Reduction of growth and reproduction of Cyperus esculentus by specific crops

To assess the potential contribution of growing highly competitive crops to control Cyperus esculentus in the Netherlands, the influence of silage maize, hemp, winter barley and silage winter rye on emergence, growth and reproduction of this weed was studied in two field experiments. Growth and reproduction of C. esculentus (determined both as tuber production per plant and density of the weed in the subseuent crop maize) was reduced by each crop, compared to plots in which no crop was grown. In hemp, hardly any tuber production was observed. By growing a green manure crop after harvest of the winter barley the tuber production of the weed was reduced to about 40% of that in winter barley followed by fallow. Decreasing the light regime in a greenhouse experiment in the same order as was observed in the crops winter barley, maize and hemp, caused a similar reduction of tuber production to that found in the field. Therefore, it is suggested that competition for light is the main factor explaining the observed crop effects on the population dynamics of the weed. It is concluded that growing competitive crops as hemp in rotation may effectively complement chemical control of C. esculentus.     

Effect of stale seedbed preparations and subsequent weed control in lettuce (cv. Iceboll) on weed densities

The effects of stale seedbed preparations and several weed control methods on the emergence of weeds in lettuce were studied. The specific goal was to evaluate the use of a stale seedbed in combination with chemical or mechanical weed control methods in the field. Depending on location and year, stale seedbed preparations followed by weed control prior to planting reduced the amount of weeds during crop growth by 43–83%. Control of the emerged seedlings after a stale seedbed preparation was more effective with glyphosate than with a rotary harrow. Covering the rotary harrow during control to prevent light reaching the soil improved its effect on the weed density during crop growth in two of 3 years. Radiation with far red light (FR) did not reduce the number of emerging weeds in this study. Mechanical control by finger weeder, torsion weeder and hoe was applied without stale seedbed preparations. These measures reduced the weed densities by 88–99%, compared with the untreated control and were more effective than chemical weed control with carbetamide and chlorpropham. The results show that the stale seedbed technique in combination with mechanical control of emerging weeds can reduce the weed population during crop growth as effectively as chemical control. The technique may therefore help reduce the use of herbicides in lettuce crops in the future.     

Effects of three management strategies on the seedbank, emergence and the need for hand weeding in an organic arable cropping system

The effects of three different weed management strategies on the required input of hand weeding in an arable organic farming system, the weed seedbank in the soil and the emerging weed seedling emergence were studied from 1996 to 2003. Strategies were based on population dynamic models and aimed for (1) control of weeds as carried out in standard organic farming practice, (2) control of all residual weeds that grow above the crop and (3) prevention of all weed seed return to the soil. Under all strategies, the size of the seedbank increased during the conversion from conventional to organic farming systems. The increase under strategy 3 was significantly smaller than the increase under the other strategies. From 1999 onwards, the weed densities in plots treated with strategy 3 became significantly lower than the weed densities in plots treated with the other strategies. The time needed for hand‐weeding required to prevent weed seed return, in addition to the time needed in standard organic farming practices, reduced during the course of the study. A management strategy aimed at the prevention of seed return (strategy 3) can reduce the size of the increase of the seedbank, which is usually observed after transition from conventional to organic farming. This study provides unique real‐world data that are essential for evaluating population dynamic models. The results may contribute to the development of weed management systems based on ‘no seed’ threshold strategies and to a further decrease in the dependence on herbicides.     

Prediction of the competitive effects of weeds on crop yields based on the relative leaf area of weeds

For implementation of simple yield loss models into threshold-based weed management systems, a thorough validation is needed over a great diversity of sites. Yield losses by competition wsth Sinapis alba L. (white mustard) as a model weed, were studied in 12 experiments in sugar beet (Beta vulgaris L.) and in 11 experiments in spring wheat (Triticum aestivum L.). Most data sets were heller described by a model based on the relative leaf area of the weed than by a hyperbolic model based on weed density. This leaf area model accounted for (part of) the effect of different emerging times of the S. alba whereas the density model did not. A parameter that allows the maximum yield loss to be smaller than 100% was mostly not needed to describe the effects of weed competition. The parameter that denotes the competitiveness of the weed species with respect to the crop decreased the later the relative leaf area of the mustard was determined. This decrease could be estimated from the differences in relative growth rate of the leaf area of crop and S. alba. However, the accuracy of this estimation was poor. The parameter value of the leaf area model varied considerably between sites and years. The results strongly suggest that the predictive ability of the leaf area model needs to be improved before it can be applied in weed management systems. Such improvement would require additional information about effects of abiotic factors on plant development and morphology and the definition of a time window for predictions with an acceptable level of error.     

Real-time vision-based detection of Rumex obtusifolius in grassland

Rumex obtusifolius is a common grassland weed that is hard to control in a non-chemical way. The objective of our research was to automate the detection of R. obtusifolius as a step towards fully automated mechanical control of the weed. We have developed a vision-based system that uses textural analysis to detect R. obtusifolius against a grass background. Image sections are classified as grass or weed using 2-D Fourier analysis. We conducted two experiments. In the first (laboratory) experiment, we collected 28 images containing R. obtusifolius and 28 images containing only grass. Between 23 and 25 of 28 images were correctly classified (82–89%) as showing R. obtusifolius ; all grass images were correctly classified as such. In the second (field) experiment, a self-propelled platform was used to obtain five sequences of images of R. obtusifolius plants. We used the parameters that gave the best classification results in the first experiment. We found, after changing one of the algorithm's parameters in response to prevailing light conditions, that we were able to detect R. obtusifolius in each image of each sequence. The algorithm scans a ground area of 1.5 m² in 30 ms. We conclude that the algorithm developed is sufficiently fast and robust to eventually serve as a basis for a practical robot to detect and control R. obtusifolius in grassland.     

The role of arable weed seeds for agroecosystem functioning

A literature study was conducted to gather knowledge on the impact of weed seeds on agroecosystem functioning other than effects related to the production of weed seedlings and plants. The results of the review suggested that a larger and more diverse weed seedbank can contribute to the biodiversity of various groups of macrofauna and microbiota, with a positive or negative impact on the agroecosystem. However, relationships between weed seed availability and functional biodiversity in the field have generally not been established, with the exception of case studies of seeds sustaining populations of granivorous farmland birds or acting as reservoirs and vectors of plant pathogens. To value the contribution of different weed seeds to sustaining populations of functional biota, more detailed information on their relationships with seeds is essential. Hypotheses and related questions that can be used to explore the impact of weed seeds on functional biodiversity have been identified. The identification of weed seed species that are a key to sustaining functional biota may lead to a weed management strategy aiming to minimise the damage of weeds to crops while taking the alternative ecological roles of weed seeds into account.     

Influence of allocation and detoxification of metribuzin in Chenopodium album on the reliability of prediction of the minimum lethal herbicide dose rate

A new method for application of minimum lethal herbicide dose (MLHD) rates is based on the assumption that a MLHD for a photosystem (PS) II herbicide can be predicted for weeds when the developmental stage of seedlings, herbicide uptake and herbicide action are taken into account. Chenopodium album     

Techniques to estimate relative leaf area and cover of weeds in crops for yield loss prediction

Destructive measurements to collect input data for models that predict yield loss from relative leaf area of weeds can be laborious. Alternative methods were tested in seven field experiments with sugar beet or spring wheat. Weeds with different morphologies showed the same linear relationships between relative leaf area, meas ured destructively, and cover, assessed by means of a frame, until 3 or 4 weeks after crop emer gence. At later growth stages, differences in weed morphology resulted in different relationships. Visual estimates of weed cover corresponded only roughly with cover assessments with a frame. The possibility of estimating relative leaf area of weeds with a reflectance technique was tested, assuming that for early growth stages the leaf area index of weeds can be considered as additional to that of the crop. In spring wheat, relative leaf areas of Sinapis alba L., sown at different times and densities, correlated well with characteristics based on infra-red reflectance. In sugar beet, these relationships were not as distinct. Techniques d'estimation de la surface foliaire relative et de la couverture des mauvaises herbes dans les cultures, en vue de prédictions de pertes en rendement Des prélèvements destructifs peuvent alimenter en données les modèles qui prédisent les pertes de rendement à partir de la surface foliaire relative des mauvaises herbes, mais ils sont exigeants en temps. Des méthodes alternatives ont été testées lors de sept expériences au champ dans de la betterave ou du blé de printemps. Jusqu'à trois ou quatre semaines après la levée de la culture, des mauvaises herbes possédant différentes morphologies montraient les mêmes relations linéaires entre d'une part la surface foliaire relative mesurée de manière destructive et d'autre part la couverture mesurée grâce à une grille. Aux stades de croissance ultérieurs, du fait des différences morphologiques entre les mauvaises herbes, les relations étaient différentes. Les estimations visuelles de la couverture en mauvaises herbes ne correspondaient qu'approximativement aux mesures de couverture effectuées au moyen de la grille. La possibilité d'estimer la surface foliaire relative des mauvaises herbes avec une technique de réflectance a étéévaluée, en supposant que, aux stades de croissance initiaux, l'indice de surface foliaire des mauvaises herbes peut être considéré comme additif de celui de la culture. Dans le blé de printemps, les surfaces foliaires relatives de Sinapis alba L., semées à différentes époques et à différentes densités, étaient bien corrélées avec des caractéristiques basées sur la réflectance infra-rouge. Dans la betterave à sucre, ces relations n'étaient pas aussi claires. Bestimmung der relativen Blattfläche und des Deckungsgrads von Unkräutern zur Prognose von Ertragsverlusten Die destruktive Gewinnung von Daten der relativen Blattfläche von Unkräutern für Modelle zur Vorhersage von Ertragsverlusten sind arbeitsaufwendig. Alternative Methoden wurden in 7 Versuchen in Zuckerrübe und Sommerweizen untersucht. Morphologisch unterschiedliche Unkräuter zeigten bis zu 3 oder 4 Wochen nach dem Auflaufen der Kulturpflanzen dieselben linearen Beziehungen zwischen der destruktiv gemessenen relativen Blattfläche und dem mittels eines Rahmens bestimmten Deckungsgrad. In späteren Entwicklungsstadien führten die morphologischen Unterschiede zu verschiedenen Verhältnissen. Bonituren des Unkrautdeckungsgrads entsprachen nur grob den mit dem Rahmen ermittelten Deckungswerten. Die Bestimmung der relativen Blattfläche der Unkräuter durch Messung der Lichtreflexion wurde geprüft, wobei angenommen werden kann, daß sich in den frühen Entwicklungsstadien der Blattflächenindex der Unkräuter mit dem der Kulturpflanzen summiert. Im Sommerweizen war die relative Blattfläche von zu verschiedenen Zeiten und in unterschiedlicher Dichte gesätem Sinapis alba L. mit den Daten der Infrarotreflexion gut korreliert, auf den Zuckerrübenflächen nicht so genau.