The influence of weed competition on crop yield

Agricultural Systems 18 (1985) 81 93

The Influence of Weed Competition on Crop Yield

H. D. J. van H e e m s t Centre for AgrobiologicalResearch (CABO), Wageningen, The Netherlands

SUMMARY The mean yield of weedy crops relative to that of weed-free crops, and the critical periods of crop-weed competition are calculated for twenty-six different crop species. Experimental data from the literature are used as a base for the calculation. As a result, the crops are placed in hierarchical order of declining competitive ability, with wheat at the top of the list and carrots and onions at the foot of it.

INTRODUCTION Calculations of potential yields of various agricultural crops have appeared regularly in recent years (FAO, 1978; Buringh et al., 1979). Such calculations are valid for situations in which the supply of essential growth factors, such as water and nutrients, is not limiting and where the crop is free from weeds, pest and diseases. Calculations of crop yields in situations where one or more essential growth factors are in limited supply, and weeds, pest and diseases may interfere, are now being developed (Van Keulen & Wolf, 1985). In such calculations, used in farming systems analysis, estimates of the effect of weeds on crop yield are an essential prerequisite for determining an effective strategy of weed control and a proper cost-benefit analysis. The effect of weed competition on crop yield is influenced by the competitive ability and density of the weed and the density and competitive ability of the crop. These factors are, in turn, influenced by environmental conditions, including weather and soil conditions and 81 Agricultural Systems 0308-521X/85/$03-30 ~~ Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

82

H. D. J. van Heernst

management practices, such as fertilization level, plant spacing, crop rotation, etc. Although the behaviour of crops and weeds in mixtures can be described fairly well on the basis of their performance in monocultures (Baeumer & de Wit, 1968: Spitters & Van den Berg, 1982), the necessary information for such a quantitative description is mostly not available, and it is more practical to use empirical relationships, based on field studies. The most important information is: (a) the general effect of uncontrolled RetQtive cumulGtive frequency 99~

/ 95,-

90807570-

50403025 2O 10

5

6

6 /.t

0.02 0

Fig. 1.

50 1 1.00 Retotwe yletd of weedy crop

An example of the relative cumulative frequency of the relative yield of weedy transplanted rice, plotted on normal probability paper.

The influence of weed competition on crop yield

83

weed growth on crop yield: (b) the influence of the timing of weed control on crop yield. EFFECT OF U N C O N T R O L L E D WEED GROWTH ON C R O P YIELD Numerous studies have been made of the effect of weed removal, by hand, by mechanical means or by herbicide application, on crop yield. The results for any given crop were collected by first calculating the yield of the weedy crop relative to that of the weed-free crop in each study. These values, between 0 and 1, with class intervals of 0.1, were then placed in rank order, and the relative cumulative frequency of each interval plotted on normal probability paper, as illustrated in Fig. 1. The relative yields of each crop species were usually normally distributed, as shown in Fig. 1. The mean relative yield (/~) and the standard deviation (6) could be directly estimated from the Figure and are a measure of the competitive ability of the crop. The relatively high standard deviations reflect large variations in weed density and environmental conditions in the various experiments. The relative yields of weedy crops, for various crop species, are given in Table 1. Insufficient results were available for eight of the crops to allow estimates of the standard deviation. Of the crops investigated, the yield of wheat was least affected by weeds, while vegetables, such as carrots and onions, were affected most. I N F L U E N C E OF THE T I M I N G OF WEED C O N T R O L ON C R O P YIELD The relative competitive ability of crop plants and weeds changes in the course of the plant's life cycle. The influence of removing weeds at various times has therefore received considerable attention. Two types of treatment have been used: (i)

The crop is kept free from weeds for various periods of time from the beginning of its growth cycle, after which the weeds are allowed to develop freely. (ii) Weeds are allowed to grow uninterrupted at first, but are removed at various stages in the development of the crop and continually removed until the end of the growth cycle.

0' 75 0- 70 0.65 0"64

0.61 0.59 0.56

0-52 0"51

Sugarcane Cabbage Maize

Sorghum Transplanted rice

Relative yield weedy crop

Wheat Peas Potato Soybean

Crop

0.26 0.23

0-24

0.22

0.26 0.15

0" 13

Standard deviation

0"15

0"00 0'13

0"12

0'21 0"30

0"40 (0"27) 0"25

0.21 0.30

(0-21)

(0"19)

Critical period Start End

[6], [42], [50], [57], [77], [81] [57] [371, [57], [81] [51, [10l, [17], [18], [26], [35], [36], [51], [52], [54], [59], [66], [86], [87], [89], [103] [24], [57] [lOl] [12], [15], [23], [51], [52], [53], [57], [58], [63], [67], [69], [70], [76], [78], [82], [89], [lO2] [19], [20], [21], [22], [39], [4o], [57], [98], [lOO] [1], [27], [30], [55], [57], [62], [65], [68], [74], [91], [105]

Source (Numbers re[er to numbers in square brackets at end o f reJerences)

TABLE 1 Mean Yield of Weedy Crops Relative to Weed-free Crops, and Boundaries of the Critical Period of Crop-Weed Competition, Relative to the Length of the Total Growth Cycle. (Values in Parentheses are Estimated)

Sweet potato Flax Groundnut Beans Tobacco Red beet Okra Cassava Upland rice Yam Sugarbeet Cotton Garlic Mungbean Carrots Onions

0"45 0"42 0'39 0"39 0"34 0"29 0"29 0"28 0"25 0"25 0"23 0"12 0"12 0"10 -0"13 0"13 0.10 0.21 0.00 0.20 0.15

0"33 0"41 0"41

0.19 0-15 0.00

0-12 0.07 0.00 0.10

0"36 0"39

0"37

0"26

0"38 0"20 0"36

0.65 0.52 0'53

[lOll

(0.50)

[561 [lll, [411, [571, [601, [lOll [45], [571, [731, [981

[71, [57] [2], [3], [8], [91, [33], [43], [441, [471, [57], [721 [28], [31], [49], [691, [93] [64], [104] [46] [101] [25], [341, [71] [301, [571, [61], [83] [49] [291, [791, [94], [951, [96], [97], [106] [131, [151, [57], [80]

[49]

0"25 (0"35) 0"35 0"35 (0"39) 0"28 0"40 0"52 (0.44) 0.50 0.42 0.48

~2

e~

86

H. D. J. van H e e m s t

By combining the results of the two types of treatment, a period may be determined during which the presence of weeds has the strongest effect on crop yield, the so-called "critical period of crop weed competition" (Nieto et al., 1968). In order to allow comparison of experiments carried out under different conditions, crop yield is again expressed on a relative basis and the period over which weeds are allowed to grow is expressed relative to the length of the total growth cycle. The boundaries of the critical period are arbitrarily defined as the period during which the relative yield exceeds 0.95. Examples of this are given in Figs 2 and 3. The curves are fitted by eye. The critical period for soybeans extends between 0.12 and 0.30, and for onions between 0.15 and 0"53. Results for other crops are given in Table 1. The onset of the sensitive period is generally not very critical, because pre-emergence herbicides usually ensure weed-free conditions initially, and because most herbicide and cultural techniques can be used soon after sowing but cannot be so easily used later. However, the end of the critical period is important, since it determines the required duration of herbicide use, and determines the type of herbicide that can be used, because of crop sensitivity and weed tolerance of herbicide. It also determines the number of successive weedings necessary. SOYBEAN RetQt,ve y~eLd o f w e e d y c r o p

10

R e L a b v e y l e L d of w e e d y c r o p 10

~. o./~_8 _~_:_, ~

• ~-,,.,..

:2 ,5 Relative w e e d f r e e

"N

10 period

0

o

.~ 10 Relative w e e d y p e r r o d

Fig. 2. Relative yield of weedy soyabeans for various weed-free or weedy periods. Source: Burnside(1972)(A), Coble(1976)(©). E a t o n e t a l . ( 1 9 7 3 ) ( x ), Maun (1977)(O).

The influence of weed competition on crop yield

87

ONIONS

ReLative yieLd weedy crop 10

Relative yield weedy crop

-\

x

.l 0

i

.5 1.0 Relative weedfree period

0

i

.5 1.0 Relative w e e d y period

Fig. 3. Relative yield of weedy onions for various weed-free or weedy periods. Source: Hewson & Roberts (1971) (0), Wicks et al. (1973) (×). There are relatively few determinations of the end of the critical period for specific crops. Fortunately, there appears to be a good correlation (Fig. 4) between the relative yield of weedy crops and the period from emergence to the end of the critical period. Using this relationship, estimates can be made of the endpoint of the critical period for species when inadequate data are available (Table 1). End of critical period 1.0

05

0

0'5 1.0 ReLative yield of weedy crop

Fig. 4. Correlation between the length of time from emergence to the end of the critical period and the yield of the weedy crop, relative to the weed-free crop for various crop species (r = -- 0'82).

88

H. D. J. van Heemst

DISCUSSION The mean weedy crop yields derived here are based on data from various sources. The large standard deviations reflect the variation in the experimental conditions of the various trials. Nevertheless, the hierarchical order in which the crops are placed (Table 1) may give a good impression of the competitive ability of the various crops. Dew (1972) placed barley, wheat and flax in that order, indicating that barley is the best competitor and flax the poorest of the three, with wheat a better competitor than flax. Also, Pavlychenko & Harrington (1934) classified wheat higher than flax. This is in agreement with the order in which these crops are placed in Table 1. As data on the length of the critical period of crop-weed competition are scanty, it could be of practical use to derive such data from weedy crop yields, in the way presented here.

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