Interference Between Crops and Weeds - ACS Publications

C h a p t e r 28

Interference Between Crops and Weeds Richard J. Aldrich

Allelochemicals: Role in Agriculture and Forestry Downloaded from pubs.acs.org by FUDAN UNIV on 04/21/17. For personal use only.

Agricultural Research Service, U.S. Department of Agriculture, and University of Missouri, Columbia, MO 65211

The exact nature of weed interference between crops and weeds is s t i l l 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 l i t t l e 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. It is commonly assumed that reductions in crop yield from weeds are the direct result of competition, of allelopathy, or of the two acting together. Competition between crops and weeds is generally for the growth factors available in the space occupied by these plants rather than for space itself, except in special situations such as in root crops. As applied to weed-crop relationships, competition implies the removal of an essential growth factor by neighboring plants. Theoretically, competition could occur for any of the growth factors—light, water, nutrients, oxygen, and carbon dioxide (1). Practically, environmental conditions commonly preclude competition for 0 and C0 . This leaves light, water, and ?

2

0097-6156/87/0330-0300$06.00/0 © 1987 American Chemical Society


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nutrients as the most l i k e l y sources of competition to explain reduction i n crop y i e l d due to weeds. Based on current knowledge, the explanation for lower crop y i e l d with weed presence i s a matter of identifying which of these competed-for growth factors and/or allelopathy accounts for the observed reduction i n crop y i e l d at any given time during the growing season. Figure 1 i s a generalized response curve which shows the effect of duration of weed presence on crop y i e l d . This genera l i z e d curve may be skewed to the l e f t or to the right depending upon the weed, the crop, growing conditions, and possible other factors, but the general relationship remains. Of p a r t i c u l a r i n t e r est i s the fact that there i s a period at the beginning of the crop's l i f e cycle when y i e l d i s not reduced by weed presence (2). What causes the i n i t i a l down-turn i n the y i e l d curve? Is i t due to competition for l i g h t , water, or nutrients? Is i t due to a l l e l o p athy? Is the cause(s) of the i n i t i a l drop the same as the cause(s) for the continued drop i n the midportion of the y i e l d curve? Is the cause(s) of y i e l d reduction toward the lower portion (later i n the growing season) of the y i e l d curve the same as that e a r l i e r i n the growing season? Available data do not allow precise, unequivocal answers. However, a comparison of usage patterns for specified growth factors with the weed presence—crop response curve provides a basis for broad generalizations explaining such crop y i e l d reduction. The generalizations which follow are not intended to imply d e f i n i t i v e answers to this extremely complex question so much as they are to provide a general perspective of "most l i k e l y explanations" and to i d e n t i f y areas i n which additional research w i l l l i k e l y be most h e l p f u l i n our search for answers. Further, the question of why weeds reduce crop yields i s approached from a broad, h o l i s t i c perspective. This suggests that conclusions from s p e c i f i c studies, especially of single factors, may well d i f f e r from these generalizations . Competition f o r Growth Factors Phosphorus. Let us f i r s t consider the nutrients. Figure 2 shows the usage of phosphorus by corn (3) and soybeans (4) superimposed on the crop response—weed presence curve. The usage of phosphorus at any point i s the percentage of the t o t a l taken up by the crop plants for the entire growing season. With both crops the f i r s t eight weeks are a time of r e l a t i v e l y minor usage—about 2 5% of the t o t a l . For competition to occur there must be an overlapping of the root depletion zones of neighboring weed and crop plants. Since phosphorus moves almost s o l e l y by d i f f u s i o n i n s o i l , the potential depletion zone w i l l be e s s e n t i a l l y no larger than the size of the root system (1). In effect this means there l i k e l y would need to be a co-mingling of weed and crop roots i n s u f f i c i e n t density to deplete the available phosphorus f o r competition to occur. Although this could occur i n the young plants early i n the growing season, i t i s more apt to take place l a t e r when the respective root systems are f u l l y elaborated. Furthermore, some of the phosphorus i n weeds destroyed early, either mechanically or by herbicides and l e f t on the s o i l surface, can be expected to be available to the crop that

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same year as a result of microbial decomposition of young weeds. Phosphorus l i k e l y would not be released as readily from older less succulent weeds. The narrowness of root depletion zones, minor r e l a t i v e early need by the crop, and release of some phosphorus i n tissues of young weeds suggest that competition, i f i t occurs, does so toward the end of the y i e l d curve as shown by the v e r t i c a l lines in Figure 2. The l i k e l i h o o d of competition occurring i s indicated by the closeness of the v e r t i c a l lines. Phosphorus competition, i f i t occurs, l i k e l y contributes only a small increment of the t o t a l crop y i e l d reduction attributable to weeds. Potassium. Figure 3 shows that only a small part, 7 to 8%, of the soybean crop's usage of potassium occurs i n the f i r s t four weeks (4_). At 8 to 10 weeks, usage s t i l l represents only 1/4 to 1/3 of t o t a l usage. Usage by corn occurs e a r l i e r i n the l i f e cycle than i n soybeans. More than 50% of the corn plants need f o r Κ w i l l have occurred by eight weeks after corn emergence (3). It can thus be theorized that competition for Κ could occur e a r l i e r i n the growing season with corn than with soybeans. Even with a crop such as corn, i t seems u n l i k e l y that competition f o r Κ could account for the y i e l d reduction from weeds present only during the early part of the crop's l i f e cycle. There are several reasons f o r suggesting that competition for Κ l i k e l y does not occur early i n the crop's l i f e cycle. As discussed e a r l i e r , f o r competition of a soil-supplied growth factor to occur, there must be an overlapping of depletion zones of neighboring plants. As i s true of Ρ, Κ moves i n s o i l mainly by d i f f u s i o n , which i s a slow process. Thus, the depletion zones of neighboring plants w i l l be narrow. Further, since Κ r e mains i n solution i n the plant and i s not locked into organic com pounds i n the weed tissues, Κ i n weed plants destroyed mechanically or by herbicides and l e f t on the s o i l surface should be quickly available to crop plants. As was suggested for P, competition for K, i f i t occurs, i s most apt to occur l a t e r as depicted by the v e r t i c a l lines i n Figure 3 and accounts for only a small part of the reduction due to weeds. Water. As shown i n Figure 4, usage of water by soybeans increases steadily to a maximum 12 to 14 weeks after emergence i n Missouri. Water uptake i s shown as a percent of the maximum quantity needed and i s adapted from data on s o i l water depletion (5). Water d i f f e r s from other soil-supplied growth factors i n that i t i s continually being lost by evaporation. Any water lost by evaporation or plant use at any point i n the growing season i s permanently lost. Evapo ration may result i n water movement over considerable distances to plant roots i n s o i l . The depletion zone f o r water may therefore extend well beyond the plant roots. The combination of water usage, continual water loss, and potential f o r a s o i l depletion zone extending beyond the roots suggests that competition for water might occur e a r l i e r i n the crop l i f e cycle than f o r any other soil-supplied growth factor. Competi tion for water may account f o r a major part of the crop y i e l d reduc tion from weeds. R e a l i s t i c a l l y , i t seems unlikely that competition for water explains the e a r l i e s t observed reduction i n crop y i e l d . At least i n humid areas, the l i k e l i h o o d of a r e l a t i v e l y f u l l s o i l

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Interference Between

303

Crops and Weeds

0

4

8

12

16


Weeds Present (Weeks)

Figure 1. Generalized Crop Yield Response to Duration of Weed Presence.

0

4

8

12

16


Figure 2. Crop Response to Weed Presence, l e f t , and Relative Phosphorus Uptake, right. Closeness of the v e r t i c a l lines i n d i cates the r e l a t i v e p o s s i b i l i t y of competition for Ρ being a factor i n crop y i e l d reduction.

0

4

8

12

16


Figure 3. Crop Response to Weed Presence, l e f t , and Relative Potassium Uptake, right. Closeness of the v e r t i c a l lines i n d i cates the r e l a t i v e p o s s i b i l i t y of competition for Κ being a factor i n crop y i e l d reduction.

304


water charge at crop planting and the extension of roots with moisture drawdown i n the s o i l suggest that s o i l water should be adequate f o r both weeds and crops u n t i l some time l a t e r , possibly u n t i l about week 6 on the generalized crop yield—weed presence response curve. Nitrogen. Total Ν used by a corn crop i s r e l a t i v e l y small during the f i r s t s i x to seven weeks (3_) but increases rapidly thereafter as the corn plant approaches tasseling (Figure 5). Applied N, and that present i n s o i l , may be i n many different forms. Some forms are r e a d i l y soluble and others are not. Sooner or l a t e r the organic and insoluble forms undergo n i t r i f i c a t i o n to soluble inorganic forms i n s o i l . Water-soluble N, unlike Ρ and K, i s f r e e l y mobile i n s o i l . Thus, the depletion zone of Ν can extend well beyond the plant s roots i n s o i l . Therefore, as suggested by the v e r t i c a l lines beginning at week 6, competition for Ν may occur e a r l i e r than for Ρ and Κ but somewhat l a t e r than for water. Thus, competition for Ν may account f o r a s i g n i f i c a n t part of the t o t a l crop y i e l d reduction from weeds but l i k e l y accounts f o r l i t t l e of the e a r l i e s t observed crop y i e l d reduction i n nonleguminous crops. For the purpose of this discus sion, i t i s assumed competition for nitrogen does not occur with leguminous crops.


T

Light. Light i s the other growth factor for which weeds and crops may compete. With respect to competition, l i g h t i s very different from the other growth factors because i t i s nontransferable i n the plant whereas the other growth factors are r e l a t i v e l y mobile. Therefore, anytime a single leaf or an entire plant i s shaded, i t i n effect suffers competition for l i g h t . Such competition during the f i r s t few weeks of the crop's l i f e cycle may not necessarily result in reduced crop y i e l d because most crop plants w i l l compensate for some early loss i n t o t a l photosynthesizing area. Although as sug gested i n Figure 6, competition for l i g h t could occur throughout the time weed presence reduces crop y i e l d , i t seems unlikely that compe t i t i o n for l i g h t could by i t s e l f account for the e a r l i e s t observed reduction i n crop y i e l d . However, competition for l i g h t may account for much of the remaining reduction i n crop y i e l d due to weeds. Allelopathy Allelopathy i s the remaining direct factor currently used to explain crop y i e l d reductions. For the purpose of placing allelopathy i n context with competition i n understanding the cause(s) of crop y i e l d reduction from weed presence, only allelopathy associated with weeds present i n the crop w i l l be considered. For such weeds, at least i n humid temperate regions, allelochemicals contained i n weeds can be assumed to enter the crop's environment by exudation, leaching, decomposition or some combination of entry modes. Evidence to date i s c o n f l i c t i n g r e l a t i v e to when allelopathy may be a factor i n crop response to weed presence. In closed-system controlled studies, some results show seedling weeds to be a l l e l o pathic towards seedling crop plants (6) and some results show them not to be (7).

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0

4

8

12

16



Figure 4. Crop Response to Weed Presence, l e f t , and Relative Water Uptake by Soybeans, right. Closeness of the v e r t i c a l lines indicates the r e l a t i v e p o s s i b i l i t y of competition for water being a factor i n crop y i e l d reduction.

Figure 5. Crop Response to Weed Presence, l e f t , and Relative Nitrogen Uptake by Corn, right. Closeness of the v e r t i c a l lines indicates the r e l a t i v e p o s s i b i l i t y of competition for Ν being a factor i n crop y i e l d reduction.

Figure 6. Crop Response to Weed Presence. Closeness of the v e r t i c a l lines indicates the r e l a t i v e p o s s i b i l i t y of competition for l i g h t being a factor i n crop y i e l d reduction.

305

306


It has also been suggested by some researchers (8) that a l l e l o chemicals produced by velvetleaf which emerged with soybeans reduced soybean branching. Since branching i s i n i t i a t e d r e l a t i v e l y early (stage - V or 3 to 4 weeks after emergence), this provides c i r cumstantial evidence for release of the allelochemicals by young velvetleaf plants. However, since the branches were counted at midseason (10 to 12 weeks after emergence) and velvetleaf was present to soybean maturity, the p o s s i b i l i t y of reduced branching i n response to interference for l i g h t cannot be ruled out. On the basis of results to date, i t i s concluded, as shown i n Figure 7, that allelopathy could be a source of crop y i e l d reduction throughout the time crop yields are reduced by weed presence, but i t i s most apt to be a factor from the midpoint of the curve on. Thus, the contribution of allelopathy to crop y i e l d reduction l i k e l y i s less than that of competition for l i g h t , water, and nitrogen. The reduction i n crop y i e l d could be the result of i n h i b i t i o n i n growth, i n h i b i t i o n i n reproduction, or some combination of the two.


5

Direct Feedback Response to Light The remainder of this discussion examines the p o s s i b i l i t y of a direct feedback mechanism i n response to l i g h t as an explanation for crop y i e l d reduction from early weed presence. Three types of data w i l l be examined: 1) results of our research on velvetleaf i n t e r ference with l i g h t i n soybeans; 2) a comparison of observed and estimated soybean y i e l d reductions for weed presence versus leaf removal; and 3) the poor correlation between weed control and crop yields. Velvetleaf interference i n soybeans. In 1984, a f i e l d study was conducted on the Agronomy farm near Columbia, Missouri to examine the effect of velvetleaf height and duration on soybean yields. Other weeds were kept out for the entire test period. Williams 82 soybeans were d r i l l e d i n 10-inch rows May 24 and overseeded with velvetleaf the same day. The area was f e r t i l i z e d to excess prior to planting and i r r i g a t e d regularly throughout the growing season to eliminate these as factors i n soybean y i e l d . Velvetleaf c l i p p i n g to 75, 100, or 125% of the soybean height was begun the week of soybean emergence and continued for the prescribed 3 and 6 weeks. Since velvetleaf i n the 125% height treatment did not a t t a i n this height by 3 weeks, only the 6-weeks data w i l l be presented. As can be seen i n Figure 8, keeping velvetleaf 25% below the top of the soybean canopy for the f i r s t s i x weeks resulted i n yields only s l i g h t l y (7%) below yields of soybeans kept free of velvetleaf for the entire season. Maintaining velvetleaf at the same height as soybeans for s i x weeks resulted i n a 16% reduction and overtopping velvetleaf a 64% reduction i n soybean yields compared to weed-free status. Since i t i s assumed nutrient and water needs were f u l l y met, this leaves allelopathy and l i g h t as possible factors to explain y i e l d results. A comparison of soybean yields and velvetleaf weights at 6 weeks for 100% and 125% heights supports the conclusion that a direct effect of shading the top portion of the soybean plant, and not allelopathy, i s responsible for the observed y i e l d reduction. The dry weight of velvetleaf at 6 weeks was nearly


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307

i d e n t i c a l for the 100% and 125% heights (99.0 and 100.00 g/m , respectively). Yet, as we saw i n Figure 8, soybean yields where velvetleaf overtopped (125% height) were less than half those where weeds were at the same height as soybeans (100% height). Weight of velvetleaf at 6 weeks under the 100% height was not less than weight under the 125% height apparently because of increased l a t e r a l branching and growth i n response to removal of some terminals on the main stem. There i s evidence that an a l l e l o p a t h i c effect i s d i r e c t l y related to the biomass of the a l l e l o p a t h i c species (9, 10, 11, 12) and a l i g h t effect to the r e l a t i v e heights of the involved species (1). Since the velvetleaf biomass was the same f o r the 100% and 125% heights, this suggests that a difference i n l i g h t i n t e r ference between the two velvetleaf heights was responsible for the lower y i e l d under the 125% height. In addition to lower t o t a l y i e l d , branches per soybean plant and number of pods per plant were less under the 125% height. As previously mentioned, other research (8) has shown allelopathy to be involved i n reductions i n soybean yields by velvetleaf. Since our research suggests that reductions i n soybean yields from early velvetleaf presence are l i k e l y due to a direct effect of l i g h t interference, i t i s theorized that the observed a l l e l o p a t h i c effect reported by Dekker and Meggitt (8) occurs l a t e r and i s l i k e l y the result of o v e r a l l i n h i b i t i o n of growth, not a direct effect on reproduction. Overall i n h i b i t i o n of growth could be expected to increase coincidentally with an increase i n velvetleaf biomass associated with continued presence of velvetleaf (refer to the closeness of v e r t i c a l lines i n Figure 7 for a diagrammatic representation). Weed presence versus leaf removal. A comparison of observed and expected reductions i n soybean yields f o r periods of weed presence and leaf removal provides additional circumstantial evidence f o r a direct effect of l i g h t interference. Table I shows several such comparisons. The observed y i e l d i s that reported f o r a given weed present i n soybeans for the indicated period follow Lag soybean emergence. The estimated y i e l d i s from the National Crop Insurance Association charts showing reduction for given degrees of plant damage at d i f f e r e n t stages of soybean plant development. The soybean stage of development i s based on the average number of days required f o r development (13); an adjustment was made f o r determinant growth-type v a r i e t i e s . In every case, the observed y i e l d reduction exceeded that estimated for 50% leaf removal. For venice mallow (Hibiscus trionum) and velvetleaf, observed reductions exceeded that estimated for 100% leaf removal for the same period. F i f t y percent leaf shading i s suggested as an arbitrary degree of leaf shading one might expect with a dense stand of the weeds i n Table I by 4 to 6 weeks after emergence. I f anything, this l i k e l y i s on the high side. For example, Oliver (14) found that a velvetleaf stand of 32,000 plants per hectare had less leaf area than soybeans 8 weeks after emergence for either early- or late-planted soybeans. Theoretically, the crop can suffer competition for l i g h t any time weeds shade i t s leaves. However, i f the effect i s that of simple competition for l i g h t , e.g., a reduction i n photosynthesis, one might expect the observed y i e l d to be somewhat comparable to that for 50% leaf removal. The observed reduction i s consistently


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A L L E L O C H E M I C A L S : ROLE IN A G R I C U L T U R E AND FORESTRY

Figure 7. Crop Response to Weed Presence. Closeness of the v e r t i c a l lines indicates the r e l a t i v e p o s s i b i l i t y of allelopathy being a factor i n crop y i e l d reduction.

Figure 8. Effects on Soybean Yields of Three Velvetleaf Heights Maintained for the F i r s t Six Weeks after Soybean Emergence Compared to No Height Control ( l e f t bar) and Weed Removal for the Entire Season (right bar). Velvetleaf height (75%, 100% and 125%) i s r e l a t i v e to soybean height.

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Table I. Comparison of Observed Soybean Yield Loss from Weed Presence with Estimated Loss for Leaf Removal for the Same Period Soybean Y i e l d Reduction


Weed

Venice mallow Venice mallow Sicklepod T a l l morningglory T a l l morningglory Velvetleaf

Weeks Present

5 6 4 4 6 4

Soybean Stage

7" « V -V V^ V^ 8" 9 V ~V

V

v

g

V

~/ 39ψ u i ' 6j-' ~/ 341/

2 2

9

V

4

Observed

1 5

5

1

Estimated -^ 50% leaf 100% leaf removal removal (%) 6

1 7

6-7 5 5 6 - 7

4-5

17-20 15 15 17-20 21-22

—'From National Crop Insurance Association "Soybean Loss Instructions," NCIA Publication 6302, Revised in 1979. —^From Eaton, et a l . (15) ^From Thurlow and Buchanan (16) -^From Oliver, et a l . (17) — From Aldrich unreported data.

greater; by as much as 7 times i n the case of velvetleaf. If the shaded 50% of the leaves serve as a sink for photosynthate produced by the remaining 50%, i t could be theorized that the shaded leaves are acquiring photosynthate needed for branch and pod production and i n this way are reducing y i e l d . However, i t has been shown that a mature soybean t r i f o l i o l a t e i s an inconsequential sink for photosyn thate produced i n other t r i f o l i o l a t e s (18). Thus, i t i s suggested that the reduction i n soybean yields from early weed presence i s not due to either allelopathy or simple competition for l i g h t . Rather, i t may be due to a direct effect manifested through fewer branches and pods per soybean plant. Studies are underway to determine the effect on branching of shading s p e c i f i c t r i f o l i o l a t e leaves and combinations of t r i f o l i o l a t e leaves on the soybean plant. Weed control and crop y i e l d . One other type of circumstantial e v i dence emphasizes the importance of l i g h t i n weed interference and suggests that the effect of shading may not be simple competition for l i g h t . If the effect of weeds was due to simple competition, one would expect a rather close relationship between extent of weed control and crop y i e l d . An analysis of crop yields f o r postemergence herbicide studies reported i n the 1983 and 1984 North Central Weed Control Conference—Research Report shows that there are no differences i n crop y i e l d within the range of 70 to 90% or better control (Table II). Y i e l d data from a l l reported experi ments, which met certain constraints, were included regardless of crop. To eliminate as much confounding as possible, the following constraints were used i n deciding what data to include: (a) only post-emergence treatments were included; (b) only data involving increasing rates of application of a single herbicide were used;

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(c) only treatments made at the same stage of crop growth were used; and (d) treatments involving any additional weed control were excluded. Crop y i e l d s for reported weed control of 90% or better were compared with yields for reported control of 80% to 89%, 70% to 79%, and 50 to 69%. This provided 6 9 , 5 1 , and 25 y i e l d comparisons for 90% or better versus 80 to 89% control, 70 to 79% control, and 50 to 69% control, respectively. The average y i e l d for 90% or better control for the two years compared to the average y i e l d for 80 to


89% c o n t r o l was

100.5;

i n 1 9 8 3 i t was

9 9 . 6 and

i n 1 9 8 4 i t was

101.2.

Although no s t a t i s t i c a l treatment was attempted, the 0.5% difference c e r t a i n l y i s inconsequential. The comparison of 90% or better control with 70 to 79% control was 1 0 0 . 3 ; i t was 100.8 i n 1983 and 99.1 i n 1 9 8 4 . Here, too, the difference of 0.3% i n y i e l d i n favor of 90% or better control i s inconsequential. The comparison of 90% or better control with 50 to 69% control was 1 1 8 . 0 indicating an advantage for 90% or better control.

Table II.

Relationship Between Percent Weed Control and Crop Yield, North Central Region, 1983 and 1984

Weed Control Range

Crop Yields for 90% or Better Weed Control Compared to 3 Lower Control Ranges 1983 1984 Average a

(%) 80 to 89% 70 to 79% 50 to 69%

99.6 100.8 115.8

101.2 99.1 121.1

100.5^ 100. 118.ol'

The average obtained after dividing the y i e l d for 90% or better weed control by the y i e l d i n the given lower weed control range. •—^69 y i e l d comparisons. —/51 y i e l d comparisons. — ' 2 5 y i e l d comparisons. a

Why are crop yields not more c l o s e l y related to weed control within the 70% or better range? It i s suggested that treatments e f f e c t i v e enough to provide 70% or better control s u f f i c i e n t l y suppress height of the weeds not controlled to prevent their i n t e r f e r ing with crop y i e l d . There are some reports i n the l i t e r a t u r e which support this suggestion. For example, i t has been shown that broadleaf weed escapes from herbicides i n sugar beets are smaller than those i n untreated plots and less damaging to sugar beet y i e l d s than untreated weeds ( 1 9 ) . It has also been reported that giant f o x t a i l suppressed early i n the growing season by sublethal rates of h e r b i cides applied post-emergence did not reduce soybean yields even though the giant f o x t a i l dry weight exceeded that on the weedy check late i n the growing season ( 2 0 ) . For most of the y i e l d comparisons in Table II, the herbicides were applied to r e l a t i v e l y small weeds early i n the crop's l i f e cycle. The implication i s that preventing

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early shading of crop plants by weeds avoids any direct effect on reproduction.


Summary There i s much circumstantial evidence that reduction i n crop y i e l d from weeds may not be adequately explained by either competition, as currently defined, or allelopathy, especially the reduction from weeds present only during the early part of the crop growing season. S p e c i f i c a l l y , the removal of l i g h t by weeds, i n keeping with the current d e f i n i t i o n of competition, may not f u l l y describe the e f fects of l i g h t interference. A direct effect of shading on reproduction i s suggested as a possible third explanation i n soybeans. Additional research i s needed to prove or disprove this suggested explanation. Additional research i s also needed to ascertain the s p e c i f i c reason for y i e l d reductions due to allelopathy, e.g., i s i t due to direct i n h i b i t i o n of reproduction or i n d i r e c t l y due to overa l l i n h i b i t i o n of growth of the crop plant. An alternative way of viewing the l i g h t effect on soybean y i e l d is to consider competition as being of two types, what may be termed passive and active competition. Passive competition would apply to reduced crop growth, and thus y i e l d , as a result of l i g h t capture (removal) by associated weeds. The effects on crop growth, and thereby y i e l d , of such competition would increase steadily with continued shading by weeds. Active competition would apply to a direct and immediate i n h i b i t i o n of reproduction i n response to altered l i g h t quality and/or quantity. The effects of active compet i t i o n on crop y i e l d would be as a consequence of immediately losing the potential y i e l d from the new branch or pods prevented from forming. The suggested o v e r a l l chronology explaining crop y i e l d reduction from weed presence i n soybeans thus i s summarized as follows: E a r l i e s t y i e l d reduction from weed presence

= direct effect of shading on a x i l l a r y bud d i f f e r e n t i a t i o n and development.

Yield reduction i n the midportion of the y i e l d drop curve

= direct shading effect, + competition (for l i g h t and water i n that order) and + allelopathy.

Yield reduction i n the l a s t part of the yield-drop curve

= direct shading effect, + competition (for l i g h t , water, phosphorus, and potassium i n that order) and + allelopathy.

This chronology of explanations helps understand why the curve r e l a t i n g crop y i e l d to weed presence drops as steeply as i t does. As indicated i n the explanation, there i s a compounding of effects. E a r l i e s t reduction i n crop y i e l d potential may be due to only one factor, e.g. direct feedback. The effect of this factor i s v i s u alized as persisting for the remainder of the crop growth cycle

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FORESTRY

although there may conceivably be some compensation depending upon the plant process(es) affected and when the weeds are removed. The second factor in yield adds to the effect of the first factor as does the third factor, and so on for a l l factors involved. The reverse would be true for the direct effect of shading; if this effect of light interference is avoided during this early period, i t will not be a factor in crop yield and the potential yield will be proportionately higher. It remains for future research to corrobo rate, modify, or discard this proposed chronology of explanations. Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Trenbath, R. R. In "Multiple Cropping"; ASA Special Publica tion No. 27; Stelly, M., Ed.; American Society of Agronomy, Madison, WI., 1976; pp. 129-169. Zimdahl, R. L. "Weed-crop Competition: A Review." Interna tional Plant Protection Center, Oregon State University, Corvallis, 1980. Aldrich, S. R.; Leng, E. R. "Modern Corn Production"; F. and W. Publishing Corporation: Cincinnati, 1965. Scott, W. D.; Aldrich, S. R. "Modern Soybean Production"; F. and W. Publishing Corporation: Cincinnati, 1970. Aman, Α.; Scrivner, C. L. Submitted for publication to Soil Sci. Soc. Am. J. Schumacher, W. J.; T h i l l , D.C.;Lee, G. A. J. Chem. Ecol. 1983, 9, 1235-1245. Bell, D. T.; Koeppe, D. E. Agron. J. 1972, 64, 321-325. Dekker, J.; Meggitt, W. F. Weed Res. 1982, 23, 91-101. Rietveld, W. J.; Schlesinger, R.C.;Kessler, K. J. J. Chem. Ecol. 1983, 9, 1119-1133. Hall, A. B.; Blum, U.; Fites, R. C. J. Chem. Ecol. 1983, 9, 1213-1222. Shettel, N. L.; Balke, Ν. E. Weed Sci. 1983, 31, 293-298. Drost, D.C.;Doll, J. D. Weed Sci. 1980, 28, 229-233. Fehr, W. R.; Caviness, C. E. "Stages of Soybean Development." Iowa State University, Special Report No. 80, Ames, 1977. Oliver, L. R.; Frans, R. E.; Tolbert, R. E. Weed Sci. 1979, 27, 183-188. Eaton, B. J.; Feltner, K.C.;Russ, O. G. Weed Sci. 1973, 21, 89-94. Thurlow, D. L.; Buchanan, G. A. Weed Sci. 1972, 20, 379-384. Oliver, L. R.; Frans, R. E.; Talbert, R. E. Weed Sci. 1976, 24, 482-488. Thaine, R.; Ovenden, S. L.; Turner, J. S. Aust. J. Biol. Sci. 1959, 12, 349-375. Schweizer, Ε. E. Weed Sci. 1981, 29, 128-133. Biniak, Β. M.; Aldrich, R. J. Submitted for publication to Weed Sci. 1985.

RECEIVED December 26, 1985

Interference Between Crops and Weeds - ACS Publications

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