Arndt, Wellmann; Walter, R.,Schäufele; Bernward, Märländer
A two-year field trial in Göttingen investigated various parameters for their suitability for forecasting the harmful effects of infestations of Chenopodium album and Chamomilla recutita at different times, leading to findings of the economic damage thresholds. Owing to the elastic response of the weeds to an increasing development lead of the sugar beets, the yield losses of the latter were primarily deter-mined by the density and time of appearance of the weeds. For this reason, a forecast of the harmful effects based solely on the weed density is not possible in sugar beets. However, the combination of weed density and the stage of sugar beet development, in which the weeds sprout, offers the possibility of forecasting sugar beet yield losses.
The degree of weed coverage and the relative degree of weed cover-age adequately reflected at a given evaluation point the effect of the time of appearance on the occupancy-loss relationships of C. album or C. recutita when the plants had sprouted between the two-leaf and nine-leaf stages. However, when the weeds were powerfully sup-pressed by the beets, as in the case of C. album sprouted in the 10-13- leaf stage, the loss relationship clearly diverged. The degree of weed coverage was not suitable as the sole parameter for forecasting the harmful effects because the occupancy-loss relationships depended very much on the time of evaluation. However, the differences in the occupancy-loss relationships at different evaluation dates were sub-stantially reduced in C. album, and almost completely eliminated in C. recutita, by resort to the relative degree of weed coverage. How-ever, application of the latter for forecasting the harm to sugar beets is probably useful only in the case of uncompetitive weeds and with herbicides which allow later deployment in respect of the weed de-velopment stage than those currently available.
Although the technical quality of sugar beets was affected in part by weed competition, this aspect may be disregarded in loss forecasting
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