Integrated Weed Management in Vegetable Production: Evaluation of Seed Bank Ecology, Cover Crops and Hand Weeding

Abstract

<p>Weeds compete with vegetable crops for resources including nutrients, soil moisture and light, and can interfere with crop growth. Some weed species act as hosts to some of the worst insect pests and diseases of vegetables. Consequently, weeds result in lower crop yields, can reduce the quality of produce through stunted growth, staining or contamination, and contribute to increased crop management costs. Together, these impacts can have significant impacts on farm profitability.</p> <p>The high potential for weed growth in vegetables is a consequence of the highly disturbed soils and regular nutrient amendment and irrigation that are characteristics of these systems. Weeds common in vegetable production feature high levels of fecundity and precocity, and in most cases are characteristic of ruderal species such as <i>Amaranthus palmeri</i> S.Wats., and <i>Daucus carota</i> L. Their propagules undergo seasonal dormancy and can form large seed banks in soil, allowing them to escape unfavourable conditions and to remain viable anywhere from several months to several decades depending on species, thus conserving seeds for future growth and reproduction. Weed management practices such as herbicides and tillage, which are common in intensive production systems generally will not provide adequate weed control if used alone. An integrated weed management (IWM) framework, using a diverse range of direct and indirect weed management options in a coordinated fashion, provides a more effective and sustainable weed management.</p> <p>This thesis presents a general introduction to the field of research on weed management in vegetable production systems, a research chapter on the diversity and ecological traits of weeds present in the soil seed bank of a sample of vegetable farms in Australia, two research chapters on non-chemical weed management methods, and a general discussion of the overall research findings.</p> <p>Weed management strategies are often improved if they focus on the weed seed banks, which form a major source of weed problems in cropping systems including vegetables. In Chapter 2 I present the findings from a survey of the weed seed banks carried out across seven major vegetable growing regions of Australia, including a total of 36 farms. In the survey I recorded the abundance and diversity of weeds in seed banks, with soil cores collected to a depth of 20 cm and divided into 0-5 cm, 5-10 cm and 10-20 cm depth increments. The seed banks were dominated by broadleaved weeds in terms of the class and as expected by annual weeds in terms of their perenniality. The average size of the weed seed bank among the sampled farms varied from 2,964 to 12,867 seeds m², while species richness varied from three to nine across species per site across the sample. Weed seed abundance and diversity tended to be greater at 10-20 cm, with no differences in the upper layers due to a combination of soil mixing with inversion tillage and lower seed emergence at greater depths.</p> <p>The dominance of soil weed seed banks by annual species is associated with their r-selected strategies that allow them to adapt to the frequently and intensely disturbed soils and nutrient rich environment common to intensive vegetable production systems. The effect of different management practices on abundance and species richness were not found to be consistent across the study sites, although including a fallow in the crop rotation increased soil weed seed bank abundance. IWM strategies that focus on depletion of the soil weed seed bank (reducing weed seed inputs and increasing removal) are necessary to reduce the weed burden in vegetables in the medium to longer term.</p> <p>Chapter 3 contains results of a controlled environment trial focusing on cover cropping, one of the important integrating components of IWM. Cover crops can contribute to weed suppression at various stages of the weed life-cycle outside the commercial cash crop periods within a vegetable farm’s crop cycle, as well as impacting on weed seed bank dynamics. While cover crops have been widely researched in general, a deeper understanding of cover crop traits and the relative timing of emergence can improve the effectiveness of cover crops for weed management. This experiment evaluated the effect of two types of cover crops – a brassica (<i>Eruca vesicaria</i> (L.) Cav., Nemat) and a grass species (<i>Secale cereale</i> L., cereal rye) – on two types of weeds, a broadleaved weed (<i>Chenopodium album</i> L., fat hen) and a grass weed (<i>Lolium rigidum</i> Gaudin, annual ryegrass). The experiment included three weed emergence times: before, with and after cover crop emergence.</p> <p>The results showed that late emerging weeds (after the cover crop) were more effectively suppressed by the cover crop treatments than early emerging weeds, while Nemat was more effective at suppressing the two weed species than cereal rye. The suppressive effect of the cover crops was higher for annual ryegrass than fat hen. Early emerging cover crops had a longer growing period, facilitating early canopy development relative to the weed species, contributing to their greater suppressive ability. The species-specific traits of Nemat, including higher growth rate and early canopy closure, made it more effective than cereal rye in suppressing the two weed species. Similarly, the tall growth habit and broader leaf area of fat hen was more competitive with the cover crops than the low stature, narrow-leaved annual ryegrass. The findings highlight the importance of selecting appropriate cover crop species for particular circumstances (in this case, predominant weed types and their pattern of growth), and also highlighted the importance of timely cover crop sowing to encourage early emergence relative to weeds, therefore helping to ensure greater weed suppression.</p> <p>In high- value vegetable crops hand weeding is another important component of IWM, which helps to both avoid competitive yield losses and prevent addition to the weed seed banks. Hand weeding requires considerable physical labour and thus has a high economic cost, yet it is a common weed control method in small-scale production systems and is used in many large-scale production systems to follow up on weeds that have survived earlier control activities. In Chapter 4 I discuss a field evaluation of the performance of a selection of hand weeding tools. In this study, the agronomic, ergonomic and economic performance of four hand weeding tools – chipping hoe, stirrup hoe, rotating hoe and a combined tool – were evaluated at two sites (representing contrasting soil types) and for two weed growth stages.</p> <p>The light-weight chipping hoe was more effective in controlling weeds and required less physical effort. This was mainly due to the flexibility the tool offers in being able to operate around crop plants. As expected, weeding while the weeds were young was found to be more effective agronomically and also had significant economic benefits, with almost 50% lower labour costs than weeding older weeds. Older weeds are physically more difficult to remove due to their larger root structure, require higher work intensity, and take more time. Integrating hand weeding in overall weed management strategies for vegetable production will require careful selection of implements to match the capacity of the field worker and the weed load to improve the ergonomic performance, as well as careful attention to timing of operations for better agronomic and economic performance.</p> <p>To conclude, this research has revealed that seed banks of vegetable fields in Australia featured highest density and diversity in the deepest soil layers (100–200 mm) compared to the surface layers and confirmed that Australian vegetable farms are infested with some of the most problematic weeds of agricultural systems from around the world, and that these weeds have traits that are well adapted to the agroecological conditions present on vegetable farms.</p> <p>Sustainable weed management strategies aimed at depleting the weed seed bank in the longer-term are required. This can be achieved by adopting a flexible IWM approach which integrates a range of chemical, mechanical and cultural methods without relying overly on particular methods. However, implementation of IWM systems is knowledge intensive, site/farm specific, and can pose challenges for farmers to adopt without reliable information and extension support.</p> <p>The incorporation of optimised non-chemical methods into IWM, such as cover cropping and hand weeding, can not only reduce weed pressure but also help build more resilient and sustainable agroecosystems. Using cover crop species with rapid initial growth rates and early canopy development and facilitating the timely emergence of cover crops relative to weeds will maximise their capacity to suppress growth and reproduction of weeds, which can reduce additions to the weed seed bank. Similarly, hand weeding using a light-weight and ergonomically optimised tool, and scheduling hand weeding at early stages in the weed life-cycle will reduce the work load of field workers and overall weeding costs. As a back-up method after tillage and herbicides, hand-weeding can also contribute significantly to preventing weed seed production and input into the soil seed bank. Integration of these practices with other methods of weed management as part of IWM can make important contributions to sustainable and more productive vegetable farms, making it easier for the vegetable industry to meet growing future demand.</p>