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 was completed that determines the optimal cover crop species composition (cereal:legume) and poultry litter rate and placement method that minimize weed competition and maximize nitrogen use efficiency in corn. The second year’s field experiment was completed and crop, soil, and weed metrics were collected. This experiment has attracted a broad multi-disciplinary team within ARS as well as land grant universities and has been included in several grant applications. Two University of Maryland M.S. students have selected this experiment for their thesis and have initiated two tangential projects within this study. The first thesis will focus on assessing N and C spatio-temporal dynamics while the other student will assess greenhouse gas emissions within the project. A report on the role of the small regulatory protein RsmA in production of the phytotoxins syringomycin, phaseolotoxin, and tabtoxin, and other virulence factors, was published in a peer-reviewed journal. A decision was made to increase the scope of study regarding the regulatory network involved in the production of the phytotoxin tagetitoxin by the potential weed biological control agent Pseudomonas syringae pv. tagetis. This required that resources be shifted away from studies directed at determining the efficacy of natural herbicides towards this work. Experiments were initiated with an ARS collaborator at the Beltsville Agricultural Research Center to finalize sequencing of the P. syringae pv. tagetis genome. This will allow mapping of genes mutated in mutants no longer capable of production of tagetitoxin. A collection of such mutants was developed previously by scientists in the Sustainable Agricultural Systems Laboratory. These sequencing experiments will potentially identify genes involved in regulation of tagetitoxin and also identify genes involved in the synthesis of this phytotoxin. Experiments were continued to determine the role of components in the Gac/Rsm regulon in production of tagetitoxin. Plasmids have been created to overexpress five rsmX genes in P.syringae pv. tagetis. Creation of plasmids to demonstrate the roles of rsmY and rsmZ continues to be a challenge. Results from previous allelopathy research have provided new insights on the activity of allelochemicals in soil. For example, solution-phase availability was found to be critical and may need more careful evaluation. Consequently, we deferred initiation of microplot and allelopathy research designated in the project plan and have begun discussions on a more appropriate line of research to begin next year. A two-phase, two year greenhouse pot experiment was completed. The first trial examined 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 the first experiment were used to design the subsequent weed-crop competition experiment using the same soils. Both trials have been completed and a manuscript is in preparation.
1. Cover crop-based, organic rotational no-till soybean production requires a multi-tactical weed management approach to ensure consistent grain yields. Traditional tillage-based organic soybean production practices have restricted the ability of existing producers from expanding their operations at a time when demand for organic grains is high. This is due to high labor requirements to manage weeds for existing producers and an aversion to tillage due to risks of lowering soil quality by farmers using no-till practices. Traditional tillage-based organic systems can also reduce soil quality and increase susceptibility to erosion. Several experimental results at Beltsville and with collaborators in the mid-Atlantic region over the past 4 years have elucidated several key principles of the effects of cereal rye cover crop mulches on weed management in the organic rotational no-till soybean system. The effects of cultural management practices (fertility, establishment method, planting and termination timing, and seeding rate) on the weed suppression potential of cereal rye mulches was demonstrated to be species-specific (ineffective on perennial weeds), less effective as weed seedbanks and residual soil fertility increase, and proportional to early spring ground cover and cover crop biomass at soybean planting. This work will serve as a framework to managing weeds in cover crop-based, organic rotational no-till soybean production.
2. Soil abundance and availability of phytoxic benzoxazinoids from rye cover crops is low. ARS researchers at Beltsville, Maryland, showed that phytotoxic benzoxazinoid compounds are gradually (2 week period) released from rye cover crop and at low but measurable amounts. Also, the more dominant chemical form released appear to arise from the roots of the rye; and diffusive soil movement of the more common benzoxazinoid forms was minimal contrary to accepted theory that predicts that because of their higher solubility and less particle binding than pesticides they should move more freely through soil. Physical binding and soil fate of these compounds need to be more carefully assessed before a clear picture about their role in cover crop weed suppression is established.
3. An acccurate water solubility for MBOA was determined. The methoxy-substituted benzoxazolinone, MBOA, is important as a defense chemical found in several graminae species; however, its water solubility has never been reported. The environmental fate and exposure of this compound is significantly controlled by this property. The maximum water solubility for MBOA, as determined using established methods, was 0.54 g/L versus the published value of 8.9 g/L for the non-methoxy benzoxazolinone, BOA. These values were found to be very important in explaining their availability in soil after release from rye cover crop material.
Kong, H., Roberts, D.P., Patterson, C.D., Kuehn, S., Heeb, S., Lakshman, D.K., Lydon, J. 2012. Effect of overexpressing rsmA from Pseudomonas aeruginosa on virulence of select phytotoxin-producing strains of P. syringae. Phytopathology. 102:575-587.