Site-specific weed control techniques have gained interest in the precision farming community over the last years. Managing weeds on a subfield level requires measuring the varying density of weeds within a field. Decision models aid in the selection and adjustment of the treatments, depending on the weed infestation. The weed control can be done either with herbicides or mechanically. A site-specific herbicide application technology can save large amounts of herbicides. Mechanical weed control techniques adapting to the weed situation in the field are applicable to a wide spectrum of crops.Site-specific techniques for the detection and management of weeds are presented. A system for the discrimination of different weed species and crops from images is described, which generates weed maps automatically. Models for the yield effect of weeds are developed and applied in onfarm-research experimental setups. Economic weed thresholds are derived and used for a herbicide application with a patch sprayer.

Field experiments using precision farming technology and Geographic Information Systems, following a so-called Precision Experimental Design, were conducted in maize, winter barley and winter wheat and compared with two randomised plot experiments in maize to quantify yield effects attributed to weed competition and weed control. Fields were divided into cells, and weed densities for all weed species, soil conductivity and grain yield were measured in each cell. Untreated plots and herbicide treatments against grass weeds or broad-leaved weeds were included in all three experiments. Chenopodium album, Polygonum spp. and Echinochloa crus-galli were the dominating weed species in maize. Stellaria media, Veronica hederifolia, Matricaria chamomilla, Alopecurus myosuroides and Galium aparine were the most abundant weed species in the winter barley and winter wheat fields. All species were distributed heterogeneously within the fields with den-sities ranging from 0 to more than 200 plants m )2 . In the Precision Experimental Design, it was found that grass-weed competition and herbicide application had a significant effect on grain yield, using a linear mixed model with spatial correlation structure to determine the effects of groups of weed species, soil variability and herbicide application on grain yield separately. When a conventional plot experiment was set up in the same field, no statistically significant grain yield difference between the treatments was found. The results highlight the benefits of Precision Experimental Design for studying weed-crop competition. Data can be used to calculate yield loss functions for groups of weed species and to create a decision support system for site-specific weed control.

Herbicide resistance in Alopecurus myosuroides causes severe problems in Western European cropping systems. Costs of herbicide resistance were investigated in this study by analysing variable production costs and sales revenues. Three farms were selected for this study, with winter wheat as the dominating crop in all farms. Resistance in A. myosuroides populations was verified at all locations. Four farming approaches were simulated over a period of 20 years: (i) continuing the actual cropping system without increase of resistance, (ii) continuing the actual cropping system with increase of resistance, (iii) changing cropping practice to overcome resistance and (iv) changing cropping practice to prevent resistance. Contribution margins representing the proportion of sales revenue that is not consumed by variable costs were calculated for all approaches. Comparative static simulations showed that average contribution margins in a cropping system with more than 60% winter cereals and reduced tillage practice dropped from 807 € ha À1 a À1 without herbicide resistance to 307 € ha À1 a À1 with herbicide resistance. Alopecurus myosuroides population densities increased to more than 1000 plants m À2 . Diverse crop rotations, including spring crops, clover-grass leys and intensive tillage, suppressed A. myosuroides populations, and average contribution margin was 630 € ha À1 a À1 . Preventive methods with rotations of winter cereals and spring crops with less clover-grass leys resulted in an average contribution margin of 691 € ha À1 a À1 . In conclusion, rotations of winter cereals and spring crops combined with inversion tillage and herbicides provide stable yields and can prevent weed population increase.

Gutjahr C, Sökefeld M & Gerhards R (2012). Evaluation of two patch spraying systems in winter wheat and maize. Weed Research52, 510–519. Summary Fifteen field experiments were conducted in winter wheat and maize to analyse the spatial distribution of weed species and to evaluate two systems of site‐specific weed control. Fields were divided into cells and weed densities for all weed species were counted in each cell. Weed species were then grouped into annual broad‐leaved species, grass weeds and Galium aparine in winter wheat and annual broad‐leaved species, grass weeds and perennials in maize. Weed control thresholds were set for each group of weed species and different herbicides were sprayed in cells where the weed control threshold was exceeded. All three weed species groups were distributed heterogeneously within and between the fields, with densities ranging from 0 to more than 2000 plants m−2. Site‐specific weed control was realised with a GPS‐controlled multiple‐tank sprayer, containing three separate spray lines for each of the three application maps. In winter wheat, site‐specific weed control using a multiple‐tank sprayer resulted in 70% untreated area compared with uniform applications across the whole field. Simulated site‐specific weed control using a conventional sprayer with a tank mixture only resulted in 37% untreated area. In maize, 47% of the area remained untreated using the multiple‐tank sprayer and 6% using a conventional sprayer and tank mixtures. Efficacy of site‐specific weed control was good in all fields. The results demonstrate the need for combining weed mapping and new application technologies for patch spraying.

Summary In 393 field experiments in Baden‐Württemberg region in south‐western Germany, herbicide efficacy, yield loss and crop tolerance of maize (Zea mays) were investigated between 1981 and 2011. The collected data served to determine changes in weed frequencies, in herbicide use, yield loss functions and economic thresholds (ETs). Over 60 weed species were reported. Chenopodium album and Galium aparine were the most frequent broad‐leaved weeds, the former becoming more frequent over time. Species of the genera Lamium, Polygonum, Veronica and Matricaria occurred in about every fifth trial. Alopecurus myosuroides and Echinochloa crus‐galli were the most frequent grass weeds; the former declining in frequency by 1.1% per year, the latter increasing by 1.5%. Results suggest a weed population shift towards thermophilic species. aceto‐lactate‐synthase and 4‐HPPD‐inhibitor herbicides became important in the 1990s. Pendimethalin and bromoxynil have been integral components of weed control since the 1980s. ETs, the point at which weed control operations provide economic returns over input costs, ranged between 3.7% and 5.8% relative weed coverage. Without weed control, no yield increase was found over 24 years. Yield increased by 0.2 t ha−1 year−1, if weeds were controlled chemically. Despite intensive use of effective herbicides in maize, problematic weed species abundance and yield losses due to weed competition have increased in Baden‐Württemberg over a period of 30 years.