The final Component in NP 304 is the synthesis of ARS’ collective knowledge to develop weed
management systems. Since reliance on any single management tactic may select for weed populations that escape control, many of the approaches used herein employ multiple tactics. In addition, strategies were developed to prevent future infestations by minimizing the opportunities for weeds to invade. Also included in this component is a higher risk research effort dealing with the potential to use indigenous microbial seed antagonists for weed control.
Problem Area X-A - Cultural and Mechanical Control
ARS identified mulch properties controlling weed suppression by cover crops. In previous research, cover crops were demonstrated to influence many of the micro-climatic factors that control weed seed germination and emergence from soils as well as impede seedling emergence and light penetration of mulch materials. Research during this project cycle has focused on the role of toxins released from decomposing cover crop residue in suppressing weeds. Release of water-soluble phytotoxins from fresh cover crop residue following cover crop kill contributed to weed suppression for a relatively short time until leaf tissue had decomposed. Release of hydroxamic acids could account for toxicity of extracts derived from rye at early growth stages but unknown compounds were probably responsible for the toxicity of extracts of rye at later growth stages. Combining residue and herbicide (metolachlor) amounts that are marginally effective when used alone, a high degree of weed control was achieved. These results could open a new area of research that would target combinations of cover crops and low levels of selected phytotoxins to maximize weed control with minimal environmental impact.
ARS established that the suppressive effects of leaf litter generated during the harvest of the previous crop on the subsequent ratoon crops were attributed to delayed spring emergence caused by increased soil moisture, lower soil temperature, and the liberation of benzoic acid, a known allelochemical, during the decomposition of the leaf litter and that these suppressive effects could be mitigated by the partial removal of the leaf litter from the row top if done in a timely fashion. Before leaf litter generated during the harvest of sugarcane can be used as a cultural method to suppress weeds the following year, deleterious effects on the crop must be overcome. A 5-15 percent reduction in yield associated with the failure to remove leaf litter generated during the harvest of sugarcane equates to a $25-75 million loss in revenue to the Louisiana sugarcane industry. The research shows that by repositioning the leaf litter away from the planted line of sugarcane, growers can still realize some weed suppression while
minimizing the impact of the litter on the re-emerging crop. This research has lead to the development of residue management practices that have been adopted by Louisiana sugarcane growers.
Cultural practices using killed cover crop residues as mulch for weed suppression in vegetable production were assessed. Weed control is a major limitation to the implementation of sustainable and organic vegetable production. These studies demonstrated that organic mulches formed when cover crops were killed and left in place effectively reduced weed growth. Mulch impact on soil borne diseases and beneficial and pest insects were also assessed. These studies demonstrated the feasibility of using an alternative cropping system where vegetable crops were transplanted into mulches formed from cover crop residues to reduce weed growth.
Six cowpea genotypes were identified that are superior to commercial varieties for use as a weed suppressing cover crop. Cowpea is an excellent cover crop for the southern United States, because it suppresses weeds and produces nitrogen and biomass that benefits rotational crops. Only one cover crop cowpea variety, Iron Clay, is currently marketed in the United States. Following an extensive evaluation of cowpea germplasm collection, six genotypes that have characteristics superior to Iron Clay were selected for development as new cover crop varieties. When available to growers they will improve the effectiveness of cowpea as a weed suppressing cover crop.
Water hyacinth, a rapidly-growing floating aquatic weed, is managed successfully in many infested lakes worldwide. Viability of water hyacinth fragments generated in the field by mechanical cutting/chopping was assessed. Fragments from viable meristems tissues are readily dispersed through tidal movements
in the Sacramento-San Joaquin Delta, and by wind/wave action. This information provides a scientific basis for evaluating the impacts of mechanical harvesting and chopping methods so that short- and long- term benefits and risks can be assessed.
Management of sugarcane residues by burning is undesirable for public and environmental health reasons. ARS developed an eco-friendly formulation consisting of a consortium of autochthonous lignocellulosic microbes that enhanced the unformulated microbial degradation rate by 67 percent in laboratory bioassays. Thick residue blankets can result in a delay in emergence of new ratoons, which in turn may cause losses in sugar yields of up to 1.25 t/had. A cost-effective microbial based formulation to remove field cane trash should provide several benefits to sugarcane growers. The formulation is currently undergoing field testing. Benefits to growers include: 1) reduced sugar yield losses, 2) improved health for workers and nearby communities, 3) improved soil fertility, 4) a possible increase in harvester efficiency and cane quality if applied prior to harvest, 5) winter weed control since the residue is allowed to slowly degrade during the winter months, and 6) entitlement to Natural Resources
Conservation Service (NRCS) credits for incorporating a Best Management Practice into their farming practice.
Research provided a fundamental understanding of weed/crop interactions that heretofore was largely unknown. Knowledge was gained that is critical to development of thresholds for weed management in sweet corn. ARS determined extent to which competitive traits in sweet corn could improve weed management and demonstrated the significance of planting date on crop competitive ability with weeds. This research offers several practical ways to reduce reliance on herbicides and improve sustainability of sweet corn production by bolstering cultural tactics and multiple approaches for weed management. Commercial hybrids with competitive canopy architecture offer a tangible method to improve weed management in sweet corn. The sweet corn industry recognizes planting date can be used to improve the crop’s ability to compete with weeds.
Problem Area X-B - Integrated Weed Management in Cropland
The ARS Biological Control Documentation Center was modernized and maintained for storage and retrieval of information on invertebrate and microbial biological control agents of invertebrate, weed and microbial pests. User-friendly, object-oriented PC data entry systems were developed and tested for (1) importation of foreign invertebrates and microbial biological control agents; (2) for domestic quarantine consignment, shipment and release of imported invertebrate biological control agents and for the field release of imported microbial biological control agents; and (3) for the non-quarantine culture, shipment, release and recolonization of introduced biological control agents. Scientists can input, gather, track, exchange information on importations and releases, determine where organisms were collected and released, as well as track current and past activities on a site. The database can also be queried by the general public via the Web.
The evolution-of-increased-competitive-ability (EICA) hypothesis is one model for describing the
relationship between evolution and invasion. It posits that invasive plant species, with fewer enemies in their new range, will evolve to allocate fewer resources to defense, and more resources to competitive ability. Using genetic material from ARS’s National Center for Genetic Resources Preservation, ARS performed the first multi-species test of the EICA hypothesis, finding broad evidence for increased allocation to growth by invasive genotypes, but not increased competitive ability. This provides scientists with a new understanding of how evolution influences invasion, suggesting that evolution will contribute to success in disturbed environments rather than in intact plant communities. Publication of this research led to an interview of the investigator by Science.
ARS also published a novel hypothesis suggesting a mechanistic link between two widely accepted causes of weed invasion: high resource availability and enemy release. It states that plants from habitats with high availability of resources (e.g., light, water, and N) are often poorly defended, nutritious, and strongly inhibited by herbivores. When introduced to a new range, these high-resource species may therefore benefit most from escaping from herbivores. Because the same weed species are likely to benefit directly from high-resource availability, resources and release from herbivore pressure may act in
concert to facilitate plant invasion. If correct, this hypothesis has a variety of ramifications for
understanding and managing invasive plants. It suggests that increases in resource availability may favor exotic over native species, and play a larger role in invasion than previously thought. It also predicts that biological control may be particularly successful against invasive species adapted to high resource availability. Over the long-term, this work should lead to more effective targeting of biological control efforts, and increased awareness of the importance of reducing resource availability to control invasive plants.
Diversified rotations reduce weed seed populations and increase crop yields for organic farmers. This research determined the dynamics of soil weed seed population over 6 years in the Farming Systems Project, a long-term experiment that compares conventional and organic cropping systems. Of three organic rotations, weed seed populations and weed abundance were lower in longer and more phenologically diverse rotations. Mortality of typical annual weed seed was high, which led to rapid decline of seed bank populations during rotation years with good weed control. Organic corn yield loss because of weed competition was reduced by approximately one-half by using longer, more diverse rotations. This information has confirmed the requirement for diverse rotations in the National Organic Standards and aided organic producers to identify crop rotations for reducing weed populations and increasing yields.
ARS demonstrated that weed community density declines across time with rotations comprised of two cool-season crops followed by two warm-season crops, in contrast with rotations of fewer crops. This trend with rotation design was consistent across several long-term rotation studies in the Great Plains. This information provides a foundation for producers to design weed management systems based on disrupting weed population dynamics. Weed community densities are so low in fields of producers who follow this four-crop rotation design with no-till practices that herbicides are not needed to prevent yield loss due to weeds.
The impact of tillage level in sweet corn on volunteer potato control achieved with mesotrione and fluroxypyr was determined. This research provides producers with knowledge and information on the effect of tillage level on herbicide efficacy on volunteer potato and offers practical ways to manage this weed, combining tillage and herbicides in an integrated approach. Conservation management systems for soybean to maximize weed control, minimize herbicide input, and improve net returns were
developed. Components of the systems included cover crop, narrow row spacing, no-tillage (NT), and herbicide-resistant cultivars. Cover crops provided partial weed control in soybean enabling effective weed control with half rates of herbicide, but a rye cover crop-based soybean production was less profitable than a no-cover crop system due to additional seed costs. Narrow row soybean reduced herbicide input by half, effectively suppressing weeds, and increasing yields and net returns compared to wide rows under conventional tillage and NT.
Integrated systems of weed and crop management in cropping systems that include peanut and cotton were developed. Herbicide input needs were greater (due to increased densities of perennial weeds), whereas spotted wilt and stem rot occurrence were lessened in reduced tillage systems compared to conventional tillage systems due to. In conventional tillage peanut production, efficiency of weed control and peanut yield are improved by using stale seedbed tillage before planting and seeding peanut in narrow rows. Research from this project was used to develop an integrated system for spotted wilt management, along with cost-effective systems of weed management unique to reduced tillage systems. ARS scientists demonstrated the plasticity that allows weed species to proliferate when in competition with crops, and results showed that physiological characteristics of weeds were stable in the presence of competition from crops. Reductions in cotton row spacing can increase the competitiveness of cotton towards weeds resulting in reduced branching and seed production. However, the higher populations used in narrow row cotton were more susceptible to seedling death and yield reductions when weeds were present. Studies demonstrated the positive and negative consequences of conventional tillage and no-tillage cotton production practices and the use of rye cover crop and preemergence herbicides in glyphosate tolerant cotton. Unfavorable weed shifts to highly competitive grasses resulted when tillage was reduced and cover crops were used.
ARS determined the advantages of different of weed control systems for certified organic watermelon production. Planting watermelon into black plastic mulch and season long cultivation between rows provided superior weed control and yields than planting into a mowed rye/vetch cover crop mulch and using acetic acid and mowing between crop rows for weed control. The research also determined the natural mulch system produced as good or better quality (sugar and lycopene content) watermelon, than the plastic mulch system. Further research is being directed to isolating the influence of specific
production factors on phytonutrient quality, and on isolating the cause of increased plant stress on increased sugar and lycopene content. The research provided organic growers with the first scientific evaluation and recommendations combining weed control systems, variety selection, and potential impact on phytonutrient content.
ARS initiated a new ecological approach to integrate the use of natural soil microbial populations as antagonists of seeds of two important weed species, velvetleaf and giant ragweed, in the development of biological-based methods for weed management. Improved experimental approaches allowed more systematic examination of microbial assemblages that effectively mediate seed decay in the environment and take into account both microbial- and seed biology. ARS is currently conducting unique multi-year field- and laboratory-based studies designed to identify key microbial populations and the environmental factors that regulate seed-microbe interactions in soil. The immediate products include the identities of natural microbial populations from soil that affect seed mortality and fundamental knowledge of the underlying mechanisms of microbial-mediated seed bank depletion under natural soil conditions. The long term impact will be a successful technology that integrates existing weed management tactics with enhancement of natural seed decay activities by native soil microorganisms. Results from these studies have contributed to a growing database of bacterial and fungal species found associated with seeds of different weed species that is accessible through the National Center for Biotechnology Information (NCBI). Other researchers will benefit from improvements in experimental design to examine seed decay processes in the laboratory and field, and data analysis methods that yield more meaningful insight into the biotic and abiotic factors associated with microbe-seed interactions.
It was demonstrated that biodiversity of cropland can be manipulated purposefully through careful Roundup-Ready management. The wide breadth of the research, from Minnesota to Louisiana, gave credence to the results and maximized its impact amongst scientists and agrichemical company
personnel in the USA. One of the criticisms of Roundup-Ready technology, especially in Europe, is that weed control is so good that biological diversity is diminished to the point that agroecosystems might fail. For example, the over-winter dependence of endangered birds, rodents, insects, etch. on weed seeds may be compromised with the extensive use of Roundup-Ready crops. It was demonstrated to other scientists, the agrichemical industry, and environmental groups that judicious use of this technology could be used to enhance biological diversity in comparison to practices that were standard prior to the
introduction of Roundup-Ready technology. ARS also showed that Roundup-Ready technology can fail if delayed weed emergence is not taken into account by farm managers.
Weed control and economic benefits of transgenic weed control technology vs. conventional weed control systems were determined and characterized the impact of continuous use of transgenic weed control systems on weed species and population shifts. Results demonstrated that one or two applications of glyphosate alone controlled most common weeds and preemergence herbicides were not necessary for total weed control in glyphosate-resistant (GR) soybean under both conventional tillage and no-tillage systems. Glyphosate-only programs reduced herbicide inputs and were as profitable in GR soybean as were other herbicide programs in conventional soybean. Within 3 years, continuous bromoxynil-resistant cotton system resulted in weed shifts towards common purslane, sicklepod, and yellow nutsedge and yield decline and that was mitigated by rotating with GR cotton system.
Redvine and trumpetcreeper are invasive, perennial, woody, deep-rooted, difficult-to-control viny weeds in the Mississippi delta. Although a few herbicides are active on these weeds, they kill only foliage and have no effect on underground rootstocks. Their biology (seed dormancy and germination, seedling emergence, rootstock sprouting potential, growth, leaf surface, and epicuticular wax amount and
composition) as well as the nature of interactions of glyphosate and other herbicides in these weeds was characterized to develop effective control measures. ARS research demonstrated the mechanisms for
seed survival and the basis for a greater susceptibility of trumpetcreeper to glyphosate than redvine. Control of these weeds with glyphosate was only temporary and regrowth from underground rootstocks occurred because of insufficient translocation of glyphosate to rootstocks. A strategy was developed to manage these vines by integrating glyphosate in glyphosate-resistant soybean with fall deep tillage. Improved weed control with new formulations compared to existing products could reduce pesticide usage and costs to the farmer.
Problem Area X-C - Integrated Weed Management in Noncropland
Saltcedar (Tamarix spp.) provides poor habitat for birds relative to native riparian vegetation. ARS has implemented a saltcedar biological control program by releasing the saltcedar leaf beetle at several experimental sites throughout the arid western United States. The program has shown great promise, but its implementation has been restricted due to concerns about the endangered Southwestern willow flycatcher, some of which have been found to nest in saltcedar in some Arizona and New Mexico
locations. Birds at sites with high densities of saltcedar leaf beetles and at nearby paired sites where the beetles were absent were censused. Numbers of both birds and bird species were significantly greater where beetles had established. The demonstration that insects released for the biological control of saltcedar provide an important food resource that increases the quality of saltcedar habitat for birds helps justify the expansion of the biological control program. Because saltcedar invasion substantially reduces the resource value of western riparian systems, this research is important to livestock producers and others interested in maintaining healthy rangelands that provide badly-needed water resources. Saltcedar is thought to increase soil salinitiy through salt deposition on the soil surface. ARS found that