INTRODUCTION
Winter canola (Brassica napus
L.) is one of the main oilseed crops in the world. It received the
highest attention of Iran`s agriculture policy makers as an alternative
crop for self-sufficiency in vegetable oil production. Uncontrolled weed
populations can substantially reduce crop yield and farmers rely primarily
on herbicides for weed control. Herbicide use is an essential component
of successful crop production. However, reducing use of herbicide for
weed control may lessen the impact of herbicides on non target organisms,
development of weed resistance and ground water contamination. Furthermore,
development of weed control strategies that lessens our reliance upon
herbicides for weed control may prove to be more cost effective (Hall
et al., 1992).
Integrated Weed Management (IWM) involves a combination
of cultural, mechanical, biological and chemical methods for effective
and economical weed control (Swanton and Weise, 1991). The principle of
IWM should provide the foundation for developing optimum weed control
systems and efficient use of herbicides. The Critical Period for Weed
Control (CPWC) is a key component of an IWM program (Knezevic et al.,
2002). This is the period during the life cycling of a crop when it must
be kept weed free in order to prevent a specific level of yield loss (Van
Acker et al., 1993). Weed presence before and after CPWC should
not significantly reduce yields (Martin et al., 2001). Knowledge
of the CPWC and its affecting factors are essential for making decisions
on the appropriate timing of weed control and in achieving the efficient
use of herbicide (Knezevic et al., 2002; Van Acker et al.,
1993).
The CPWC is determine by calculation of the time of interval
between two separately measured competition components: the critical duration
of weed interference, the maximum length of time before early-emerging
weeds can grow and interfere with the crop before unacceptable yield loss
is incurred and the critical weed-free period, the minimum length of time
required for the crop to be maintained weed free before yield loss caused
by subsequent emerging weeds is no longer of concern (Knezevic et al.,
2002).
Martin et al. (2001) indicated that canola must be
kept weed-free in most cases until the four-leaf stage of crop (17-38
days after crop emergence [DAE]) and in one early-seeded experiment, until
the six-leaf stage of the crop (41 DAE), in order to prevent >10% yield
loss. After the four-to six-leaf stage of the canola crop, a few weeds
emerged and late-emerging weeds accumulated little shoot biomass. Other
research has been shown that an infestation of wild mustard could remain
in a canola crop until four to six-leaf stages of the crop without causing
irrevocable yield loss (McMullan et al., 1994). Blackshaw et
al. (2002) exhibited four wild radish (Raphanus raphanistrum
L.) per meter square emerging with canola reduced canola yield 9 to 11%,
whereas 64 wild radishes per meter square reduced canola yield 77 to 91%.
At 64 wild radishes per meter square, canola yield was reduced 91, 65,
56 and 19% when wild radish emerged 0, 2, 4 and 7 weeks after canola,
respectively. Wild radish that emerged 10 weeks after canola did not reduced
crop yield. They represented wild radish did not directly reduce canola
quality, but if wild radish seed were not separated from canola seed,
the amount of erucic acid and glucosinolates will increased above marketable
level in some cases.
The critical weed-free period was influenced by crop sowing
date relative to the emergence of the weeds. Delayed sowing reduced the
length of the critical weed-free period because a few weeds emerged after
the late planting date (Martin et al., 2001). The critical period
of weed control influenced by the many factors that affect weed interference
intensity, including the diversity of weed species, weed density, distribution
and emergence periodicity, the nutrient status of the soil, weather and
cultural practices (Hall et al., 1992; Swanton and Weise, 1991).
The objective of this study was to determine the critical
period of weed control for winter canola in a semi arid region (Tehran
west).
MATERIALS AND METHODS
Experiment was conducted at the research field of Agriculture
Faculty of Tarbiat Modarres University (35° 42´N, 50° 71´E; 1215 m above
sea level) in 2004-2005. This location (West of Tehran Metropolitan) is
a represent of a semi arid region. Total annual precipitation in experiment
year was 350 mm. Mean daily temperature during growing season varies from
25°C in early fall to -5°C mid winter and 25°C end of spring. The soil
texture was sandy loam with pH = 7.4 and less than 1% organic matter.
The experiment was arranged in a randomized complete blocks with four
replications. Fourteen experimental treatments of divided to two separate
groups represent weed control and weed interference duration respectively.
In the first set crop was kept weed-free from emergence time to two-leaf
stage (V2), four-leaf stage (V4), six-leaf stage (V6), eight-leaf stage
(V8), early flowering (IE), 50% of silique set (50% SS) and final harvest
(H) [A standardized growth stage scale developed by BASF, Bayer Ciba-Geigy
and Hoechst called BBCH decimal system provides an accurate and simplified
approach to describing canola growth stages]. In Interference treatments
all weed species were allow to grow in canola field till above mentioned
growth stages of canola. Both full season weed removal and interference
treatments considered to control treatments. Each plot consists of five
4 m rows, spaced 0.3 m apart.
During land preparation in early summer, 300 kg ha-1
triple super phosphate fertilizer incorporated to the soil (with mold
board plow) followed by disking. One commonly used commercial variety
of winter canola (Var. Okapi) was direct seeded in September 22nd, 2004.
Canola seedlings thinned at 3 leaves stage to a final population of 650000
plant ha-1. Nitrogen fertilizing divided to three stages involved
two-four leaves, end of rosette phase and initiation of grain filling.
Field was irrigated to meet crop water requirement (seven times before
rosette in autumn and eight times in spring before physiological maturity).
An area of three m2, corresponding to the central
area of the middle two rows of each plot was hand harvested at maturity.
Canola was manually threshed and the constant weight before final yield
was recorded. The Gompertz Eq.
1 was used to describe the effect of increasing lengths of weed-free
period on canola yield (Ratkowsky, 1990; Hall et al., 1992; Martin
et al., 2001).
where, Y is the yield (% of season-long weed-free plot), A is the yield
asymptote, B and K are constants, T is time in DAE (days after emergence)
and exp refers to e (the base of the natural logarithm). Also Logistic
Eq. 2 was used to describe the effect of increasing duration
of weed infestation on the yield of canola (Ratkowsky, 1990; Hall
et
al., 1992).
where, Y is yield (% of season-long weed-free plot), T is the DAE (days
after emergence), X is the point of infection (DAE) and D, F and K are
constants coefficient (Hall
et al., 1992). In order to predict
constant coefficients in above functions a mathematical computer program
(SIGMA PLOT) was used. According to the curve derived from Gompertz and
Logistic equation, the critical length of the weed-free and weed-interference
period for canola based on canola days after emergence calculated for
specific yield loss levels of 5 and 10%, respectively (Martin
et al.,
2001).
RESULTS AND DISCUSSION
Field observation was resulted the high variation in weed
species and their level of infestation throughout the season. It brings
from their difference in ecological and physiological properties. Among
fall emerged species, common purslane (Portulacca oleracea L.)
had produced high density and biomass between weeds in autumn followed
by Volunteer wheat (Triticum aestivum L.), Redroot pigweed (Amaranthus
retroflexus L.), Common lambsquaters (Chenopodium album L.)
and Jimsonweed (Datura stramonium L.) respectively (Table
1). In early winter all of summer weeds disappeared except volunteer
wheat that was a winter weed. In spring London racket (Sisymbrium irio)
did appeared but never produce enough biomass because of dense and highly
established canola canopy.
Critical weed-free period:
The results of this experiment exhibited that weed-crop
competition for radiation capture was too important in four-leaf growth
stage of canola. In fall (canola vegetative growth) some species of weeds
passed of canola canopy as increasing their height (e.g., Redroot pigweed
and Common lambsquaters) and received enough radiation but other weeds
grew under the canola canopy (e.g., Hogweed and Volunteer wheat) and competed
only for soil water and nutrient.
The critical period of weed interference corresponded directly
to reductions in radiation quantity due to weed shading effect (Weaver
and Tan, 1983). However, yield losses occurred even in absence of shading
and it is likely that water stress also played a role in interference
(Weaver and Tan, 1987). In spring the canola canopy height increased (>1
m) as flowering shoots growth (generative growth) and all of the weeds,
even were tall, shaded intensively. The growth of late-emerging weeds
is generally reduced due to crop shading effect (Weaver and Tan, 1987).
In this experiment weed density increased even though after
four-leaf stage. In contrast the weeds single plant weights were consistent.
Probably, the canopy closure by canola never can to prevent establishing
the weeds but the inter and intra-specific competition (often for solar
radiation) after four-leaf stage caused to little biomass accumulation
in weeds. Martin et al. (2001) reported after four-leaf stage of
canola, few weed emerged and those accumulated little biomass and canopy
closure by the canola may have prevented weeds from establishing after
the four-leaf stage. Minimum weed-free period [minimum length of time
after sowing that a crop must be kept weed-free so that later emerging
weeds do not reduce yields (Knezevic et al., 2002), obtained 25
days after canola emergence (585 GDD) with 5% yield loss (Fig.
1).
| Table 1: |
Weed average
density and biomass in control and interference treatments measured
end of fall and early spring |
 |
 |
| Fig. 1: |
Canola (Brassica
napus L.) yield response to increasing length of weed-free
period (•) or duration of weed infestation
(o) in days after emergence of the crop (DAE). Development stages
of the crop, indicated by arrows, were two-leaf (2), four-leaf
(4), six-leaf (6), eight-leaf (8), early flowering (ef), pod
set (ps) and harvest (h) |
Critical timing of weed removal:
Based on the results of this study, weeds can remain in
canola up to the four-leaf stage (25 DAE) with 5% yield loss. Maximum
weed-infested period [maximum length of time that weeds which emerges
with the crop can remain before they become large enough to compete for
growth resources (Knezevic et al., 2002), obtained 25 days after
canola emergence (585 GDD) at 5% yield loss (Fig.
1). This means that canola tolerate the early season weeds interference
until 25 DAE and repair the weeds damages. Weaver and Tan (1987) reported
maximum weed-infested period in field seeded tomatoes varied 5-6 weeks
after sowing. Bukun (2004) represented that maximum weed tolerated in
early season in cotton was about 100-170 GDD after planting at 10% yield
loss level.
Critical period of weed control:
Critical period of weed control obtained with combination
of both critical timing of weed-removal and weed-free period as Logistic
and Gompertz models curves. Knezevic et al. (2002) described three
relationships that can exist in critical studies. The first is when the
critical weed-free period is of longer duration than the critical timing
of weed-removal; the crop must be kept free of weeds between these timings
to prevent yield loss. In the second relationship, the crop must be kept
weed-free for the same duration that a weed infestation can be tolerated
(i.e., the critical weed-free period and the critical timing of weed removal
are equivalent in term of developmental stage or days after crop emergence
(DAE). In this situation, yield loss will be avoided if weed control is
performed at this one critical time. The third relationship exists when
the critical timing of weed removal is longer than the critical weed-free
period. In this case, yield loss will not occur if weeds are controlled
at any point between these critical stages.
In this experiment, the critical weed-free period and the
critical timing of weed removal are equivalent in days after crop emergence.
Therefore, the critical time of weed control that occurred on 25 DAE (between
four-to six-leaf stages) and a time weed control is enough for avoid yield
loss. However, if we had not performed any weed control, canola yield
loss will reduces just 7% because of lack the powerful canola competitor
species in experiment field. The coefficient for the equations which defined
the critical period of weed control are listed in Table
2 and 3.
Martin et al. (2001) showed when there were high levels of weed
pressure, weed removal prior to the four-leaf stage (17-38 DAE) of spring
canola was not required to prevent yield loss >5% and that canola growth
and development are sufficiently plastic at the four-leaf stage to recover
yield potential after weeds are removed. After
| Table 2: |
Parameter estimates for Gompertz
equation and standard error* |
| |
| * Y = Aexp (-Bexp(-KT); Y = Yield
(% of season-long weed free corn); A = Asymptote (% of season
long weed-free corn); B, K = Constants; T = Time from emergence
(days) |
| Table 3: |
Parameter estimated for the logistic
equation and standard error* |
| |
| * Y = ((1/(Dexp(K(T-x))+F))+((F-1)/F))100%;
Y = Yield (% of season-long weed-free corn); T = Time from emergence;
x = Point of infection (days); D, F = Constants |
CPWC, shading by the crop will normally prevent the establishment
of late-emerging weeds, or the crop might tolerant the reduced competitiveness
of these weeds (Martin et al., 2001).
Hall et al. (1992) represented the beginning of the
CPWC varied from 3-to 14-leaf stages of corn (Zea mays L.) development
and the end of CPWC was less variable and ended on average at the 14-leaf
stage. He suggested weed interference reduced corn leaf area by reducing
the expanded leaf area of each individual leaf and accelerating senescence
of lower leaves. Bukun (2004) reported the beginning of CPWC in Cotton,
(Gossypium hirsutum L.) ranged from 100 to 159 GDD and the end
from 1006 to 1174 GDD, depending on the weed species and density. Ngouajio
et al. (1997) showed the CPWC beginning in Common bean (Phaseolus
vulgaris L. [cv. Maringue]) occurred emergence to first trifoliate
and end of second trifoliate leaf to pod filling stages of bean. He offered
the CPWC in bean was less than 25 days at 5% yield loss.
Practical implication:
The development of any IWM (integrated weed management)
system requires knowledge of the behavior of weeds in the agro ecosystem,
including possible effects on crop yields (Amador-Ramirez, 2002). Knowledge
of CPWC and morphological changes occurring in the crop may provide useful
information upon which to base future weed control recommendation (Hall
et al., 1992). The result of this study suggests that weeds must
be controlled during the four-to six-leaf of canola in growing season
to prevent yield loss and utilize the post emergence herbicide with less
survival in CPWC will be useful for weed control in canola.
ACKNOWLEDGMENTS
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.
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