Use of growth indices on the mechanisms of competition between
barnyardgrass (Echinochloa crus-galli) and redroot pigweed
(Amaranthus retroflexus) with dry bean (Phaseolus vulgaris)
E. Izadi Darbandi., M. H. Rashed , M. Nassiri
Ferdowsi University of Mashhad, Faculty of Agriculture. P.O.Box 91775-1163Mashhad, Iran
Abstract
To study the mechanisms of competition between barnyardgrass and redroot pigweed with dry bean, an additive
experiment was conducted at the Mashhad experimental station of Ferdowsi University. The experiment was laid
out as a completely randomized block, with three replications. Treatments included three levels of redroot
pigweed densities (4,8,12 plant/m2) and three levels of barnyardgrass (10,20,30 plant/m2) planted in a dry bean
crop with a density of 20 plant/m2. A weed free control was included in each block. Results showed that as a
consequence of competition by the weeds total dry matter (TDM), growth rate, height and leaf area of the bean
crop decreased significantly. For all above parameters, redroot pigweed had a stronger competitive effect on dry
bean than barnyardgrass. Except for the early season growth, redroot pigweed growth index curves exceeded that
of bean. For barnyardgrass, except for the end of the season (during the physiologycal ripening period of bean)
growth index curves were lower than for bean. Greater height and a better leaf position of redroot pigweed in the
mixed canopy from early season on, resulted in redroot pigweed being more competitive for light than bean.
Barnyardgrass had a much weaker competitive ability for light, mainly due to a lower height and a less
favourable leaf position. Apparently, barnyardgrass imposed its competitive effect mainly through root
competition, whereas redroot pigweed imposed its competitive force through roots and mainly foliage.
Introduction
Understanding the mechanisms of competition between species can increase our knowledge about the outcome
of competition. This understanding might be useful for weed management and can be applied in crop breeding
for improved weed suppression (Fontyn & Mahall, 1981). The success of weeds in farming systems depends on
their growth strategy including: high growth rate, strong and efficient root system and good canopy structure
(Maun, 1977). For example the species that have greater height, higher leaf area and more horizontal leaves are
more successful in absorbing radiation than other species (Holt, 1995). This study was conducted to investigate
the mechanisms of competition of barnyardgrass and red root pigweed with dry bean.
Materials and Methods
An experiment was conducted in a randomized complete block design with 3 replications at Mashhad
Experimental Station of Ferdowsi University in 2001. Treatments included three levels of redroot pigweed
densities (4, 8, 12 plant m-2) and three levels of barnyardgrass densities (10, 20, 30 plant m-2) planted separately
in bean (20 plant m-2). A weed free control crop was added to each block. Destructive sampling was done weekly
and height, leaf area, dry matter and crop growth rate were measured for all species. Leaf area density in three
canopy layers (0-25 cm, 25-50 cm and 50 cm up wards) was determined after canopy closure To study
competition for light, the CROP SYS model was used (Nassiri & Kropff, 1999).
Results and Discussion
Both weeds significantly (p< 0.01) reduced total dry matter, growth rate, height and leaf area of bean. For all of
the above parameters, redroot pigweed had a stronger effect on bean than barnyardgrass. Barnyardgrass growth
index curves were lower than bean except at the end of the season (physiological ripening period of bean).
Redroot pigweed growth index curves significantly exceeded that of bean and barnyardgrass except during the
early growth period (data not shown). Redroot pigweed established 70 percent of its leaf area above the bean
canopy throughout the season. Only at the end of the season, barnyardgrass established 40 percent of its leaf area
above the bean canopy (Figure 1).
50-125
25-50
Canopy layers
0-25
Barnyardgrass
50-125
25-50
0-25
Redroot pigweed
50-125
25-50
0-25
100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100
weed
bean
Percentage of leaf area
Figure1: Distribution of leaf area in three canopy layers for each species after canopies closure
This indicates that the shoots of barnyardgrass had less effect on competition. Perhaps barnyardgrass imposed its
competitive effects via partitioning of assimilates to the roots (Maun.1977). For redroot pigweed it was shown
that competition for light was one of the most important aspects in its competition with dry bean (Figure 2),
which was due to its greater height, its better leaf area position (above the bean canopy) and its horizontal leaves
(Holt, 1995). Results from the CROPSYS model also indicated that redroot pigweed throughout the season
absorbed 60 percent of the radiation, whereas the share of dry bean was only 20 percent. Barnyardgrass Only at
the end of the season barnyardgrass was able to absorb 45 percent of the total radiation
.
93
112
Canopy height (cm)
102
A
83
92
B
73
82
63
72
62
53
52
43
42
33
32
23
22
13
12
3
2
80 70 60 50 40 30 20 10
Bean
0
10 20 30 40 50 60 70 80
Barnyardgrass
80
70
60
50
40
30
20
10
Bean
0
10
20
30
40
50
60
Redroot pigweed
Radiation(A),
absorption
(%)
Radiation
absorptionabsorption
(%)
Figure 2. Cumulative
radiation
in mixed canopies of bean , barnyardgrass
and redroot
pigweed
(B), for each of the species after canopies closures
References
Fontyn, P. J., & B. E. Mahall.1981. J. Ecol. 69:883-896.
Holt, J. S. 1995. Weed Sci. 43:474-482.
Maun, M. A.1977. Weed Sci. 25:128-131.
Lantinga, E. A., M. Nassiri,., & M. J. Kropff. 1999. Agric. and fore. Met. 96:71-83.
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