Improved crop tolerance to weeds is an important component of sustainable
agriculture system. If crop cultivar can tolerate weeds, it may be reduces
need for synthetic herbicides (Christensem, 1995). Many factors influence
of weed-crop competition include weed interference time and duration,
weed and crop density, nutrient level and environmental circumstance (Zimdahl,
2004). Time of weed emergence during a growing season will influence growth
and reproductive as well as competitiveness (Blackshaw and Harker, 1998;
Knezevic and Horak, 1998). Last research is indicated that weed interference
on crop is not same in various growth and development stages therefore
weed-crop competition capability is different in their life cycling (Tollenaar
et al., 1994). Ngouajio et al. (2001) reported that weed
removal in early season in tomato is not necessary because of there are
less competition between weed and crop. Weaver et al. (1992) suggested
that the length of time that a crop can tolerate early-season weed competition
is related more to the availability of soil moisture, or possibly essential
nutrients, than to light limitations. Weaver et al. (1992) has
pointed out the length of maximum weed infested period depends upon how
quickly weed begin to shade the crop and compete for water or nutrients
and The length of the minimum weed free period, depends upon how quickly
the crop develop a closed canopy as well as the germination patterns of
the weed species present. Early emerged weeds are more effective in contrast
late emerged weeds. Bensch et al. (2003) reported the maximum soybean
(Glycine max L. Merr.) yield loss occurred at pigweed (Amaranthus
retrofilexus, A. palmeri and A. rudis) first planting
date (companion soybean) and 8 plants m-1 of row density. The
second pigweed planting date (two weeds later) did not significantly reduce
soybean yield. Blachshaw et al., (2002) represented Wild radish
(Raphanus raphanistrum L. RAPRA) that emerged 10 weeks after canola
did not reduce canola yield and had no directly effect on canola quality,
but the Wild radish seeds that mixed with canola seeds, the amount of
erucic acid and glucosinolate was increased above marketable levels in
some cases. Evans et al. (2003) found that an increase in nitrogen
applied early growing season increased corn tolerance to presence of weeds
even when no yield response to nitrogen was observed. They believed increasing
nitrogen aid in more timely corn leaf area expansion and improving the
resiliency of corn leaf nitrogen content to effect of weed interference.
Early-emerging large crabgrass reduced snap bean biomass by 10-28% and
snap bean pod numbers by 44-60% depending on the density. Snap bean pod
number and biomass were reduced as the density of early-emerging redroot
pigweed increased. Regardless of the density, late-emerging redroot pigweed
had less effect on snap bean growth and yield than did early-emerging
redroot pigweed (Aguyoh and Masiunas, 2003).
Eleftherohorinos et al. (2002) reported red rice interference
affected panicle number more than 1000-grainweight in both rice cultivars. The reduction of all yield components
was greater in Thaibonnet than in Ariette.
Dry weight and stem or panicle number of red rice plants grown with either
of the two rice cultivars increased with increasing red rice density and
the values were higher most of the time when grown with Thaibonnet than
The objective of this study were determine the effect of periodical weed
interference on yield and yield components and find the most sensitive
yield component to weed periodical interference.
MATERIALS AND METHODS
Experimental location: Field study was conducted at the Agriculture
Faculty of Tarbiat Modarres University farmland in West of Tehran in 2004. That
region had arid climatic condition with 300 mm annual average precipitation.
Mean daily temperature during growing season was varied from 25°C in early
fall to -5°C in mid winter to 25°C end of spring. The soil type was
sandy loam with pH 7.4 and less than 1% organic matter. Canola production in
the area is conducted under irrigated conditions. Fields are generally furrow-irrigated
immediately after seeding and throughout the season, weekly.
Experimental procedures: After plowing in early summer, 300 kg
ha-1 triple phosphate fertilizer was added to field and after
disking and making furrows the plots established. A canola commercial
variety (Var. Okapy) was direct seeded in Sep. 22. After emergence, the
canola thinned at 3 leaves stage to get proper density of 650000 plant
ha-1. Nitrogen fertilizer added at three times involved 2-4
leaves, rosette quitting and pod filling stages. For control the canola
aphid (Brevicorine brassicae) systemic pesticide was applied in
March 30 and May 10. Irrigation in furrow system had performed weekly.
Two weeks before harvest, irrigation was stopped for harmony in ripening.
Experimental design: We used a randomized complete block design
with four replications. We had two sets of treatments. In first set, the
weeds allowed to interfere with the crop for increasing duration until
two-leaf, four-leaf, six-leaf, eight-leaf, early flower and mid pod set
stages and removed totally to end of season. In the second of treatment,
the canola kept weed-free for increasing duration until above stages.
Both full season weed removal and interference treatments considered to
control treatments. Plots in this experiment were 1.5 wide by 4 m long
involved five furrows. The furrow arranged 30 cm wide.
Data collection: In harvest, 10 canola plants were selected randomly
of each plot and the pods number on main and lateral branches were counted
separately. Ten pods of main and lateral branches were selected randomly
and their seeds number per pod counted. Seed thousand weight were measured
by divided 1000 seeds with counter and weighted. A 3 m2 of
each plot was divided and canola hand cut from ground surface and canola
seed yield was measured.
Statistical analysis: Data on yield and yield components were
subjected to analysis of variance using the PROC MIXED function of Statistical
Analysis System (SAS) to assess the effect of weed-interference duration
on canola yield and yield components.
RESULTS AND DISCUSSION
In this experiment some species of weeds emerged in fall that involved
Hogweed (Purtulaca oleracea L.), Common lambsquarters (Chenopodium
album L.), Redroot pigweed (Amaranthus retroflexus L.), Volunteer
wheat (Triticum aestivum L.) and Jimsonweed (Datura stramonium).
The London racket (Sisymbrium irio) emerged in early spring. Most
populated weed species was Hogweed which was emerged in fall. In early
winter and canola rosette initiation all of summer weeds disappeared and
only Volunteer wheat was remained and passed through the winter.
The results indicated that periodical weed interference had not significant
effect on seed thousand weight (p>0.05). However with increasing in
weed interference duration, seed thousand weight reduced. Seeds number
per pod in main stem significantly affected by periodically weed interference
(p<0.01) and seed number per pod in lateral branches significantly
affected by weed interference duration (p<0.01) (Table
1). Both pods number in main and lateral branches significantly influenced
by weed periodically interference (p<0.01) (Table 1)
i.e., with delaying in weed removing, number of pods per stems in canola
were reduced. Lateral branches numbers were not significantly affected
by periodically weed interference. Canola seed yield had reduced significantly
as influenced by weed periodically in 5% (Table 1).
Data showed that the highest seed thousand weight of canola was happen
in weed interference until two leaf and lowest was occurred in full season
weed interference (Table 2).
Evans et al. (2003) reported seed hundred weight was negative correlation
with weed interference duration and positive with weed free duration.
||Analysis of variance in canola yield and yield component
|* and ** significant level in 0.05 and 0.01%, respectively.,
ns: non significant
||Effect of periodical weed interference on canola yield and
|WI V2, V4, V6 and V8, weed interference up to 2, 4,
6 and 8 leaf; WI IF, weed interference up to initiation of flowering;
WI 50%P, weed interference up to 50% pod set; WI H, weed interference
up to harvest. Means within a column followed by the same letter(s)
are not significantly different according to Duncan`s multiple range
test at the 0.05 level
Because of weeds low density and species, weeds had no intensively effect on
seed thousand weight but highest reduction in seed thousand weight of canola
was happened under weed interference up to 2-8 leaves treatments. Canola canopy
in vegetative stages has low height and those weed species grew faster and taller
will be dominant in competition for light and will shaded on lower level of
canopy and this will induced canola yield loss. Martin et al. (2001)
and Yaghoobi (2005) reported critical period of weed control happened in this
duration (e.g., the critical period of weed control in canola was in 4-6 leaves
stage). In present study increasing in weed interference duration up to two
leaves stage of canola reduced seeds number per pod in main stem but before
that did not. Evans et al. (2003) reported the yield component most sensitive
to both nitrogen and interference from weeds was seed number per ear.
In early growth stages of canola (i.e., 2, 4 leaf stages) the canopy
closure was not established and weed and crop had been competing for nutrient
and moisture more than light but after canopy closure weed and crop compete
for light intensively (Weaver et al., 1992). Use of irrigation
and fertilizer avoid intensive competition for water and nutrient in irrigated
farmland but after canopy closure for canola (after 2, 4 leaf stages)
competition increase for light intensively.
Reduction in light intensity will decrease in photosynthesis and reduce
seeds number per pod. In Table 2 major reduction of
seeds number per pod happened in primary growth stages. This indicates
that the most influence of weed competition for light begun up to 4 leaf
stage of canola and was continued but previous stages (before 4 leaf)
had not affected significantly.
Evans et al. (2003) reported increasing the duration of weed interference
reduced seed number per ear as sigmoidal decline which was intensified
at the lowest nitrogen rate. Seed numbers per pod in lateral branches
were not significantly influenced but reduced with increasing the weed
interference duration. Knezevic et al. (1997) pointed out Redroot
pigweed (Amaranthus retroflexus L.) and Sorghum-grain (Sorghum
bicolor L. Moench) competition caused to reduction in seed number
Pod number per main and lateral branches significantly influenced periodical
weed interference (p<0.01) and this effect was intensive in compare
with other yield components (Table 2). Weed interference
in first growth stages of canola had less influence on pod number per
main and lateral branches.
But after passing these stages (i.e., 6 and 8 leaf stages) increasing
on light competition between weeds and crop, caused to decline in pod
number in all stems. Weed interference after 6 and 8 leaves and early
flowering had same effect on pod number per stems. Weed competition during
these stages (i.e., after canopy closure) caused to major loss on pod
per stems of canola. Probably competition for light in late season had
greatest influence in compare with competition for moisture and nutrient
which are available in irrigation-cropping. Martin et al. (2001)
demonstrated that critical timing of weed removal in canola occurred in
about 4 leaves stage. Aguyoh and Masiunas (2003) reported Snap bean pod
number and biomass were reduced as the density of early-emerging redroot
Canola yield differed significantly between various periodical weed interferences
after 8 leaf stage (Table 2). Bailey et al.
(2003) showed cotton yield loss increased with velvetleaf (Abutilon
theophrast) density e.g., maximum yield loss was 84% at 3.5 velvetleaf
plants m-1 of row.
In addition the seed thousand weight and lateral branches number
per plant did not influenced by weed periodical interference significantly
but seed per pod in main and lateral branches and pod number in main and
lateral branches influenced by weed durational interference significantly.
The pod number in stem between other yield components was must sensitive
to periodically weed interference. In present study weed species of studied
farmland were not sufficiently powerful for influence on winter canola
yield because of the canola has planted for a few years in that field
and more competitive weed species had not populated sufficiently in practical
We gratefully acknowledge Mr. Davood Yaghoobi for his practical
aids in performance of experiment and Research Deputy of Tarbiat Modarres
University for his financial and technical support.