critical period for weed control in big onion (allium …

TROPICAL AGRICULTURIST, VOL. 164, 2016

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CRITICAL PERIOD FOR WEED CONTROL IN BIG ONION (ALLIUM CEPA L.) AT TWO DIFFERENT AGRO-ECOLOGICAL ZONES OF SRI LANKA

D.P.P. LIYANAGE,1 D.R. WENAKA,2 D.L.WATHUGALA,2 H.R.C.NIROSHANI1, S.D.S. YAPA3

1Grain Legumes and Oil Crops Research and Development Center, Angunakolapelessa.2 Mahaweli Authority of Sri Lanka, Walawe Special Area, Embilipitiya.

2Department of Crop Science, Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya,

3Adaptive Research Station, Department of Agriculture, Weerawila.

ABSTRACT

Critical period for weed control (CPWC) is the foundation of integrated weed management. It can be considered as the first step to design weed control strategy. To determine critical periods for weed control of big onion, field experiments were conducted during 2014 and 2015 at two different agro-ecological zones, namely, DL1b and DL5 in Hambantota districts, Sri Lanka. A quantitative series of treatments comprised of two components, (a) increased duration with weed interference and (b) increased length of weed-free period, were imposed. Critical period was determined through Logistic and Gompertz equations. Fresh bulb weight was recorded at harvesting. The CPWC was determined by fitting Logistic and Gompertz non-linear equations to relative yield loss data. Four acceptable yield loss levels (5, 10, 15 and 20%) were considered. The CPWC of big onion in DL5 zone during Maha season was 15-100 days after transplanting (DAT) for acceptable yield loss level.. Meanwhile, 9-68 DAT were observed as CPWC in DL1b zone during Yala season. These results revealed that weed is a serious threat to big onion cultivation in both DL1b and DL5 regions and fields should be kept weed free for much of the growing season to reduce considerable yield reductions. However, these periods can be varied according to weed densities, agronomic practices and climatic conditions. Application of integrated weed management practices throughout the growing season instead of total dependence on herbicides can be recommended.

KEY WORDS: Big Onion, Critical period for weed control, Yield loss ,

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WEED CONTROL IN BIG ONION

INTRODUCTION

Big onion (Allium cepa L.) is one of the important condiment crops in Sri Lanka. Onion also have an impact on the National economy in Sri Lanka, as large number of farmers engaged in big onion cultivation and every year government spends large amount of foreign reserves to import big onion to meet the national requirement. According to Department of Census and Statistics (DCS), 2014 report, 15, 278 farmers had cultivated big onion in year 2014 on 5,743 ha of lands and produced 98,900t with an average yield of about 17 t/ ha. In general, the average big onion yield in Sri Lanka is lower than the maximum potentials of respective varieties. There are several reasons for yield reduction. They are weeds, pests and diseases and other abiotic stresses. Among them the main reason for lower than expected yield is weed competition, because of the peculiar canopy structure of the onions.

It has been estimated globally that weeds are considered to be responsible for 10% yield loss of main cultivating crops (Frud-Williams, 2002). The knowledge of weed competition on crop productivity and development is an essential tool in the case of taking decisions on weed control. The critical period is an integral part of integrated weed management (IWM) and can be considered the first step to design weed control strategy which is useful in identifying crop growth stages that are most sensitiveto weed competition. The critical period for weed competition (CPWC) can be defined as the time period crop must be weed-free after crop emergence in order to prevent unacceptable yield losses (Nieto et al., 1968). In theory, presence of weeds before or after CPWC will not pose a threat and should not cause significant yield loss. Thus, crop yield obtained by weeding during CPWC is almost similar to that obtained by the full season weed-free conditions. In general, one-third of the crop life cycle is considered as critical for weed control. The critical period for weed control has been determined for main crops such as maize, wheat, rice, soybean grown in different parts of the world. A long critical period is an indication of less competitive crop or more competitive weeds and vice versa. Many authors have reported that big onion plants are poor weed competitors (Carlson and Kirby, 2005; Dunan et al., 1996; Menges and Tamez, 1981; Qasem, 2006; Smith et al., 2008, 2011). The poor competitive ability of big onion with weeds has been attributed to its initial slow growth and lack of adequate foliage to smother weeds (Wicks et al., 1973). Rajendra et al. (1986) explained big onion’s inherent characteristics such as short stature, non-branching habit, sparse foliage, shallow root system and extremely slow growth in initial stages make the onion to be a poor weed competitor. Therefore, this crop must be maintained weed free for a longer period to avoid yield reduction (Glaze, 1987). Studies have been conducted on yield reduction of big onion due to weed competition all over the world


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and the values obtained are varying with a wider range. For an example, Bhalla and Patel (1983) reported that 48% to 85% reduction in bulb yields in India due to weed competition. Qasem (2005) obtained yield losses of 86% -100% from weed-infested crops for the whole growing season in during 1996/1997 growing season. Loses up to 96% in big onion due to weed competition have been reported from United Kingdom (Peter et al., 2007). Babiker and Ahmed (1986) reported that 26- 48% yield reduction in transplanted big onion and Labrada (1977) reported a 54% yield reduction while Prakash et al., (2000) reported a 81% yield reduction compared to weed-free condition. According to Ibrahim et al. (2011) the yield was decreased by 54% compared to weed free condition when the field left with weeds for the entire season.

The yield reduction due to weed competition depends on various factors such as weed species, their densities, and agricultural operations etc. at a particular region. However, all these research findings emphasize that weed control is an important management practice for big onion production that should be carried out to ensure an optimum yield out of the crop. The CPWC is likely to be unique for every crop species because of their morpho-physiological make up, but it is not an inherent property of the crop rather a measurement of crop-weed-environment interaction. As most of the agronomic research conducted on big onion in Sri Lanka has been focused on the main crops and as information are meagre on CWF P for big onion and weed is the most critical problem in big onion cultivation in Sri Lanka.In order to provide more precise information for grower’s, CPWC should be determined specifically for a particular regions by considering the weed composition and climatic conditions (Rajcan and Swanton, 2001) for maximizing the yield of particular region. Use of herbicides is the main weed control method practiced by big onion growers in Sri Lanka. The present study therefore, aimed to determine the critical weed free period in big onion under two different agro-ecological zones, namely, DL1b and DL5 in Hambantota district, Sri Lanka and to find out the yield loss due to weeds in big onion in particular locations.

MATERIALS AND METHODS

Experimental site The experiment was carried out at the Grain Legume and Oil Crops Research and Development Centre (GLORDC), Angunakolapelessa and Agriculture Research Station (ARS), Weerawila during Maha season 2014/15 and Yala season 2015. These areas belong to DL1b and DL5 Agro-ecological regions in the Dry zone, respectively. Both areas show bi-modal distribution of rainfall. Expected annual rainfall in DL1b and DL5 agro-ecological zones are higher than 900mm and 650mm, respectively. The soil type of these two locations is RBE (Reddish Brown Earth).

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Treatments and the experimental design Randomized Complete Block Design (RCBD) with 4 replicates was used as experimental design. To determine CPWC, quantitative series of treatments were applied.

Treatments:(a) Increasing duration of weed interference (referred to as weedy plots)Weeds were allowed to compete with big onion from seedling establishment until 2, 3, 4, 5, 6, 7, 8, and 9 WAT .Then plots were maintained weed-free until harvest. (b) Increasing length of weed-free period (referred to as weed-free plots)Increasing length of weed free period was maintained by removing weeds until 2, 3, 4, 5, 6, 7, 8 and 9 WAT. Then, subsequent emerging weeds were allowed to compete for the remaining periodr of the growing season. In addition, whole season weedy plot and weed-free plot were maintained as controls in each block. Timing of weed removal was based on the number of weeks after transplanting (WAT) of big onion seedlings. Weeds were removed by manually. The experiment was conducted under naturally occurring population of mixed weed species.

Field establishment Popular big onion variety “Galewela selection” was used in the experiment. Nurseries were prepared according to the recommendations of Department of Agriculture (DOA), Sri Lanka. Nursery sterilization was done by surface burning by using rice straw and rice husk. Inorganic fertilizer mixture (Urea 15g, TSP 30g and MOP 15g) was incorporated to the surface of each bed before establish onion seeds. Seeds at the rate of 40-50 g/bed were applied to the depth of 1 cm in rows and 10 -15 cm apart. Then beds were covered with straw mulch. Water was applied daily until germination. Mulch materials were removed after germination. Beds were covered using white poly-ethylene to protect from rains and sun light during initial stage of seedlings. The 42 days hardened seedlings, with 3-4 leaves, 15-18 cm height with minute bulb were used for field planting. Field preparations were also done according to the DOA recommendations. The bed size was 1X3 m with 15 cm height. Fertilizer was incorporated into the soil 2-3 days before planting at the rate of urea 65kg/ha, TSP 100kg/ha and MOP 50kg/ha. Selected seedlings were dipped in the Thiophanate methyl 50% + Thiram WP 18g/ 10 liters of water for 20-30 minutes prior to transplanting. Irrigation was done as required. First top dressing was applied at the rate of urea 65 kg/ha after 3 WAT and second top dressing was applied at the rate of urea 65 kg/ha and 25 kg/ha after 6 WAT. Two weeks before harvesting, irrigation water supply was stopped.


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Data collection Growth parameters i.e. Height of plant was taken at 7weeks after transplanting and yield measurements i.e. fresh bulb weight, dry bulb weight, total weight (Leaves+ bulbs) of big onion, diameter of bulbs, number of bulbs at harvesting and weather parameters were taken as data. Fresh bulb yield of big onion in each plot were used for this analysis.. Determination of critical weed free period Non-linear equations were used to describe the crop yield response against weed interference. The three parameter’s Gompertz equation (Hall et al., 1992; Knezevic et al., 2002 and Johnson et al., 2004) was used to describe the effect of increasing duration of weed-free period on onion yield.

RY= a*exp [-exp (-(x-x0)/b)] -------------------------- [1] Where, RY is the relative yield (% season-long weed-free yield), “a” is the upper limit, x0 is the days to give 50% yield, “x” is the number of days and “b” is the slope. A3-parameter Logistic equation was used for describing the effect of increasing the length of weed-interference on relative yield (Hall et al., 1992; Knezevic et al., 2002 and Johnson et al., 2004):

RY = if(x<=0, if(b<0,0,a), if(b>0, a/(1+abs(x/x0)^b), a*abs(x/x0)^(abs(b))/(1+(abs(x/x0))^(abs(b)))------------------------------[2] Where, RY is the relative yield (% season-long weed-free yield), “a” is the upper limit, x0 is the days to give 50% yield, “x” is the number of days and ” b” is the slope. The Gompertz and Logistic equations were fitted with the SigmaPlot software. The logistic equation was used to determine the beginning of the CPWC, and the Gompertz equation was used to determine the end of the CPWC for yield loss levels of 5% 10% and 20% chosen arbitrarily (Hall et al., 1992; Martin et al., 2001)

RESULTS AND DISCUSSION

Climatic conditions of the experimental sites Monthly rainfall, daily mean temperature and relative humidity during the study period are shown in figure 1. The daily mean temperature variation in both regions during the study period was 25-30 ºC. Relative humidity also changed in the range of 75-85%. Except in June in all other months comparatively higher relative humidity levels were recorded in DL1b region. Higher

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rainfall was recorded in DL1b region in all months except April and October. The highest rainfall was recorded in September compared to all other months in DL1b region. The DL5 region was also experienced comparatively high rainfall in October. The lowest rainfall was recorded in July at DL5 region.

Big onion seedlings were transplanted on 23rd January at agro-ecological zone DL5 and harvested on April. In the vegetative period of the crop, was received low rainfall compared to other months. But, at the harvesting period, a higher rainfall was received. Onion seedlings were established on mid of March for the early Yala season at the DL1b agro-ecological zone and harvested at end of the June. Although While the month April received highest rainfall in D1b region, June received the lowest rainfall than other months. Onion seedlings were established on 3rd of July for Yala season at the DL5 agro-ecological zone and seedlings were established on 8th July at DL1b region. Yield was harvested on mid of October at both locations. There were substantial occasions of stormy weather conditions prevailed at both locations during the season and thus, crops was severely affected by both anthracnose and bulb rot disease. The DL1b region was received 1,006 mm rainfall while 892.5 mm of rainfall was received at DL5agro-ecological zone from January to October.

Figure 1: The variations of monthly rainfall (mm), temperature (ºC) and relative humidity at DL1b& DL5 locations during study period.

Previous studies have illustrated that the competitive ability of crops and weeds is heavily dependent on the environmental conditions (Lindquist et al., 1999). In addition to influencing the emergence patterns, the environment can play a large part in regulating the crop-weed competitive relationships. For example, weeds and crops respond differently to the variation in temperature, water availability and soil fertility (Black et al., 1969). Hence, plant response to variations in each of these factors needs to be predicted. The CPWC can vary on the management strategies that are imposed on the system.


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Growers can influence the biological system they work with through various management practices. One of the most critical factors affecting the CPWC is the time of planting. For example, early seeding of canola resulted in the need for a somewhat longer CPWC because field operations were conducted relatively early in the emergence period of the weeds resulting in higher weed infestation levels for a longer time (Martin et al., 2001). In addition, the manipulation of edaphic factors, including the alteration of soil nutrient supply and soil available water are believed to influence the crop-weed interference relationships, especially in determining the critical time of weed removal (CTWR) (Weaver et al., 1992). For example, the competitive ability of crops and weeds can be significantly influenced by the amount of fertilizer applied, its location, and the time of application (Di Tomaso 1995; Evans 2001; Tollenaar et al., 1994). Big Onion is highly sensitive to temperature and photoperiod. Bulb formation is favored by relatively high temperature and longer photoperiod (Thompson and Kelly, 1957). De Ruiter (1986) examined the relationship between bulb size and thermal time from emergence to harvesting for six cultivars at different sowing dates. The relative growth rates of bulbs were linearly related to accumulate thermal time.

Table 1: Parameter estimates of the Gompertz and Logistic models used to determine the critical timing of weed removal for fresh bulb yield. (The models were fitted to relative yield of big onion (expressed as the percentage of the season-long weed free))

Parameter estimatesLocation Season a (SE) b (SE) X0(SE) R2 Gompertz equationDL1b Early yala 173.6 68.4 51.0 0.746 (78.3) (32.1) (33.0) DL5 Maha 95.8 10.2 26.9 0.931 (3.7) (1.5) (1.0) DL1b Yala 108.1 22.8 21.7 0.837 (8.4) (4.4) (2.6) DL5 Yala 105.1 11.8 32.6 0.862 (6.6) (2.3) (1.6) Logistic equationDL1b Early yala 97.5 2.0 46.7 0.757 (6.4) (0.4) (4.5) DL5 Maha 102.7 3.2 33.5 0.969 (2.6) (0.2) (0.9) DL1b Yala 100.8 1.9 33.9 0.915 (4.1) (0.2) (2.1) DL5 Yala 81.9 6.5 48.9 0.818 (3.9) (1.6) (1.9)

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The Gompertz and Logistic equations were also fitted to the relative yield data and the critical periods of weed crop competition for fresh bulb yield (Figure 2 and 3, Tables 1). Big onions were cultivated by commercially grown farmers during Yala season at DL1b agro-ecological zone. For Gompertz and Logistic models, r2 of fresh bulb yield were 0.75 and 0.76, respectively in the early Yala season and 0.84 and 0.92, respectively in the Yala season. Probability level of fresh weight was <0.05. The duration of the critical periods of weed competition was 8-68 DAT at 5% and 11-60 DAT at 10% yield loss levels, respectively for fresh bulb yield in the Yala season at DL1b.

Gompertz Equation: RY=108.19*exp(exp((x-21.7333)/22.8168)) R2= 0.8376 P<0.05 Logistic Equation: RY= 100.8574/ (1+abs(x-33.9876) ^33.9876) R2= 0.915, P<0.05

Fig 2: influence of weed interference on relative yield of big onion in the DL1b agro-ecological ecological zone during Yala season. Increasing duration of weed interference data fitted to the logistic equation; increasing weed-free period data fitted to the Gompertz equation.

The duration of the critical periods of weed competition in the early Yala season at DL1b was 9-86 DAT at 5% and 16-86 DAT at 10% yield loss levels, respectively for the fresh bulb yield. Similar to DL1b, parameters of Gompertz and Logistic model have shown a significant effects (P<0.05) on fresh bulb yield of onion in the Maha season atDL5. The r2 of fresh bulb yield was 0.93 for Gompertz models and 0.97 for Logistic models. Probability level of fresh weight was <0.05. Therefore, fresh bulb yield of Gompertz and logistic model was significant in the DL5 region. In Maha season of the DL5, results shows that the duration of the critical periods of weed competition was 15-76 DAT at 5% and 18-56 DAT at 10% yield loss levels, respectively. These results suggest that big onion crop should be kept weed free until 86 days after transplanting or whole period of the crop in order to prevent more than 5% yield loss.


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Gompertz Equation: RY= 173.61*exp(-exp(-(x-51.084)/68.44)) R2= 0.7467 Logistic Equation: RY= 97.5337/ (1+abs(x-46.7214) ^2.0836) R2= 0.7571, P<0.05

Fig 3: influence of weed interference on relative yield of big onion in the DL1b agro- ecological zone during early Yala season. Increasing duration of weed interference data fitted to the logistic equation; increasing weed-free period data fitted to the Gompertz equation.

Gompertz Equation: RY=95.836*exp(-exp((x-26.931)/10.2245) R2= 0.9313 P<0.05 Logistic Equation: RY= 102.7108/ (1+abs(x-33.5416) ^3.277) R2= 0.9699 P<0.05

Fig 4: influence of weed interference on relative yield of big onion in the DL5agro-ecological zone during Maha season. Increasing duration of weed interference data fitted to the logistic equation; increasing weed-free period data fitted to the Gompertz equation.

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Gompertz Equation:

RY=105.1329*exp(-exp(-(x32.6057)/11.834)

R2= 0.8620

P<0.05

Logistic Equation:

RY= 81.9602/ (1+abs(x-48.9049) ^6.5979)

R2= 0.8187

P<0.05

Fig 5: influence of weed interference on relative yield of big onion in the DL5agro-during Yala season. Increasing duration of weed interference data fitted to the logistic equation; increasing weed-free period data fitted to the Gompertz equation. Table 2:The critical period of weed control (CPWC) for big onion calculated from Logistic and Gompertz equations at four acceptable yield loss levels (AYL) expressed as days after transplant (DAT) of fresh yield of onion bulbs in the DL1b and DL5 agro-ecological zones CPWCLocation Season 5% 10% 15% 20%DL1b Early yala 2015 9-86 16-81 19-72 23-66DL1b Yala 2015 8-68 11-60 14-55 17-49DL5 Maha 2014/15 15-76 18-56 21-49 23-45DL5 Yala 2015 0-60 0-55 0-51 28-48

In the Yala season of DL5, the r2 of Gompertz and logistic model of fresh bulb yield was 0.86 and 0.82, respectively. Probability level of fresh weight was <0.05. Thus, fresh bulb yield of Gompertz and Logistic model was significant. The duration of the critical periods of weed competition was 0-60 DAT at 5% and 0-55 DAT at 10% yield loss levels, respectively.

Big onion can tolerate to weed interference up to a certain period before it causes significant yield loss. In both experiments in early Yala season of DL1b, 100% marketable yield reduction did not observe but in the Yala season, 100% yield reduction was observed. However, data showed that weeds cause 77-87% and 84-90% yield reduction in the case of no control of


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weeds during early Yala season and Yala season, respectively. Similar results were reported by Prakash et al, (2000) where they reported that season-long crop-weed competition reduced bulb yield by 81.2% compared with the weed-free condition. Rameshwar et al. (2001) also reported that bulb yield loss can be increased with duration of weed competition and maximum loss (94.7%) occurred due to full season competition. Weeds compete with crop plants for growth factors and impair crops growth and productivity. However, crops as well as weeds are different in their competition effects. Onions are generally considered as weak competitors, with weeds and suffer badly in competition (Bleasdale, 1959; Wicks et al., 1973; Menges and Tamez, 1981; Karim et al., 1998). The competition effect, even for a short period after onion emergence, can harm crops and may result in severe yield losses and growth reduction which is in most cases unrecoverable even with addition of higher levels of growth factors mainly water and nutrients (Williams et al., 1973; Zimdahl, 1980). The factors affecting the weakness of big onion are short stature, low above ground canopy, slow growth and very shallow and small root system. Moreover, type of weed, weed density, and their arrangement in the field also greatly affect competitive ability of the crop. Uncontrolled weed growth caused 49-86% reduction in the bulb yield compared with best herbicide treatment (James et al., 2010). Despite high yielding cultivars, adequate seedling per unit area, adequate irrigation and other cultural practices, yield is very low due to weed infestation. Big onions do not compete well with weeds; hence, weed control is essential for a successful onion production.

The beginning and end of the CPWC determined using the functional approach depends on the level of acceptable yield loss (AYL). Many studies have reported 2 to 5% as the maximum AYL (Hall et al., 1992; Van Acker et al., 1993). But the AYL can be adjusted depending on the market price of the crop and the cost of weed control for a particular field. (Knezevic et al., 2002) Since the 5% yield loss level would not be practical from economic view point but 10% yield loss is considered excellent in terms of economic return. This level can be achieved by application of post-emergence herbicide (Hakim et al., 2013) or weeding between 11-60 DAT for Yala season at DL1b agro-ecological zone. CPWC of DL5agro-ecological zone was 18-56 and 0-55 for Maha season and Yala season, respectively.

Length of CPWC is maximum in the early Yala season of DL1b agro-ecological zone. CPWC for a crop is a measure of crop/ weed and environmental interaction. Results revealed that the CPWC of big onion is varied from region to region and season to season. Bandula Premalal et al. (1997 and 1998) observed a considerable variability associated with critical period of weed interference in common beans in two different seasons of Mid country Intermediate Zone of Sri

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Lanka. According to them the main differences between two seasons were the amount of rainfall and temperature. Bandula Premalal et al. (1997 and 1998) differentiated mainly by differences in annual rainfall patterns.

Onion growth and yield loss may be unrecoverable if weeds were allowed to compete at early stages after planting. Competition between weeds and crop plants at seedling stage is strong, since weed population could normally appear early in the season and individual tends to capture resources as much as possible to ensure growth and survival of them. Although there were some differences in available or provided growth factors at both locations in terms of water, nutrients, light, weed population, composition and density, critical period was found to be similar in both locations at Yala season. The beginning and duration of the CPWC can vary depending on several factors, including the characteristics of crop and weeds, environmental variables (Hall et al., 1992), cultural practices, and the assumptions made regarding the methods employed to determine the CPWC.

CONCLUSIONS

According to the field studies conducted at two locations in two agro-ecological zones of Sri Lanka, namely, DL1b and DL5 weed infestation is a serious threat to big onion production. Data showed that weed causes 77-100% of yield losses in big onion cultivations in the case of no weed control. If farmer can tolerate a 10% yield loss, the CPWC in DL1b zone during Yala and early Yala seasons were 12-60 and 11-78 DAT, respectively and in DL5 zone during Maha season it was 18-62 DAT. This suggests that big onion in these two regionss should be maintained without weeds for a long period after transplanting. However, these periods can be changed according to weed type, weed density, agronomic practices and climatic conditions of the respective location.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Mr. W.A.K. Karunathilake, Additional Director, GLORDC, Angunakolapelessa for instructions given and technical assistance to make this study a success


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