1a. Objectives (from AD-416)
1) Develop integrated multi-tactic weed management programs which will include cover crops, cultural practices, and bio-herbicides for reduced-tillage production systems. 2) Determine the ecological mechanisms by which crop and soil management impact crop and weed establishment and growth processes.
1b. Approach (from AD-416)
A multi-tactic approach will be developed for reduced-tillage corn production that combines the optimum hairy vetch-rye cover crop mixture for suppressing weeds with poultry litter injection that supplements nitrogen required by the crop, but localizes it in space and time to minimize utilization by weeds. Soil properties, mulch characteristics, and weed sizes will be determined that optimize the postemergence destruction of weeds with a high-residue cultivator, minimize disturbance of the surface residue, and maximize soybean yield. Methods will be developed to augment and purify naturally-produced bacterial phytotoxins and test their efficacy as a postemergence herbicide. Basic research will be conducted to increase our understanding of weed suppression by cover crop mulches in relation to potentially allelopathic compounds leached into the soil and their duration of soil activity. Basic research also will be conducted to determine the effect of soil organic matter on mitigating the competitive effects of weeds on crops.
3. Progress Report
A component experiment evaluating the optimal combination of grass and legume cover crop mixture proportions and poultry litter rate and placement method was initiated. The first year’s field experiment was completed and crop, soil, and weed metrics were collected. This experiment has generated much interest among collaborators and has been included in several grant applications. A University of Maryland M.S. student has designed several tangential projects within this study and has begun developing methodology for assessing N and C spatio-temporal dynamics. The role of the regulatory protein RsmA in regulating production of the phytotoxins syringomycin, phaseolotoxin, and tabtoxin was determined. A manuscript detailing the role of RsmA in regulation of these phytotoxins was prepared and submitted. A decision to shift resources away from studying the efficacy of natural herbicides toward this work was made due to significant progress that has also been made regarding the role of the Gac/Rsm regulon in regulating the production of tagetitoxin in the potential bioherbicide Pseudomonas syringae pv. tagetits. Using molecular genetic techniques it was shown that the component genes of the Gac/Rsm regulon gacS, gacA, rsmA, and rsmE all function in regulating this phytotoxin. However, creation of the plasmid to demonstrate the roles of rsmX, rsmY, and rsmZ has been challenging and requires additional resources to accomplish. Resources were redirected to studies determining the regulatory mechanisms for phytotoxin production in Pseudomonas syringae pathovars syringae, phaseolicola, and tabaci. Regulatory mechanisms must be determined first, so that strategies can be devised to enhance phytotoxin production by these potential weed biocontrol agents. Two manuscripts dealing with allelopathic properties of a rye cover crop were prepared and submitted. One manuscript focused on the behavior of benzoxazinoid compounds in soil after released from killed rye. The second focused on the suppression of weed emergence and growth by rye and the apparent lack of causal involvement of benzoxazinoid compounds. These analyses have provided a new perspective on our allelopathy research and led to a reassessment of our project approach for allelopathy-related objectives. Consequently, we deferred initiation of microplot and allelopathy research designated in the project plan, and will design a more appropriate line of research to begin next year. Two greenhouse pot experiments were conducted examining the effects of soil from long-term conventional and organic cropping systems on the functional densities required for nitrogen utilization of crop and weed species. Results from these experiments have informed refinements in upcoming weed-crop competition experiments with these soils. These experiments also provided a basis for an honor’s thesis project by an undergraduate University of Maryland student.
1. The regulatory protein RsmA is of major importance in regulating production of phytotoxins in pathovars of Psuedomonas syringae. Phytotoxin-producing strains of the bacteria, Pseudomonas syringae, are being studied with regard to their potential use as biological control agents of pernicious weed species. Prior to development as biological control agents, the regulatory mechanisms that control phytotoxin production, and the environmental factors that influence production of these phytotoxins, must be determined. Using molecular genetic approaches, ARS researchers at Beltsville, Maryland, in collaboration with molecular biologists at the University of Nottingham, United Kingdom, showed that excess cellular levels of RsmA turned off production of phytotoxin in three unrelated strains of P. syringae. This work demonstrates for the first time the role of the RsmA regulatory protein in production of phytotoxins in P. syringae. Knowledge of regulatory networks, and factors that influence them, will allow scientists to devise strategies that enhance consistency and efficacy of bioherbicidal activity of Pseudomonas syringae strains used in the control of weeds.
2. Benzoxazinoid compounds do not explain weed suppression by rye. Rye cover crops are known to produce phytotoxic benzoxazinoid compounds which are thought to explain observed weed suppression by this species. ARS researchers at Beltsville, Maryland, showed that the most abundant benzoxazinoids found in soils were not present in sufficiently high concentrations to account for the observed levels of weed suppression. Results suggest that, within the soil environment, the associations between suppression of bioassay species and rye benzoxazinoid concentrations were probably not causal, and that other compounds released with a similar dynamics were likely responsible. These results will require plant scientists to reevaluate the phytotoxins responsible for weed suppression by rye and give closer attention to the behavior and availability of these compounds in soil.
3. Quantifying synergistic interactions in ecologically based weed management. Exclusive reliance on herbicide technology can lead to unintended consequences including off-site environmental toxicity and the development of resistant weed species. Integrated weed management will require the use of multiple tactics including cultural as well as herbicide technologies to achieve a sustainable level of weed control. ARS researchers at Beltsville, Maryland, in collaboration with weed ecologists at Penn State University developed a general framework for mathematically assessing potential synergistic interactions among tactics and, thereby, identifying the most compatible tactics for optimizing weed management. This approach will be utilized by agroecologists and weed management specialists as a more rational approach to integrating weed control tactics and achieving reliable weed control with judicious use of herbicides.Ryan, M.R., Mirsky, S.B., Mortensen, D.A., Teasdale, J.R., Curran, W.S. 2011. Potential synergistic effects of cereal rye biomass and soybean planting density on weed suppression. Weed Science. 59:238-246.