Crop yield reduction allelopathy

The exact nature of weed interference between crops and weeds is still inadequately understood. It has been assumed that direct crop yield reductions from weed presence were the result of competition, of allelopathy, or of these two acting together. Further, many attempts have been made to explain the extent of crop yield reduction in terms of weed thresholds (numbers). Information is presented that suggests that neither numbers of weeds nor their influence through competition and allelopathy adequately explain the effects of weeds on crop yield. A third influence of weeds, what may be termed "direct feedback response to light," is introduced as a possible factor in yield reduction. 

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. 

Allelopathy is the reiaalning direct factor currently used to explain crop yield reductions. For the purpose of placing allelopathy in context with competition in understanding the cause(s) of crop yield reduction from weed presence, only allelopathy associated with weeds present in the crop will be considered. For such weeds, at least in humid temperate regions, allelochemicals contained in weeds can be assumed to enter the crop s environment by exudation, leaching, decomposition or some combination of entry modes. 

On the basis of results to date, it is concluded, as shown in Figure 7, that allelopathy could be a source of crop yield reduction throughout the time crop yields are reduced by weed presence, but it is most apt to be a factor from the midpoint of the curve on. Thus, the contribution of allelopathy to crop yield reduction likely is less than that of competition for light, water, and nitrogen. The reduction in crop yield could be the result of inhibition in growth, inhibition in reproduction, or some combination of the two. 

Figure 7. Crop Response to Weed Presence. Closeness of the vertical lines indicates the relative possibility of allelopathy being a factor in crop yield reduction.

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