Crop yield reduction

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. 

The combination of water usage, continual water loss, and potential for a soil depletion zone extending beyond the roots suggests that competition for water might occur earlier in the crop life cycle than for any other soil-supplied growth factor. Competition for water may account for a major part of the crop yield reduction from weeds. Realistically, it seems unlikely that competition for water explains the earliest observed reduction in crop yield. 

Figure 2. Crop Response to Weed Presence, left, and Relative Phosphorus Uptake, right. Closeness of the vertical lines indicates the relative possibility of competition for P being a factor in crop yield reduction.

Therefore, as suggested by the vertical lines beginning at week 6, competition for N may occur earlier than for P and K but somewhat later than for water. Thus, competition for N may account for a significant part of the total crop yield reduction from weeds but likely accounts for little of the earliest observed crop yield reduction in nonlegumlnous crops. For the purpose of this discussion, it is assumed competition for nitrogen does not occur with leguminous crops. 

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. 

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. 

The suggested overall chronology explaining crop yield reduction from weed presence in soybeans thus is summarized as follows ... 

Conservation agroecosystems developed in the Great Plains of the U.S. to control soil erosion are characterized by the presence of varying quantities of plant residues on the soil surface. This residue mulch protects the soil from the erosive forces of wind and water, resulting in improved stream water quality and soil conservation. Conservation tillage systems also help maintain soil productivity and reduce energy requirements of crop production (15). However, crop yield reduction has been observed with conservation wheat production in some areas of the U.S. (16-18) and with rice culture in the Far East (, 20). 

Enhanced biodegradation Is the accelerated rate of pesticide degradation, by adapted soli microorganisms, observed In soil following previous application of It or a similar pesticide. Enhanced degradation may result In pest control failure and crop yield reduction."... 

Africa, currently much less densely populated than Europe or South and East Asia, will become the region of the world with fastest population growth in the twenty-first century. Climate change is expected to cause forest fires, crop yield reduction, and increased water shortages and drought. 

The nonvisual or subtle effects of air pollutants involve reduced plant growth and alteration of physiological and biochemical processes, as well as changes in the reproductive cycle. Reduction in crop yield can occur without the presence of visible symptoms. This type of injury is often related to low-level, long-term chronic exposure to air pollution. Studies have shown that field plantings exposed to filtered and unfiltered ambient air have produced different yields when no visible symptoms were present (5). Reduction in total biomass can lead to economic loss for forage crops or hay. 

It has been estimated that of the total U.S. increase in farm output between 1940 and 1955, 43 percent is attributable to increased crop yields per hectare, 27 percent to increases in value added by livestock production, 23 percent to reduction in farm-produced power, and 7 percent to changes in the amount of capital used. While it is not possible to isolate the effect of a single input, it is estimated that increased use of fertilizer accounted for more... 

This analysis has demonstrated that pesticide use in the world could be reduced by approximately 50% without any reduction in crop yields (in some cases increased yields) or the food supply. This effort would require applying pesticides only-when-necessary plus using various combinations of the nonchemical control alternatives currently available (34). Although food production costs might Increase slightly (0.5% to 1%), the added costs would be more than offset by the positive benefits to public health and the environment (15). 

Irrigation has been practiced from the ancient time it is only in the twentieth century that the importance of the irrigation water quality was recognized [7]. The use of saline water may result in the reduction of crop yields. A high sodicity of water for irrigation may cause the deterioration in the physical properties of soils... 

In spite of these difficulties the available data on crop performance (see Table 1) shows that the external concentration at which yield reduction becomes critical varies according to the crop. The approach to improving the salt resistance depends upon the external concentration at which improvement is sought. 

A weed, which is a form of a natural resource, is simply a living organism whose presence conflicts with the interests of people. Undesirable as they may be, weed species are natural occurrences in the agricultural ecosystems created by man s food and fiber production. Over 300,000 species of plants inhabit the earth, but only 30,000 of these are weeds. About 1,800 weed species cause serious economic losses in crop production, and about 300 weed species are serious in cultivated crops throughout the world. Most cultivated crops are plagued by 10 to 30 weed species that must be controlled to avoid yield reductions (1). 

Early work with sulfur dioxide showed a linear relationship between visible injury and reduction in yield for many crop species. The assessment was made that no reduction in yield would be found unless visible injury were noted. Definitive research with ozone, other oxidants, or mixtures of these pollutants with other gases has not been done. Thus, we do not know whether such relationships between visible injury and yield hold for the oxidants, but data in Table 11-3 suggest that for acute exposures there may be good correlations between injury and yield reductions. Many researchers have hypothesized that the oxidants may have an effect on plants that will produce a yield reduction with little or no visible injury. Such studies need to be designed in a more defmitive manner before it is concluded that yield reductions without visible symptoms are clearly acceptable. Projections of yield losses have made use of some of the data reported earlier. ... 

Oshima has developed a methodology for evaluating and reporting economic crop losses that involves continuous air monitoring, chamber exposures, and monitoring plant species. He has attempted to weld these into a comprehensive method of determining yield reductions. He has reported yield reductions for some species, but has not folly clarified the procedure. He presented no economic values in either report. This type of approach needs to be considered further. 

---