The effects of density and the timing of removal on interference between barley, white mustard and wild oats.
The results are reported of glasshouse trials in which the interspecific effects of barley, white mustard (Sinapis alba) and wild oats (Avena fatua) grown in pots were studied. In mixtures of the three species wild oats reduced the growth of barley more than did mustard. In two-species mixtures the yield of barley was depressed by wild oats and by high densities of mustard. The highest barley yields were obtained by complete pre-emergence removal of associated species. Partial removal of mustard appeared to be advantageous to barley, though only if the removal was at the pre-em. stage. On the other hand partial control of wild oats was almost worthless. In a three-species mixture barley benefited from the removal of wild oats before emergence but the removal of wild oats after emergence or the removal of mustard at any stage did not appear to be particularly advantageous. Barley was more damaging than wild oats in a stand of mustard and was more damaging than mustard in a stand of wild oats.In a discussion of the results, it is concluded that the effects of weeds on a crop are not strictly additive, increases in the density of a weed population producing a stress which is shared with the crop and among the weeds themselves. In the case of weed mixtures, the associated species can have a lesser effect than might be predicted by calculating the sum of the activity of each species on its own.