Concern about potential increases in weed populations without the use of herbicides has limited the uptake of organic farming. However, as both public demands for organic produce and the pro®le of organic farming have increased in recent years, so too has the range of weed control options. Progress in cultural methods of weed control has included the use of novel weedsuppressing cover crops, and the identi®cation of speci®c crop traits for weed suppression. Direct weed control has also seen developments, with new implements appearing on the market that could bene®t in the future from sophisticated machine guidance and weed detection technology. Advances in novel techniques such as steaming have also been made. Many weed control operations in organic systems present the grower with con¯icts, and both these and many of the most recent developments in organic weed control are reviewed. An increase in our understanding of weed biology and population dynamics underpins long-term improvements in sustainable weed control. The outcome of these studies will bene®t conventional and organic growers alike. Emphasis is given to the need for¯exibility and a combination of weed biology knowledge, cultural methods and direct weed control to maintain weed populations at manageable levels.
In relatively short-term studies it is often difficult to identify the factors that are important in determining the patterns of emergence for different weed species. When results from longer-term studies are averaged over years, they demonstrate that some weed species follow characteristic, and potentially predictable, patterns of annual emergence. In recent years, our understanding of the seasonal changes in dormancy and germination behaviour, and the interaction of these processes with the environment, has advanced considerably. In particular, the capacity for computer-based statistical analysis and the ability to handle large datasets has made it possible to use this information in a predictive way. Currently, there are several approaches to developing predictive weed emergence modelling. Some researchers have taken an empirical approach, seeking to identify correlations between environmental variables and observed emergence patterns. Others have taken a reductionist approach, subdividing the emergence process into its component stages of dormancy, germination and pre-emergence growth, to work towards achieving an eventual understanding of the physiological processes involved. Despite recent advances, a number of major challenges remain (e.g. population variability, dormancy and the quality of the input data) that must be overcome before these emergence models can be implemented in practice. The level of complexity and degree of parameterization incorporated within such models must also be addressed in relation to the intended use of the model.
Summary 1.Weeds play an important role in arable and horticultural habitats, and models are being developed to improve our understanding of their population dynamics. The position of a weed seed in the soil profile influences the probability of a seed germinating, emerging successfully and its relative time of emergence. Identifying a relationship between the shape or weight of a seed and its ability to emerge from depth may allow the development of generic models. The aim of this study was to quantify seed response to burial depth, to improve the wider application of existing seedling emergence models. 2. A field experiment used weed seeds sown at different depths and densities in a standard substrate. In addition, two laboratory studies used pre-germinated seeds of the same range of species, buried at a range of depths in optimum conditions using the same substrate. These studies explored the effects of seed size, shape and sowing density on seedling emergence and also enabled reserve-dependent pre-emergence mortality to be quantified. 3. The largest and heaviest of the seeds tested overall, Veronica hederifolia , emerged from the greatest depth (8 cm). In contrast, Tripleurospermum inodorum and Veronica arvensis , the two smallest species, showed a sharp decline when burial exceeded 1 cm. However, the link between seed shape or weight and the ability to emerge from depth suggests a complex relationship. Given optimum conditions, some species ( Stellaria media and Chenopodium album ) have the physical reserves to emerge from a wider range of burial depths than normally observed in the field, suggesting an ability to exploit opportunities when they occur. 4. For some species, emergence was reduced at high seed densities (e.g. Veronica arvensis ). These responses may be associated with traits that have evolved to counteract sibling competition. 5. Synthesis and applications . Generic models identifying the maximum depths for seed germination and emergence have a number of practical applications. For example, they can be used to target cultivation to deplete the weed seed bank or to prescribe the optimum depth of mulches to favour certain species. Our model showed that, in general, larger-seeded species emerged from deeper in the soil, but the relationship between seed size and shape and emergence was complex, possibly species specific. Our germination data may also assist our understanding of the relative importance of different causes of seed losses, particularly from different zones of the soil profile, such as the soil surface. Understanding the component processes of seed behaviour and germination is essential to developing sustainable weed management practices in agriculture and horticulture, and the work reported here contributes to a larger programme modelling weed seed bank population dynamics.
The germination characteristics of Stellaria media (common chickweed) were investigated over a range of constant temperatures and degrees of moisture stress in order to assess the suitability of hydrothermal time as a basis for modelling germination under field conditions. Maximum percentage germination occurred over a much narrower temperature range around the optimum temperature than previously seen for cultivated crop seed. The entire final percentage germination response to temperature in water was well described by two probit curves, and this model was extended to describe the data at all water potentials at a temperature close to the optimum. The implications of the reduction in germination at nonoptimal temperatures are discussed with respect to the interpretation of germination progress curves and conditional dormancy. After adjusting for maximum percentage germination, a hydrothermal time model was found to fit the data set well within the conditions normally encountered in horticultural seedbeds. This separation of the final percentage germination presents a flexible modelling approach that allows for the different levels of dormancy typically expressed within weed populations. By contrast with many previously reported species, S. media had a synchronous germination rate within the population at any given temperature\water potential combination. This synchronous germination of at least a proportion of the population over a wide range of temperature and water potentials might have ecological significance for the opportunistic germination behaviour of this weed species.
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