Pesticide use in the United States is concentrated in the Corn Belt, mainly in the form of herbicides on corn (Zea mays) and soybean (Glycine max). If this chemical load to the environment is to be reduced, without appreciably affecting crop yields, an intimate understanding of weed ecology is necessary. For annual weeds in the U.S. Corn Belt, critical ecological information includes seedbank density, seed dormancy, seedling emergence, and environmental variables that regulate these factors. For three weeds (Setaria spp., Amaranthus retroflexus, and Chenopodium album) in Minnesota, this information is summarized and its application to weed management is illustrated. If weed seedbank densities (0-10 cm depth) are <100 seeds/m prior to spring tillage operations, subsequent seedling populations are too low (<40 seedlings/m ) to require control. Seedbanks ranging from 100-1000 seeds/m produce seedling populations (<400 seedlings/m ) that can be controlled mechanically. In contrast, pre-tillage seedbanks >1000 seeds/m typically result in such high seedling populations that nonchemical control measures alone cannot prevent large crop yield losses. About one-half of all arable fields harbor seedbanks with densities >1000 seeds/m . The proportion of buried seeds giving rise to seedlings is controlled by seed dormancy, which in turn is governed partially by April temperatures (in Minnesota), whereas the timing of subsequent seedling emergence is determined by soil temperature and soil water. EMERG models simulate daily percentages of emerging weed seedlings for April through June, when most Corn Belt crops are sown. If seedbeds of fields with potential seedling densities of 100 seedlings/m are prepared and sown when simulated emergence exceeds °80% for C. album, 85% for A. retroflexus, and 99% for Setaria spp., then mechanical weed control sufficiently reduces weed populations and crop yield losses are negligible. Accordingly, mechanical weed control may be substituted economically for chemical control, but only when accompanied by adequate understanding of the ecology of pertinent weed species.

Context. While the exact acceleration mechanism of energetic particles during solar flares is (as yet) unknown, magnetic reconnection plays a key role both in the release of stored magnetic energy of the solar corona and the magnetic restructuring during a flare. Recent work has shown that special field lines, called separators, are common sites of reconnection in 3D numerical experiments. To date, 3D separator reconnection sites have received little attention as particle accelerators. Aims. We investigate the effectiveness of separator reconnection as a particle acceleration mechanism for electrons and protons. Methods. We study the particle acceleration using a relativistic guiding-centre particle code in a time-dependent kinematic model of magnetic reconnection at a separator. Results. The effect upon particle behaviour of initial position, pitch angle, and initial kinetic energy are examined in detail, both for specific (single) particle examples and for large distributions of initial conditions. The separator reconnection model contains several free parameters, and we study the effect of changing these parameters upon particle acceleration, in particular in view of the final particle energy ranges that agree with observed energy spectra.

At locations in Argentina and the United States, solaria (miniature, portable, plastic greenhouses or a plastic sheet approximately 1 m2) were placed on field soils in autumn or late winter in an attempt to predict summer annual weed densities. Initial emergence of summer annual weeds covered by solaria commenced weeks before that of weeds in exposed seedbeds. Cumulative emergence of many species in solaria reached asymptotes before crops were sown. At asymptotic cumulative emergence, densities of dominant weeds in solaria (common lambsquarters, green foxtail, and large crabgrass) were correlated with weed densities occurring 4 wk after sowing, the typical time for making postemergence weed control decisions. These results indicate that solaria may supplement seedbank-sampling techniques for predicting weed densities in crops.

Context. In solar flares, a large number of charged particles is accelerated to high energies. By which physical processes this is achieved is one of the main open problems in solar physics. It has been suggested that during a flare, regions of the rapidly relaxing magnetic field can form a collapsing magnetic trap (CMT) and that this trap may contribute to particle energisation. Aims. In this Research Note we focus on a particular analytical CMT model based on kinematic magnetohydrodynamics. Previous investigations of particle acceleration for this CMT model focused on the non-relativistic energy regime. It is the specific aim of this Research Note to extend the previous work to relativistic particle energies. Methods. Particle orbits were calculated numerically using the relativistic guiding centre equations. We also calculated particle orbits using the non-relativistic guiding centre equations for comparison. Results. For mildly relativistic energies the relativistic and non-relativistic particle orbits mainly agree well, but clear deviations are seen for higher energies. In particular, the final particle energies obtained from the relativistic calculations are systematically lower than the energies reached from the corresponding non-relativistic calculations, and the mirror points of the relativistic orbits are systematically higher than for the corresponding non-relativistic orbits. Conclusions. While the overall behaviour of particle orbits in CMTs does not differ qualitatively when using the relativistic guiding centre equations, there are a few systematic quantitative differences between relativistic and non-relativistic particle dynamics.

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