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Dr. Arnold P. Appleby Department of Agronomic Crop Science Oregon State University


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Arnold P. Appleby

Department of Agronomic Crop Science Oregon State University


In speaking to a sophisticated audience of weed workers such as this, it nrust seem simplistic in the extreme to use a place on the program to tell you that weeds reduce yield and quality of our farm products. Indeed, I considered the idea of merely making the announcement that they do indeed reduce yield and quality an~ we could all have a long coffee break. But on further reflection, I believe there are some important ramifications that may be well for us to examine in more detail. I am hopeful that we can look at some of the ways in which weeds affect crops and perhaps somewhat set the stage for the following speakers.

No one can argue against the fact that weeds interfere with crop growth. The total cost of weeds to crops in the world is unknown but no one disagrees that it is enormous. Perhaps the most conservative estimate of food losses in recent years has been made by Parker and Fryer (8) who readily admit that their figures are only rough approximations. They have estimated a total food loss per year of 287,500,000 metric tons. The extent of interference, even in a particular site, varies from near zero to nearly 100% of the crop, depending on a wide range of factors. This extreme variability from one field to the next, from one year to the next, from varying cultural practices, etc., adds to the difficulty of making any firm estimates of weed losseso

I have selected some examples, almost at random of data illus-trating loss of crop yields due to weeds throughout the world. In two locations in flooded rice in the Philippines (Table 1).

Table 1. Weed Control in Flooded Rice, Philippinesa Yield (ton/ha) Best herbicide treatment

Hand-weeded twice Check Loe. 1 Loe. 2 7.8 908 6.9 9.5 0 8.1

~ata from Internat. Rice Res. Instit., 1974 Ann. Rpt., p. 226. Trials conducted in farmers' fields.

Yield losses were dramatic at one location at which weeds elimina-ted rice in the untreaelimina-ted plots completely. At a similar location, with a different density of weeds, differences between weeded and unweeded plots were nruch less dramatic. In upland rice, the potential yield of the rice was considerably lower (Table 2), but the stand of weeds in the untreated plots still eliminated rice production.


Table 2. 2. Weed Control in Upland Rice, Philippinesa Best herbicide treatment

Hand-weeded twice Check Yield (ton/ha) 3.2 2.6 0 8vata from Upland Rice, Internat. Rice Res. Instit.,

P• 175. 1975.

In some of our work at Oregon State University in winter wheat in western Oregon, losses of wheat yields due to Italian ryegrass and wild oats have been enormous (Table 3).

In an average of 12 experiments, yields were reduced below the best herbicide treatment by an average of 46 bu/A, representing a loss in gross income of $138/A, assuming $3/bu for wheat.

Table 3. Grass Control in Winter Wheat, Western Oregon Wheat Grain Yield

Loe. 5 Loe. 9 Avg of 12a kg/ha bu/A kg/ha bulA kg/ha bu/A Best treatment 6500 97 9000 134 6500 97

Check 800 12 2800 42 3450 51

aAvg of 12 experiments conducted in 1974-75 and 1975-76, Oregon State University.

Further, the average yield from the best herbicide treatment, not yet available commercially, exceeded the yield of the most commonly used herbicide treatment by more than 32 bu/Ao This is perhaps another example of the fact that there are still many unsolved weed problems in our so-called highly developed agricultural systems.

In a survey made of wheat production in 13 countries in North Africa and the Middle East (Table 4), losses were estimated to exceed

5,000,000 tons.

Table 4. Wheat Yield Loss from Weeds in 13 North African and Middle East Countriesa

Wheat production

Estim. loss from weeds



Tons of Wheat 26,600,000

5,130,000 19.4%


This is not a plea to use more herbicides. In some crops, herbi-cide treatments are not always the optinrum for highest yields. In an experiment in cassava in Colombia (Table 5), hand-weeding operations gave higher yields than herbicide treatments but both of them outyielded unweeded plots by a very wide margin.

Table 5. Weed Control in Cassava, Colombiaa

Best herbicide treatment (no hand labor) Hand-weeded 3 times


aData from 1974 Ann. Rept. CIAT, p. 97.

Yield (ton/ha) 21.0

31.1 2.8

Not all of the injury to crop yield is from direct interference. In studies in Nigeria (Table 6), Moody and Whitney found that insect damage to cowpeas was considerably greater in weedy areas than in weed-free areas. There are many similar examples for other insects and for plant diseases and vertebrates.

Table 6.

Weedy Weed-free

Influence of Weeds on Insect Damage to Cowpeas, Nigeria

% Seeds Damaged by Insects 64.2

41.5 From Moody & Whitney, unpublished data.

We could continue to quote similar examples from now until next Thursday but perhaps we have sufficiently satisfied everyone that weeds can indeed reduce crop yields (which we knew before we started this

dis-cussion in the first place). It may be helpful at this point to digress a bit and discuss some of the principles of how these reductions occur. The term "competition" refers to the struggle for limited resources among individuals within the same habitat. "Allelopathy" is the harmful effect of one plant on another through the production of chemical compounds that escape into the envirorunent (9). In our weed control classes in the past few years, we

have been following the suggestion of Muller (6) that the term "interference" be used to refer to the overall deleterious effects of one plant on another, thus encompassing both allelopathy and competition. I suspect that allelopathy is nruch more a factor in interference than is commonly recognized. One

reason for this is that it has been very difficult to separate in the field the effects of allelopathy and competition. Bell and Koeppe (2) have

developed a technique for studying allelopathy separately from competitive effects. Using a stairstep arrangement, they found that giant foxtail

could inhibit the growth of corn by approximately 35% through an allelopathic mechanism.


The factors making up competition are usually considered to be water, nutrients, and light with CO2 and space as possibly being factors in some situations. Although this is a familiar topic to all of you,

I sincerely believe that competition is surrounded



fuzzy thinking, and perhaps lack of attention to scientific principles. How many of you know, for a fact, that "weeds cause most of their damage early"? This is dogma in the weed science field. Probably every beginning crop science or horticultural student is taught this as a fact. Arguments are made against postemergence herbicides because of this axiom. But how true is it, really? In our grass control work in winter wheat in Oregon, we find that we suffer little or no yield loss, even under heavy grass infestations, if the weeds are well controlled by herbicides applied as late as January or February. In a careful study in eastern Oregon, Rydrych (10) found that in October-planted wheat, significant yield

re-ductions did not occur if downy brome was removed by March. Dawson carried out detailed competition studies in beans (4) and sugar beets (5) and found that beans tolerated 8 weeks of weed competition before yields were reduced and that weeds exerted little yield-reducing competition until later than 12 weeks after planting sugar beets. This is not to say that the statement "weeds cause most of their damage early" is false in all situations. Indeed, Nieto et al. (7) found that weeds allowed to remain in corn for as little as 20 days had already caused an irreversible loss in yield. Other examples are available. The point is that the dogma of early competition is a very poor generalization and should not be accepted in all situations. A much more legitimate generalization is that weeds cause their damage at various growth stages, depending _2!! which resources~ limiting. The beans and sugar beets mentioned above were furrow-irrigated with relatively high amounts of fertilizer. There simply was no limiting factor until the weeds got to the stage of competing with the crop for light. In other situations, competition for nutrients or moisture very early in the growth cycle of the crop can be devastating. I would urge that each si.tuat ion be evaluated on the basis of the limiting factors involved.

Likewise, I am skeptical of statements in the popular press such as "Two pigweeds per foot of soybean row reduce yields by 20%" or some such information. This may be a useful technique in making a point to the general reader but I question its scientific merit. This may be true only in a

particular locality under constant conditions of irrigation methods, weather, fertilizer rate, time of weed germination, etc., etc.

As in the case of yield losses, examP,les of reduction in quality of farm products are legion and could be quoted at great length. You all know of specific examples and we certainly will not attempt to belabor you with more than just a few. Wild garlic in our small grains, quackgrass or

johnsongrass rhizomes in nursery stock, poisonous jimsonweed seeds in soybeans, nightshade fruits in canning peas, and many others. Although reduction in quality is sometimes considered more of a nuisance than an economic factor, be assured that quality factors have a very large impact in many situations. The increasing stringency of federal regulations


regard-buyers to accept less than quality produce can result in losses of markets which can have a tremendous economic impact.

What are the ramifications of all of this? Perhaps the real ramifications differ between the developed countries and the developing countries in the world, the haves and the have-nots, so to speak. In the poorer nations, the effect of weeds on yield and quality of farm products may make the difference of whether someone eats or not. Even if enough food is produced to prevent malnutrition and starvation, weeds can still have a tremendous sociological effect. For example, in Nigeria, it has been estimated that weeding is the major energy bottleneck in grain-fed upland farming, absorbing some 50-70% of the total manpower utilized (1). This means that this manpower is not available for other, more productive uses.

In the developed countries of the world, on the other hand, lack of sufficient food production to feed the populace is not generally the question. The impact is primarily economic. In 1974, agricultural exports accounted for about 22% of the dollar value of all U.S. exports (3). The role of weeds, then, becomes one of the determining factors influencing

the amount of crops that are available to be exported, the balance of payments needed by this country, the economic health of growers, and the economic health of our society in general.

Literature Cited

1. Annual Report, 1975. Internat. Instit. Tropical Agric., Ibadan, Nigeria. p. 62.

2. Bell, D. T. and D. E. Koeppe. 1972. giant foxtail on the growth of corn.

Noncompetitive effects of Agron. J. 64:321-325.

3. Brown, H. 1976. Day of reckoning: Resilient poor vso vulnerable rich? RF Illustrated 3(2):1.

4. Dawson, J. Ho 1964. Competition between irrigated field beans and annual weeds. Weed Sci. 12:206-208.

5. Dawson, J. H. 1965. Competition between irrigated sugar beets and annual weeds. Weed Sci. 13:245-249.

6. Muller,


Ho 1969. Allelopathy as a factor in ecological process. Vegetatio 18:348-357.

7. Nieto, J., M.A. Brondo, and J. T. Gonzalez. 1968. Critical periods of the crop growth cycle for competition from weeds. PANS (C) 14:159-166.

8. Parker,


and J. D. Frayer. 1975. Weed

major reductions in world food supplies.


control problems causing

FAO Plant Protection Bull.


9. Rice, E. L. 1974. Allelopathy. Academic Press, Inc., New York. 10. Rydrych, D. J. 1974. Competition between winter wheat and


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