THE INFLUENCE OF INTERSPECIFIC INTERFERENCE ON THE SEED PRODUCTION OFSTELLARIA-MEDIA AND HORDEUM-VULGARE (VOLUNTEER BARLEY)

The effects of the density and proportion of both volunteer barley (Ho rdeum vulgare L.) and Stellaria media (L.) Vill. (common chickweed) on the seed yield of both species was investigated in linseed (Linum usi tatissimum L.) and autumn-sown field bean (Vicia faba L.). A model was created to estimate these effects. It was a combination of two models . The first was a simple linear model relating weed seed number m(-2) to weed dry weight m(-2). The second was an inverse linear model relat ing weed dry weight per plant to weed density. A very good relationshi p existed between barley dry weight and number of seeds S. media dry w eight and number of seed capsules m(-2). This relationship was relativ ely consistent between experiments. The inverse linear model provided a good estimation of the relationship between weed density and weed dr y weight per plant for both barley and S. media. Model variables, howe ver, differed between experiments. Using the combined model it was est imated that, in the absence of other weed species and at a density of 800 plants m(-2), S. media would produce between 4000 and 48 000 seeds m(-2), whereas volunteer barley, at a density of 400 plants m(-2), wo uld produce between 2000 and 8000 seeds m(-2). The presence of barley always reduced S. media seed yield and a barley population of 100 plan ts m(-2) could reduce S. media seed yield by up to 79%. The presence o f S. media reduced barley seed yield in only one of three experiments, in which a S. media density of 800 plants m(-2) reduced barley seed y ield by up to 68%. It was concluded that interspecific weed competitio n should not be ignored as a factor in models of weed population dynam ics.