Crops, weed interference

How weeds produce their effect is the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (1.) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to allelopathic effects. Some of these cover the effects of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy in weed interference in crop production in the Mid-South. A second objective of this review is to indicate where research is needed and to suggest potential lines of future research, especially with respect to the developing role of conservation tillage practices in this region. 

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. 

Uses Bromacil is a colorless crystalline solid. It is used for the control of annual and perennial grasses, broadleaf weeds, and woody plants.1213 Bromacil is a herbicide used for bush weed control on non-cropland areas. It is especially useful against perennial grasses. It is also used for selective weed control in pineapple and citrus crops. It interferes with photosynthesis of plants. It is available in granular, liquid, water-soluble liquid, and wettable powder formulations. 

Peas do not compete well with weeds and efficient weed control is essential to avoid yield loss. Pereimial weeds should be dealt with before planting and thistles, oilseed rape and groundkeeper potatoes can be a particular problem. Nettles and thistles can interfere with hand picking and, in machine-harvested crops, weeds can lead to crop rejection from the processor. Inter-row cultivations may be effective when the peas are between the second and fifth node growth stage. 

Table I provides general information on the identified allelopathic weeds and the crops they affect. More detailed information on two of these weeds, johnsongrass and purple nutsedge, will be presented to show the tenuous nature of the evidence for allelopathy. These two species are undoubtedly allelopathic, at least under certain conditions. Rigorous proof that allelopathy is the agent responsible for even a specific portion of the interference exerted by them is not easily attained, however, even though these are the weeds with the best research data available of those weeds that occur in the Mid-South.

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. 

Parthenium hysterophorus L. (parthenium weed), native to North and Central America and introduced into Queensland as recently as I960 (39), is a contemporary example. It is aggressive, persistent and lowers crop yield through interference, a component of which is allelopathic ( ). 

Field studies do not demonstrate conclusive allelopathic inhibition of weeds by wild mustard or broccoli, but there are some indications of allelopathic interference. First, the main weeds in the first weeding were crucifers in all treatments, but not in the following weedings. They were stimulated to germinate only at that time. Second, broccoli production was affected by B. campestris yields were increased during the summer. Earlier planting of mustard in the fall inhibited broccoli yields, but had no effect when mustard was planted at the same time broccoli was transplanted. In addition, stimulatory effects of crucifers on other crucifers or other crops has been observed before (10, 16, 41). 

The remainder of this discussion examines the possibility of a direct feedback mechanism in response to light as an explanation for crop yield reduction from early weed presence. Three types of data will be examined 1) results of our research on velvetleaf interference with light in soybeans 2) a comparison of observed and estimated soybean yield reductions for weed presence versus leaf removal and 3) the poor correlation between weed control and crop yields. 

We should determine which plant pests inflict their damage through production of phytotoxins. It now appears that several pathogenic fungi may do this. Numerous weed species may impose interference on crop growth, at least in part through allelochemicals. More than 70 species have now been alleged to have allelopathic potential. 

An obvious place for intensive work on allelopathy is in the weed science area. Here, plant interference is either our problem or our opportunity. We should be clever enough to exploit allelopathy as a weed-suppression strategy. This could be accomplished with crops that release allelochemicals through exudation or by crop residues placed into sequential cropping systems. My research team and others have already developed some promising leads in this area. 

The N-arylalanine ester herbicides such as benzoylprop-ethyl and flam-prop-methyl and difenzoquat (Figure 2.26) prevent cell elongation in certain grass weeds, allowing the crop to overtop them. The weeds are thus outcompeted and die. The exact mode of action is not certain but it is proposed that these compounds interfere with the site of action of the auxins. 

Jordan, N. (1993). Prospects for weed control through crop interference. Ecol. Appl., 3 84—91. 

Cover Crops Ideal cover crops should suppress weeds and provide little interference with the citrus crop. Jones and Embleton (1967) recommended using legumes in young citrus orchards before weeds become thoroughly established. However, in mature orchards, other alternatives such as mustard species are used as cover crops in citrus orchards. 

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