Summary To assess the effects of timing and frequency of weed harrowing on weed abundance and crop yield, different pre‐ and post‐emergence weed harrowing sequences were applied to spring cereals and peas in field experiments performed during 2003 and 2004 in Sweden. Post‐emergence harrowing was performed at crop growth stages 2–3 and 5–6 true leaves respectively. The best weed control was obtained by a combination of pre‐ and post‐emergence harrowing, but these treatments also caused yield losses of 12–14% in spring cereals, while no yield losses were observed in peas. Pre‐emergence weed harrowing treatments alone or combined with weed harrowing shortly after crop emergence proved to be most effective against the early emerging annual weed species Sinapis arvensis and Galeopsis spp. Post‐emergence harrowing alone in peas had no effect on S. arvensis. The late emerging annual weed species Chenopodium album and Polygonum lapathifolium were most effectively controlled when pre‐emergence weed harrowing was combined with one or two weed harrowing treatments after crop emergence.

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To assess the effects of initial root size of Sonchus arvensis on sprouting and shoot development, an outdoor box experiment was performed in Sweden in 2008. Shoot emergence time, shoot numbers, rosette size and flower production were quantified as functions of root length and weight. Emergence of the first shoot per root and of later cohorts was delayed with decreasing root length and weight. Number of shoots per root increased with root length and weight, but per unit root length and weight, short roots produced more shoots. The first emerging rosettes were, for rosettes of a given age, larger for longer roots. Total rosette area per root 5 weeks after planting increased with increasing root length and weight. The number of flowers and production of mature seeds were positively related to root length and weight, because of delayed sprouting of short and light roots. The proportion of flowers leading to mature seeds declined with shoot emergence time. By clarifying relationships between root size and growth parameters, this study showed that fragmenting of S. arvensis roots delays phenological development and hampers reproduction by seeds. The information may be used to refine mechanical weed control strategies for S. arvensis.

Summary The influence of weather on the efficacy of dichlorprop‐P/MCPA and tribenuron‐methyl on annual weeds in spritig barley was studied in the field during 4 years at six locations in southern Sweden. The herbicides were applied at one‐eighth to three‐quarters of the full dose at three application times with approximately 6‐day intervals. Weather stations, placed in the experiments, recorded climatic data, Maximum herbicide efficacy was obtained when spraying 1 week after the cotyledon stage with half or three‐quarters of the recommended dose. Dose‐response curves were estimated and the ED80 doses were calculated. On average, the herbicide dose required to obtain an effect of 80% was about 40% of the recommended dose. The influence of weather was analysed for seven different periods: 7 and 2 days before and after herbicide application, 1 day before and after application, and the day of herbicide treatment. The most pronounced eftects of weather were found for the day of and the day before herbicide application, revealing the strong influence of weather on herbicide uptake and plant metabolism. High air tetnperature and low global radiation during the day of treattnent reduced the ED80 dose of dichlorprop‐P/MCPA, whereas the ED80 dose of tribenuron‐methyl increased. For both herbicides, precipitation and high soil temperature increased the ED80 dose, which reflects the importance of rain on herbicide uptake and the effect of soil temperature on plant growth. The R2 values were higher in the analyses of dichlorprop‐P/MCPA than of tribenuron‐methyl, indicating that the effect of dichlorprop‐P/MCA was more weather‐dependent than that of tribenuron‐methyl. Separate analyses of the ED80 doses for Brassica napus L., Chenopodium, album L. and Stellaria media (L.) Vill. generally resulted in increased R2 values. but otherwise gave results similar to those for the total weed population. Although the analyses revealed significant effects of indivtdual weather factors on herbicide efficacy, it was not possible to discern a consistent and causal relationship between weather and herbicide performance.