Competition, weed-crop relationship

Much research is needed on the quantitative effects on crop yields of interference by most of our serious weeds, and on the relative contributions of allelopathy and competition to the total interference by each weed species. Crop-crop relationships need to be investigated much more thoroughly to determine which crops can follow others with the least inhibitory or most stimulatory effects. More emphasis should be placed on investigations of stimulatory allelopathlc effects, because these effects have been largely Ignored in the past. Possible autotoxicity should be investigated also to determine if it is unwise to cultivate the same crop continuously without rotation. 

Why weeds reduce crop yields cannot be adequately answered. Considerable data have accumulated which relate duration of weed presence and weed density to crop yield. However, such data provide little explanation for why crop yields are reduced. The objectives of this paper are to 1) provide an overview of the time relationship of competition for growth factors and of allelopathy as factors in crop yield reduction and 2) suggest a direct feedback effect on reproduction in response to light as a possible third direct factor in explaining effects of weeds on crop yield. 

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