Analysis of competition between winter wheat and annual weed species

In 1995 to 1997 the development of weed dry weight and the so-called crop e quivalent were studied in natural weed infestation to assess competitive ef fects of annual weed species. The crop equivalent is the ratio between weed dry weight per plant and winter wheat dry weight per plant. The aim was to find reliable conversion factors, which allow transformation mixed weed in festation into weed units. This approach is necessary to predict the econom ic benefit from weed management and to determine economic threshold values. In early spring the competitive ability of most weeds against winter wheat is low. The crop equivalents range in this period from 0.1 to 0.3. As the most competitive species in early spring were discovered Galium aparine and Stellaria media. The most important period with regard to weed competition is the time after wheat heading when weight of some weeds increases to a l evel close to wheat dry weight per plant or exceeds it. In the second half of the growing seasons, there is also considerable differentiation between individual weed species. A weed species of high competitiveness in second h alf of the growing season are Galium aparine and Papaver rhoeas. Values of crop equivalents could be related to state and competitiveness of winter wh eat as were found out in different crop rotation. To recalculate weed infes tation in mixed populations into weed units are most reliable crop equivale nts from period after wheat heading. The relationship between weed units an d yield has linear course and is in most cases highly significant.