When does it make sense to target the weed seed bank

Abstract Weed seeds initiate most weed invasions of arable fields, yet there is relatively little information on the value of managing weed seed banks. Matrix population models were used to examine the relative importance of managing weed seed banks, in relation to other life stages, for four model weed species with varying life histories. Simulations for giant foxtail and common lambsquarters, summer annual weeds of arable fields; garlic mustard, an obligate biennial invasive weed of temperate forests; and Canada thistle, a perennial weed of pastures and arable fields, were run under conditions of varying population density and efficacy of seedling control. The models were subjected to elasticity analysis to determine what happened to weed populations when different life stages were targeted. Losses from the dormant seed bank were most important for summer annual weeds, of intermediate importance for biennial weeds, and of low importance for perennial weeds. More effort is needed to develop weed seed-bank management techniques for summer annual weed species as part of integrated weed management systems. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; garlic mustard, Alliaria petiolata (M. Bieb.) Cavara and Grande ALPET; Canada thistle, Cirsium arvense (L.) Scop. CIRAR.