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Popcorn (Zea mays L. var. everta) yield and yield components as influenced by the timing of broadcast flaming
Crop Protection 29 (2010) 1496e1501
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Crop Protection journal homepage: www.elsevier.com/locate/cropro
Popcorn (Zea mays L. var. everta) yield and yield components as influenced by the timing of broadcast flaming Santiago M. Ulloa a, Avishek Datta a, Sidnei D. Cavalieri b, Mario Lesnik c, Stevan Z. Knezevic a, * a
Department of Agronomy and Horticulture, University of Nebraska, Northeast Research and Extension Center, 57905 866 Road, Concord, NE 68728-2828, USA Faculty of Agronomic Sciences, Sao Paulo State University, Botucatu, Sao Paulo, Brazil c Department of Plant Protection, Faculty of Agriculture and Life Sciences, University of Maribor, Pivola 10, 2311 Hoce, Slovenia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2010 Received in revised form 11 August 2010 Accepted 13 August 2010
Farmers are interested in producing popcorn under organic production systems and propane flaming could be a significant component of an integrated weed management program. The objective of this study was to collect baseline information on popcorn tolerance to broadcast flaming as influenced by propane dose and crop growth stage at the time of flaming. Field experiments were conducted at the Haskell Agricultural Laboratory of the University of Nebraska, Concord, NE in 2008 and 2009 using five propane doses (0, 13, 24, 44 and 85 kg ha1) applied at the 2-leaf, 5-leaf and 7-leaf growth stages. Propane was applied using a custom-built research flamer driven at a constant speed of 6.4 km h1. Crop response to propane dose was described by log-logistic models on the basis of visual estimates of crop injury, yield components (plants m2, ears plant1, kernels cob1 and 100-kernel weight) and grain yield. Popcorn response to flaming was influenced by the crop growth stage and propane dose. Based on various parameters evaluated, popcorn flamed at the 5-leaf showed the highest tolerance while the 2-leaf was the most susceptible stage. The maximum yield reductions were 45%, 9% and 16% for the 2-leaf, 5-leaf and 7-leaf stages, respectively. In addition, propane doses that resulted in a 5% yield loss were 23 kg ha1 for the 2-leaf and 7-leaf and 30 kg ha1 for the 5-leaf stage. Flaming has a potential to be used effectively in organic popcorn production if properly used. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Doseeresponse curves Crop tolerance Non-chemical weed control Organic crop production
1. Introduction Popcorn (Zea mays L. var. everta) is one of the most popular snacks in the United States (Hansen, 2009). Since the year 2000, the US popcorn industry reported the steady sales of as much as $1 billion a year (The Popcorn Board, 2010). Nebraska is in the top popcorn producing state with 135,000 tonnes of shelled popcorn production, which was 34% of the total popcorn produced in the US in 2007 (Hansen, 2009). The use of agricultural chemicals in popcorn production is extensive and there are reports of finding pesticide residues in popcorn products. For example, the US Food and Drug Administration found samples of conventionally produced popcorn contaminated with phenylurea herbicides and other pesticides (FDA, 2009). Therefore, many farmers are now interested in growing popcorn under organic production, which is the fastest growing segment of the US agriculture. The sales of organic products have shown an annual growth rate of 15e20% during the last 10 years (Anonymous, 2007). * Corresponding author. Tel.: þ1 402 584 3808; fax: þ1 402 584 3859. E-mail address: [email protected] (S.Z. Knezevic). 0261-2194/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2010.08.011
Organic farmers rank weed control as the major factor limiting crop yields and quality (Wszelaki et al., 2007). For example, weed competition in non-chemical cropping systems reduced maize (Z. mays L.) yield from 50% to 87% (Magani, 1990). In order to achieve acceptable weed control levels in organic farming, the combination of many strategies has to be implemented (Wszelaki et al., 2007). Propane flaming could be a potential alternative tool in the toolbox of integrated weed management system (Knezevic and Ulloa, 2007; Ulloa et al., 2010a,b,c,d). Flame weeding with propane is an acceptable weed control option in organic production and has received the renewed interest for both organic and conventional systems (Bond and Grundy, 2001). Flaming controls weeds by heating plant tissue rather than burning it (Leroux et al., 2001). Propane torches can generate combustion temperatures of up to 1900 C, which rapidly increase the temperature inside the plant. An increase of temperature above 50 C inside the plant cells can result in the coagulation of membrane proteins leading to the loss of membrane integrity (Parish, 1990; Pelletier et al., 1995; Rifai et al., 1996; Lague et al., 2001). Consequently, flamed weeds die or their competitive ability against the crop is severely reduced. Flaming could be implemented
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as an alternative weed control method to reduce the reliance on herbicides, and other physical weed control methods such as hand weeding and mechanical cultivation. Flaming does not leave any chemical residues on plants, soil, air or water, produces no drift hazards or herbicide carry-over to the next season. Flaming can control herbicide-tolerant or resistant weeds and is considerably less costly than hand weeding (Nemming, 1994; Wszelaki et al., 2007). Crop tolerance to flaming varies with species and growth stages (Knezevic and Ulloa, 2007; Knezevic et al., 2009a,b; Ulloa et al., 2010a,b,c,d). Knezevic and Ulloa (2007) reported that grass crop species (e.g., maize and sorghum [Sorghum bicolor (L.) Moench]) exhibited higher tolerance to flaming than broadleaf species [e.g., soybean (Glycine max L.)] when treated at early growth stages. Teixeira et al. (2008) observed quick recovery of maize plants flamed at the 5-leaf stage and concluded that flaming can safely be used in maize production as an alternative weed control method. Heiniger (1998) compared flaming to mechanical cultivation with respect to overall weed control and grain yield in popcorn and reported better weed control with flaming than cultivation, but there was no difference in popcorn yields. Little information is available on the response of popcorn to broadcast flaming at different growth stages. To optimize the use of propane flaming as a weed control tool, the biologically effective dose (ED) of propane for popcorn tolerance needs to be determined. Therefore, the objective of this study was to assess popcorn tolerance to broadcast flaming as influenced by the crop growth stage and propane dose. 2. Materials and methods 2.1. Site description Field experiments were conducted in 2008 and 2009 at the Haskell Agricultural Laboratory of the University of Nebraska located near Concord, NE (42.37 N, 96.68 W). The soil type was an Alcester series silty clay loam (fine-silty, mixed, mesic, Cumulic Haplustolls) with 3.6% organic matter and pH of 6.6. The previous crop was soybean and the residues were removed before popcorn planting. 2.2. Experimental details One of the commonly grown organic popcorn hybrids in Nebraska, A448 White, was sown no-till on June 9 in 2008 and on June 2 in 2009 with a four-row planter. The experimental plot size was 10 m by 3 m. The row spacing was 76 cm and the seeding rate was 70,000 seeds ha1. The plots were kept weed-free during the entire growing season by hoeing and hand weeding. The experimental layout was a split-plot design with three replications where the main plot was popcorn growth stage at the time of flaming and the sub-plot was propane dose. The treatments consisted of an unflamed check and four propane doses applied at three different growth stages. Flaming was conducted on June 23, July 3 and July 15, which corresponded to the 2-leaf, 5-leaf and 7-leaf growth stages in 2008. In 2009, popcorn was flamed on June 17, June 30 and July 7, which corresponded to the same growth stages flamed in 2008. Flaming treatments were conducted with a small custom-built research flamer which delivered open flames and was mounted on the back of a four-wheeler (Knezevic et al., 2007a). There were four burners “LT 2 8 Liquid Torch” (Flame Engineering, 2007) mounted 76 cm apart and adjusted to flame right above the crop rows. The bottom of the burner cup was positioned 20 cm above the soil surface and angled back at 30 . Such set up provided 25 cm wide-open flames over each of the four crop rows in order to purposely cause popcorn
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injury, thus we could test crop tolerance to broadcast flaming. The flaming treatments were conducted parallel to the crop rows. To deliver the appropriate propane dose, the flamer was calibrated before the treatments were applied. The calibration was done by taking into account the propane pressure and application speed (Knezevic et al., 2007a). The application speed was 6.4 km h1, and the propane pressure was adjusted to deliver propane doses of 0, 13, 24, 44 and 85 kg ha1. Flaming treatments were conducted only when the air temperatures were between 15 and 25 C and the wind speed was less than 10 km h1. 2.3. Measurements Popcorn injury was evaluated visually at 1, 7, 14 and 28 days after treatment (DAT) using a scale from 0 to 100%, with 0 representing no crop injury (based on the unflamed plots) and 100 representing crop death. For yield components, two linear meters of the center two rows of each plot were hand harvested and number of plants m2, ears plant1, kernels cob1 and 100-kernel weight were measured. Yield was measured at crop maturity by harvesting the 4 m length of the center two rows of each plot. Yields were adjusted to 16% moisture. 2.4. Statistical analysis ANOVA was performed using PROC GLIMMIX procedure in SAS (SAS Institute, 2005) to test the significance (P < 0.05) of years, treatments, replications and their interactions on the basis of the visual crop injury ratings, yield components and yield data. There was no treatment-by-year interaction; thus, the data were combined over years. There was a significant growth stage and propane dose interaction; therefore, the data were presented separately for each growth stage. Yield components and yield data were analyzed using the four-parameter log-logistic model (Seefeldt et al., 1995):
Y ¼ C þ ðD CÞ=f1 þ exp½Bðlog X log EÞg
(1)
where Y is the response (e.g., popcorn yield), C is the lower limit (e.g., minimum yield), D is the upper limit (e.g., maximum yield), X is the propane dose, E is the dose giving a 50% response between the upper and lower limit (also known as inflection point, I50 or ED50) and B is the slope of the line at the inflection point (also known as a rate of change). Plant injury based on visual ratings and yield loss data were analyzed utilizing the three-parameter log-logistic model where the C term was fixed to zero (Seefeldt et al., 1995):
Y ¼ D=f1 þ exp½Bðlog X log EÞg
(2)
Curve fitting was done by non-linear regression using the least squares method. All statistical analysis and graphs were performed with R program (R Development Core Team, 2006) utilizing the doseeresponse curves (drc) statistical addition package (Knezevic et al., 2007b). The values of VI5 (effective dose that provides a 5% visual crop injury) or YR5 (effective dose that provides a 5% yield reduction) and VI10 or YR10 (10% visual crop injury or 10% yield reduction) were determined from the curves and used as measures of the level of crop damage by flaming treatments. A yield reduction of 5% was arbitrarily assigned as the value above which the loss was considered unacceptable (Knezevic et al., 1998). Based on our previous research, a propane dose of 60 kg ha1 was highly effective in providing 80e90% control of many grasses and broadleaf weeds (Ulloa et al., 2010b,c). Therefore, the predicted yield loss with a propane dose of 60 kg ha1 was also calculated (Table 3). It was not possible to calculate the propane doses that can
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cause a 5% or 10% reduction at each growth stage due to a small effect of flaming treatments on most yield components (Table 2). A lack-of-fit test at the 95% level was not significant for any of the doseeresponse curves (Figs. 1e3) tested indicating that the loglogistic models were appropriate (Knezevic et al., 2007b). Differences between the equation parameters for each combination of growth stage and propane dose were also determined by comparing the standard errors (SE) and t and F tests at the 5% significance level.
3. Results 3.1. Crop injury Visual crop injury symptoms included initial whitening and then browning of leaves. Stunting of growth was especially evident when the plants were flamed with higher propane doses. Most visual crop injuries, however, were transient as plants appeared to be visually recovered within few weeks. Visual crop injury was influenced by the interaction among growth stage, propane dose and time of ratings (Fig. 1). Based on the maximum visual crop injury obtained at 28 DAT (upper limit of the curve, Table 1), the 7leaf was the least affected stage to broadcast flaming. For example, the 7-leaf stage presented the lowest maximum injury (15%) followed by the 2-leaf (21%) and 5-leaf (23%) stages. Similar trends occurred when popcorn injury was compared to other stages across all evaluation dates. In general, the propane dose that caused either a 5% or 10% visual crop injury (VI5 or VI10) did not vary among growth stages. For example, at 28 DAT the amount of propane needed to cause a 5% or 10% injury was similar across the growth stages and stayed around 28 and 45 kg ha1 of propane for VI5 and VI10, respectively (Table 1). Similar trends occurred for other evaluation dates (1, 7 and 14 DAT).
Crop injury decreased from 1 DAT to 28 DAT (upper limit of the curve, Table 1). For example, about 75% injury at the 2-leaf stage was evident with the highest propane dose of 85 kg ha1 at 1 DAT compared to 21% injury at 28 DAT, suggesting that popcorn plants were able to recover over time. Plants flamed at the 5-leaf and 7-leaf stages showed similar trends (Fig. 1).
3.2. Yield and yield components Popcorn yield of the unflamed plots (control) and the plots treated with low propane doses (e.g., 0e24 kg ha1) averaged around 3.9 (0.1) t ha1, irrespective of the growth stage of flaming (Fig. 2). However, popcorn yields were reduced significantly when treated with higher propane doses (e.g., 44 and 85 kg ha1), especially when flaming was conducted at the 2-leaf stage (Table 2). Yields were 2.5 (0.2), 3.6 (0.1) and 3.5 (0.4) t ha1 for the 2-leaf, 5-leaf and 7-leaf stages, respectively, when flamed with the highest propane dose of 85 kg ha1 (lower limit of the curve, Table 2). These results showed that the 5-leaf and 7-leaf showed higher tolerance to propane flaming than the 2-leaf stage. Most yield components of popcorn were negatively affected by increasing propane dose with the exception of ears plant1 (data not shown). Number of plants m2 and 100-kernel weight were the most affected by broadcast flaming followed by kernels cob1 (Fig. 2). Plots flamed at the 2-leaf stage resulted in the highest reduction in the number of plants m2 (Table 2). The average number of plants m2 of the unflamed plots was 5.3 compared to 4.3 for the 2-leaf and 5.0 for the 5-leaf and 7-leaf stages with the highest propane dose suggesting that the 2-leaf stage was the most susceptible stage to broadcast flaming. Number of kernels cob1 was significantly reduced with increasing propane dose across all flaming stages (Fig. 2). Kernels cob1 averaged about 573 in the unflamed plots, compared
1 DAT 100
2-leaf 5-leaf 7-leaf
80 I njury (% )
80 I njury (% )
7 DAT 100
60 40
60 40
20
20
0
0 0
20
40
60
80
100
0
20
Propane dose (kg ha )
80
80
60 40
40 20
0
0 60
100
60
20
40
80
28 DAT 100
I njury (% )
I njury (% )
14 DAT
20
60
Propane dose (kg ha )
100
0
40
−1
−1
80 −1
Propane dose (kg ha )
100
0
20
40
60
80
100
−1
Propane dose (kg ha )
Fig. 1. Popcorn injury at 1, 7, 14 and 28 days after treatment (DAT) as influenced by growth stage at the time of flaming. The regression lines were plotted using equation (2), and the parameter values are reported in Table 1.
4.0
6
3.5
5 Plant s m
3.0 2-leaf 5-leaf 7-leaf
2.5
4 3
2.0
2 0
20
40
60
80
100
0
−1
20
40
60
80
100
−1
Propane dose (kg ha )
Propane dose (kg ha )
600 100-k ernel weight (g)
14.0
−1
580 Kernels c ob
1499
−2
−1
Yield (t ha )
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501
560 540 520 500
13.5 13.0 12.5 12.0 11.5 11.0
0
20
40
60
80
100
0
−1
40
60
80
100
−1
Propane dose (kg ha ) 2
20
Propane dose (kg ha )
1
Fig. 2. Popcorn yield and its components (plants m , kernels cob and 100-kernel weight) as influenced by growth stage at the time of flaming. The regression lines were plotted using equation (1), and the parameter values are presented in Table 2.
to significantly lower kernel number of 513, 547 and 531 in plants flamed with the highest propane dose (85 kg ha1) at the 2-leaf, 5leaf and 7-leaf stages, respectively (lower limit of the curve, Table 2). The weight of 100-kernel was negatively affected with increasing propane dose at all flaming stages (Fig. 2). Again, the 2leaf stage presented the largest 100-kernel weight reduction (Table 2). The average 100-kernel weight was 13.2 g in the unflamed plots compared to significantly lower 100-kernel weight of 11.0, 12.6 and 12.4 g from plant treated with the highest propane dose (85 kg ha1) at the 2-leaf, 5-leaf and 7-leaf stages, respectively.
35 2-leaf 5-leaf 7-leaf
30
In general, yield loss increased with increase in propane dose regardless of the growth stage (Fig. 3). Overall, plants flamed at the 5-leaf stage exhibited the lowest yield reduction followed by the 7-leaf and 2-leaf stages. The maximum yield reductions with the highest propane dose of 85 kg ha1 were 9%, 16% and 45% for the 5-leaf, 7-leaf and 2-leaf stages, respectively (upper limit of the curve, Table 3). In addition, propane doses that resulted in a 5% yield reduction were 23 kg ha1 for the 2-leaf and 7-leaf and 30 kg ha1 for the 5-leaf stage indicating that popcorn flamed at the 5-leaf stage can tolerate higher dose of propane for the same yield reduction compared to other growth stages. It is important to mention that none of the propane doses tested in these experiments caused a 10% yield reduction when popcorn was flamed at the 5-leaf stage. These Table 1 Propane doses (kg ha1) that resulted in 5% and 10% visual crop injury [VI5 and VI10 (SE)] at three growth stages of popcorn at 1, 7, 14 and 28 days after treatment (DAT) (Fig. 1). Regression parameters are estimated using equation (2).
25 Yield loss (%)
3.3. Yield loss
20
40
60
80
100
-1
Fig. 3. Popcorn yield loss (%) as a function of propane dose and growth stage at the time of flaming. The regression lines were plotted using equation (2), and the parameter values are reported in Table 3.
(4) (6) (8) (5)
14 27 33 42
(5) (6) (8) (4)
29 41 40 45
(4) (5) (5) (7)
6 17 21 28
(2) (5) (6) (9)
10 24 30 41
(3) (6) (6) (8)
27 31 23 36
(3) (5) (4) (5)
9 14 13 25
(3) (5) (3) (4)
14 25 30 52
(3) (5) (5) (7)
1 7 14 28
2.1 3.1 2.7 3.7
(0.6) (1.0) (1.1) (1.2)
75 55 33 21
(8) (4) (3) (2)
35 44 45 44
5-leaf
1 7 14 28
1.6 2.3 2.4 2.8
(0.4) (0.7) (0.8) (1.6)
70 43 30 23
(5) (3) (3) (3)
7-leaf
1 7 14 28
1.9 1.8 1.5 1.9
(0.4) (0.6) (0.4) (0.5)
43 25 23 15
(3) (3) (3) (1)
0
Propane dose (kg ha )
10 21 24 31
2-leaf 10
20
(5) (4) (6) (4)
Regression parameters (SE) B
0
VI10 (SE)
DAT
15
5
VI5 (SE)
Growth stage
D
I50
B: the slope of the line at the inflection point; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit.
1500
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501
Table 2 Regression parameters for popcorn yield and its components (plants m2, kernels cob1 and 100-kernel weight) as influenced by propane dose and growth stage of flaming (Fig. 2). Regression parameters are estimated using equation (1). Variable
Growth stage Regression parameters (SE) B
Yield (t ha1)
Plants m2
Kernels cob1
C
D
I50
2-leaf 5-leaf 7-leaf
6.6 (8.0) 2.0 (0.6) 1.3 (0.9)
2.5 (0.2) 3.6 (0.1) 3.5 (0.4)
3.9 (0.1) 50 (7) 4.0 (0.1) 27 (6) 4.0 (0.1) 29 (20)
2-leaf 5-leaf 7-leaf
1.6 (0.6) 18.8 (2.3) 18.1 (2.3)
4.3 (0.6) 5.0 (0.1) 5.0 (0.1)
5.3 (0.1) 19 (5) 5.3 (0.1) 33 (1) 5.3 (0.1) 18 (1)
2-leaf 5-leaf 7-leaf
1.9 (1.3) 1.4 (0.6) 3.1 (0.4)
513 (15) 547 (7) 531 (13)
571 (8) 24 (10) 576 (12) 22 (5) 572 (14) 16 (2)
100-kernel weight (g) 2-leaf 5-leaf 7-leaf
2.1 (0.9) 11.0 (0.4) 13.1 (0.3) 32 (10) 9.6 (19.2) 12.6 (0.5) 13.3 (0.1) 25 (4) 3.7 (1.8) 12.4 (0.5) 13.2 (0.1) 32 (8)
B: the slope of the line at the inflection point; C: the lower limit; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit.
results showed that the 5-leaf was the most tolerant while the 2-leaf was the most susceptible stage to broadcast flaming.
4. Discussion These experiments showed that growth stage and propane dose influenced popcorn sensitivity to broadcast flaming. These findings support previous findings that have shown that plant susceptibility to flaming varied among species and their growth stages (Ascard, 1994; Knezevic et al., 2009a,b; Ulloa et al., 2010a,b,c,d). In general, broadleaf species were more susceptible to flaming than grasses regardless of growth stage (Knezevic et al., 2009a,b; Ulloa et al., 2010b,c). In our study, popcorn plants showed maximum crop injury of 21% and 23% at the 2-leaf and 5-leaf stages, respectively, compared to much lower injury of 15% at the 7-leaf stage at 28 DAT and recovered over time after flaming treatments. Our results are similar to those of Wszelaki et al. (2007) and Ulloa et al. (2010a,b,c,d), who also reported greater crop injury in flamed plants at early evaluation dates compared to later rating dates. Similar results were also reported in field maize and sweet maize (Z. mays L. var. rugosa) (Teixeira et al., 2008; Ulloa et al., 2010d). The susceptibility of plants to flaming largely depends on their heat avoidance, heat tolerance, or both (Ascard, 1995). The extent to which the heat from the flames affects plants depend on the flaming technique and leaf surface moisture (Parish, 1990). The tolerance of different plant parts to heat injury can be influenced by several other factors, including the presence of protective layers of hair, wax, lignification, external and internal water status of the plant and the species regrowth potential (Ascard, 1995). The effect of broadcast flaming on different yield components varied with crop growth stages at the time of flaming. For example, plants flamed at the 2-leaf stage had the highest yield loss and the lowest yield components. This might be explained by the fact that the ear and tassel tissues are not differentiated at the 2-leaf stage (McWilliams et al., 1999), thus exposing the plants to the stress from heat can result in potentially shorter cobs. In comparison, flaming popcorn plants at later growth stages (e.g., 5-leaf or 7-leaf), had less effects on cob size as the ear and tassel tissues start to differentiate at the 5-leaf stage and by the 7-leaf stage, cob and tassel sizes are already pre-determined (McWilliams et al., 1999). Response of popcorn to broadcast flaming from our study is similar to the previous findings of Ascard (1994) and Ulloa et al. (2010a,b,c,d), who reported that plant size at the time of flaming had an important influence on plant sensitivity, with smaller plants
Table 3 Regression parameters and propane doses (kg ha1) that caused 5% and 10% yield reduction [YR5 and YR10 (SE)] for each growth stage of popcorn (Fig. 3). Regression parameters are estimated using equation (2). The predicted yield loss (%) with a propane dose of 60 kg ha1 is represented by PR60 for each growth stage. Growth stage
2-leaf 5-leaf 7-leaf
Regression parameters (SE) B
D
I50
2.0 (1.1) 1.9 (0.4) 1.2 (0.6)
45.1 (4.3) 9.4 (3.9) 16.3 (2.2)
65 (8) 28 (3) 45 (5)
YR5 (SE)
YR10 (SE)
PR60b
23 (3) 30 (3) 23 (2)
35 (5) NAa 67 (8)
20.7 7.7 9.4
B: the slope of the line at the inflection point; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit. a Plants flamed at the 5-leaf stage did not reach 10% yield reduction level. b Based on previous research, a propane dose of 60 kg ha1 was highly effective in providing 80e90% control of many grasses and broadleaf weeds (Ulloa et al., 2010b,c).
being more susceptible than larger ones. Smaller plants are more heat sensitive because they have thinner leaves, lower biomass and the meristems are fully exposed (Ascard, 1995). In contrast, larger plants have larger and thicker leaves, the meristems are well protected by the surrounding leaves and they possess larger amounts of reserve food (soluble sugars, proteins and lipids) in the stems and the roots providing the plant with the increased capacity for regrowth (Ascard, 1995). Based on our previous studies conducted to determine the response of various weed species to broadcast flaming, a propane dose of 60 kg ha1 provided up to 80e90% control of many broadleaf and grass species (Ulloa et al., 2010b,c). From the practical point of view, such propane dose resulted in an 8% and 9% yield reduction at the 5-leaf and 7-leaf stages, respectively, which could not be acceptable by organic farmers. These yield reductions are the result of our intentional flaming under weed-free conditions where torches were positioned directly over the crop. However, we believe that positioning flames below the popcorn canopy would reduce the exposure time to the heat and therefore should reduce popcorn yield losses. Future studies are needed to test such hypotheses. Acknowledgments We thank the Propane Education and Research Council (PERC) and the Nebraska Propane Association for the partial financial support of this work. We are also grateful for the help provided by Jon Scott, Goran Malidza, Marco Fontanelli, Nicoletta Nasso and Patrick Gunther. References Anonymous, 2007. Organic Crop Production. University of Kentucky Cooperative Extension Service. Web page: http://www.uky.edu/Ag/NewCrops/introsheets/ organicproduction.pdf (accessed 07.04.10). Ascard, J., 1994. Doseeresponse models for flame weeding in relation to plant size and density. Weed Res. 34, 377e385. Ascard, J., 1995. Effects of flame weeding on weed species at different developmental stages. Weed Res. 35, 397e411. Bond, W., Grundy, A.C., 2001. Non-chemical weed management in organic farming systems. Weed Res. 41, 383e405. Flame Engineering, 2007. Red Dragon Liquid Torches. Web page:. Flame Engineering Inc., LaCrosse, KS, USA http://www.flameengineering.com/Liquid_Burners.html (accessed 08.04.10). Hansen, R., 2009. Popcorn Profile. Agricultural Marketing Resource Center (AgMRC). Web page: http://www.agmrc.org/commodities__products/grains__ oilseeds/corn/popcorn_profile.cfm (accessed 07.04.10). Heiniger, R.W., 1998. Controlling weeds in organic crops through the use of flame weeders. Web page: http://ofrf.org/funded/reports/heiniger_94-43.pdf (accessed 30.03.10.). Knezevic, S.Z., Sikkema, P.H., Tardif, F., Hamill, A.S., Chandler, K., Swanton, C.J., 1998. Biologically effective dose and selectivity of RPA 201772 (isoxaflutole) for preemergence weed control in corn (Zea mays). Weed Technol. 12, 670e676.
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501 Knezevic, S.Z., Ulloa, S.M., 2007. Flaming: potential new tool for weed control in organically grown agronomic crops. J. Agric. Sci. 52, 95e104. Knezevic, S.Z., Dana, L., Scott, J.E., Ulloa, S.M., 2007a. Building a research flamer. North Central Weed Sci. Soc. 62, 32. Abstract. Knezevic, S.Z., Streibig, J.C., Ritz, C., 2007b. Utilizing R software package for doseeresponse studies: the concept and data analysis. Weed Technol. 21, 840e848. Knezevic, S.Z., Datta, A., Ulloa, S.M., 2009a. Growth stage impacts tolerance to broadcast flaming in agronomic crops. In: Proceedings of the 8th European Weed Research Society Workshop on Physical and Cultural Weed Control, Zaragoza, Spain, pp. 86e91. Knezevic, S.Z., Costa, C.M., Ulloa, S.M., Datta, A., 2009b. Response of corn (Zea mays L.) types to broadcast flaming. In: Proceedings of the 8th European Weed Research Society Workshop on Physical and Cultural Weed Control, Zaragoza, Spain, pp. 92e97. Lague, C., Gill, J., Peloquin, G., 2001. Thermal control in plant protection. In: Vincent, C., Panneton, B., Fleurat-Lessard, F. (Eds.), Physical Control Methods in Plant Protection. Springer-Verlag, Germany, pp. 35e46. Leroux, G.D., Douheret, J., Lanouette, M., 2001. Flame weeding in corn. In: Vincent, C., Panneton, B., Fleurat-Lessard, F. (Eds.), Physical Control Methods in Plant Protection. Springer-Verlag, Berlin, Germany, pp. 47e60. Magani, I.E., 1990. Effects of Period of Weed Interference and Season Long Weed Control in Maize (Zea mays L.). M.Sc. Thesis, Ahmadu Bello University, Zaria, pp. 93. McWilliams, D.A., Berglund, D.R., Endres, G.J., 1999. Corn growth and management quick guide. Web page: http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a1173/ a1173w.htm (accessed 08.04.10.). Nemming, A., 1994. Costs of flame cultivation. Acta Hort. 372, 205e212. Parish, S., 1990. A review of non-chemical weed control techniques. Biol. Agric. Hort. 7, 117e137. Pelletier, Y., McLeod, C.D., Bernard, G., 1995. Description of sub-lethal injuries caused to the Colorado potato beetle by propane flamer treatment. J. Econ. Entomol. 88, 1203e1205.
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The Popcorn Board, 2010. Industry facts. Web page: http://www.popcorn.org/ EncyclopediaPopcornica/WelcometoPopcornica/IndustryFacts/tabid/108/ Default.aspx (accessed 07.04.10.). R Development Core Team, 2006. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria. Rifai, M.N., Lacko-Bartosova, M., Puskarova, V., 1996. Weed control for organic vegetable farming. Rostl. Vyroba 42, 463e466. Seefeldt, S.S., Jensen, J.E., Fuerst, E.P., 1995. Log-logistic analysis of herbicide doseeresponse relationships. Weed Technol. 9, 218e227. Statistical Analysis Systems (SAS), 2005. Version 9.1. The GLIMMIX Procedure. Online Documentation. SAS Institute, Inc., Cary, NC, USA. Teixeira, H.Z., Ulloa, S.M., Datta, A., Knezevic, S.Z., 2008. Corn (Zea mays) and soybean (Glycine max) tolerance to broadcast flaming. RURALS Web page: http://digitalcommons.unl.edu/rurals/vol3/iss1/1 (accessed 08.04.10.). Ulloa, S.M., Datta, A., Knezevic, S.Z., 2010a. Growth stage impacts tolerance of winter wheat (Triticum aestivum L.) to broadcast flaming. Crop Prot 29, 1130e1135. Ulloa, S.M., Datta, A., Knezevic, S.Z., 2010b. Growth stage influenced differential response of foxtail and pigweed species to broadcast flaming. Weed Technol.. doi:10.1614/WT-D-10-00005.1. Ulloa, S.M., Datta, A., Knezevic, S.Z., 2010c. Tolerance of selected weed species to broadcast flaming at different growth stages. Crop Prot. doi:10.1016/j/ cropro.2010.07.017. Ulloa, S.M., Datta, A., Malidza, G., Leskovsek, R., Knezevic, S.Z., 2010d. Timing and propane dose of broadcast flaming to control weed population influenced yield of sweet maize (Zea mays L. var. rugosa). Field Crops Res. 118, 282e288. US Food and Drug Administration (FDA), 2009. Total diet study. Web page: http:// www.fda.gov/Food/FoodSafety/FoodContaminantsAdulteration/TotalDietStudy/ ucm184970.htm (accessed 07.04.10.). Wszelaki, A.L., Doohan, D.J., Alexandrou, A., 2007. Weed control and crop quality in cabbage [Brassica oleracea (capitata group)] and tomato (Lycopersicon lycopersicum) using a propane flamer. Crop Prot. 26, 134e144.
Contents lists available at ScienceDirect
Crop Protection journal homepage: www.elsevier.com/locate/cropro
Popcorn (Zea mays L. var. everta) yield and yield components as influenced by the timing of broadcast flaming Santiago M. Ulloa a, Avishek Datta a, Sidnei D. Cavalieri b, Mario Lesnik c, Stevan Z. Knezevic a, * a
Department of Agronomy and Horticulture, University of Nebraska, Northeast Research and Extension Center, 57905 866 Road, Concord, NE 68728-2828, USA Faculty of Agronomic Sciences, Sao Paulo State University, Botucatu, Sao Paulo, Brazil c Department of Plant Protection, Faculty of Agriculture and Life Sciences, University of Maribor, Pivola 10, 2311 Hoce, Slovenia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2010 Received in revised form 11 August 2010 Accepted 13 August 2010
Farmers are interested in producing popcorn under organic production systems and propane flaming could be a significant component of an integrated weed management program. The objective of this study was to collect baseline information on popcorn tolerance to broadcast flaming as influenced by propane dose and crop growth stage at the time of flaming. Field experiments were conducted at the Haskell Agricultural Laboratory of the University of Nebraska, Concord, NE in 2008 and 2009 using five propane doses (0, 13, 24, 44 and 85 kg ha1) applied at the 2-leaf, 5-leaf and 7-leaf growth stages. Propane was applied using a custom-built research flamer driven at a constant speed of 6.4 km h1. Crop response to propane dose was described by log-logistic models on the basis of visual estimates of crop injury, yield components (plants m2, ears plant1, kernels cob1 and 100-kernel weight) and grain yield. Popcorn response to flaming was influenced by the crop growth stage and propane dose. Based on various parameters evaluated, popcorn flamed at the 5-leaf showed the highest tolerance while the 2-leaf was the most susceptible stage. The maximum yield reductions were 45%, 9% and 16% for the 2-leaf, 5-leaf and 7-leaf stages, respectively. In addition, propane doses that resulted in a 5% yield loss were 23 kg ha1 for the 2-leaf and 7-leaf and 30 kg ha1 for the 5-leaf stage. Flaming has a potential to be used effectively in organic popcorn production if properly used. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Doseeresponse curves Crop tolerance Non-chemical weed control Organic crop production
1. Introduction Popcorn (Zea mays L. var. everta) is one of the most popular snacks in the United States (Hansen, 2009). Since the year 2000, the US popcorn industry reported the steady sales of as much as $1 billion a year (The Popcorn Board, 2010). Nebraska is in the top popcorn producing state with 135,000 tonnes of shelled popcorn production, which was 34% of the total popcorn produced in the US in 2007 (Hansen, 2009). The use of agricultural chemicals in popcorn production is extensive and there are reports of finding pesticide residues in popcorn products. For example, the US Food and Drug Administration found samples of conventionally produced popcorn contaminated with phenylurea herbicides and other pesticides (FDA, 2009). Therefore, many farmers are now interested in growing popcorn under organic production, which is the fastest growing segment of the US agriculture. The sales of organic products have shown an annual growth rate of 15e20% during the last 10 years (Anonymous, 2007). * Corresponding author. Tel.: þ1 402 584 3808; fax: þ1 402 584 3859. E-mail address: [email protected] (S.Z. Knezevic). 0261-2194/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2010.08.011
Organic farmers rank weed control as the major factor limiting crop yields and quality (Wszelaki et al., 2007). For example, weed competition in non-chemical cropping systems reduced maize (Z. mays L.) yield from 50% to 87% (Magani, 1990). In order to achieve acceptable weed control levels in organic farming, the combination of many strategies has to be implemented (Wszelaki et al., 2007). Propane flaming could be a potential alternative tool in the toolbox of integrated weed management system (Knezevic and Ulloa, 2007; Ulloa et al., 2010a,b,c,d). Flame weeding with propane is an acceptable weed control option in organic production and has received the renewed interest for both organic and conventional systems (Bond and Grundy, 2001). Flaming controls weeds by heating plant tissue rather than burning it (Leroux et al., 2001). Propane torches can generate combustion temperatures of up to 1900 C, which rapidly increase the temperature inside the plant. An increase of temperature above 50 C inside the plant cells can result in the coagulation of membrane proteins leading to the loss of membrane integrity (Parish, 1990; Pelletier et al., 1995; Rifai et al., 1996; Lague et al., 2001). Consequently, flamed weeds die or their competitive ability against the crop is severely reduced. Flaming could be implemented
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501
as an alternative weed control method to reduce the reliance on herbicides, and other physical weed control methods such as hand weeding and mechanical cultivation. Flaming does not leave any chemical residues on plants, soil, air or water, produces no drift hazards or herbicide carry-over to the next season. Flaming can control herbicide-tolerant or resistant weeds and is considerably less costly than hand weeding (Nemming, 1994; Wszelaki et al., 2007). Crop tolerance to flaming varies with species and growth stages (Knezevic and Ulloa, 2007; Knezevic et al., 2009a,b; Ulloa et al., 2010a,b,c,d). Knezevic and Ulloa (2007) reported that grass crop species (e.g., maize and sorghum [Sorghum bicolor (L.) Moench]) exhibited higher tolerance to flaming than broadleaf species [e.g., soybean (Glycine max L.)] when treated at early growth stages. Teixeira et al. (2008) observed quick recovery of maize plants flamed at the 5-leaf stage and concluded that flaming can safely be used in maize production as an alternative weed control method. Heiniger (1998) compared flaming to mechanical cultivation with respect to overall weed control and grain yield in popcorn and reported better weed control with flaming than cultivation, but there was no difference in popcorn yields. Little information is available on the response of popcorn to broadcast flaming at different growth stages. To optimize the use of propane flaming as a weed control tool, the biologically effective dose (ED) of propane for popcorn tolerance needs to be determined. Therefore, the objective of this study was to assess popcorn tolerance to broadcast flaming as influenced by the crop growth stage and propane dose. 2. Materials and methods 2.1. Site description Field experiments were conducted in 2008 and 2009 at the Haskell Agricultural Laboratory of the University of Nebraska located near Concord, NE (42.37 N, 96.68 W). The soil type was an Alcester series silty clay loam (fine-silty, mixed, mesic, Cumulic Haplustolls) with 3.6% organic matter and pH of 6.6. The previous crop was soybean and the residues were removed before popcorn planting. 2.2. Experimental details One of the commonly grown organic popcorn hybrids in Nebraska, A448 White, was sown no-till on June 9 in 2008 and on June 2 in 2009 with a four-row planter. The experimental plot size was 10 m by 3 m. The row spacing was 76 cm and the seeding rate was 70,000 seeds ha1. The plots were kept weed-free during the entire growing season by hoeing and hand weeding. The experimental layout was a split-plot design with three replications where the main plot was popcorn growth stage at the time of flaming and the sub-plot was propane dose. The treatments consisted of an unflamed check and four propane doses applied at three different growth stages. Flaming was conducted on June 23, July 3 and July 15, which corresponded to the 2-leaf, 5-leaf and 7-leaf growth stages in 2008. In 2009, popcorn was flamed on June 17, June 30 and July 7, which corresponded to the same growth stages flamed in 2008. Flaming treatments were conducted with a small custom-built research flamer which delivered open flames and was mounted on the back of a four-wheeler (Knezevic et al., 2007a). There were four burners “LT 2 8 Liquid Torch” (Flame Engineering, 2007) mounted 76 cm apart and adjusted to flame right above the crop rows. The bottom of the burner cup was positioned 20 cm above the soil surface and angled back at 30 . Such set up provided 25 cm wide-open flames over each of the four crop rows in order to purposely cause popcorn
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injury, thus we could test crop tolerance to broadcast flaming. The flaming treatments were conducted parallel to the crop rows. To deliver the appropriate propane dose, the flamer was calibrated before the treatments were applied. The calibration was done by taking into account the propane pressure and application speed (Knezevic et al., 2007a). The application speed was 6.4 km h1, and the propane pressure was adjusted to deliver propane doses of 0, 13, 24, 44 and 85 kg ha1. Flaming treatments were conducted only when the air temperatures were between 15 and 25 C and the wind speed was less than 10 km h1. 2.3. Measurements Popcorn injury was evaluated visually at 1, 7, 14 and 28 days after treatment (DAT) using a scale from 0 to 100%, with 0 representing no crop injury (based on the unflamed plots) and 100 representing crop death. For yield components, two linear meters of the center two rows of each plot were hand harvested and number of plants m2, ears plant1, kernels cob1 and 100-kernel weight were measured. Yield was measured at crop maturity by harvesting the 4 m length of the center two rows of each plot. Yields were adjusted to 16% moisture. 2.4. Statistical analysis ANOVA was performed using PROC GLIMMIX procedure in SAS (SAS Institute, 2005) to test the significance (P < 0.05) of years, treatments, replications and their interactions on the basis of the visual crop injury ratings, yield components and yield data. There was no treatment-by-year interaction; thus, the data were combined over years. There was a significant growth stage and propane dose interaction; therefore, the data were presented separately for each growth stage. Yield components and yield data were analyzed using the four-parameter log-logistic model (Seefeldt et al., 1995):
Y ¼ C þ ðD CÞ=f1 þ exp½Bðlog X log EÞg
(1)
where Y is the response (e.g., popcorn yield), C is the lower limit (e.g., minimum yield), D is the upper limit (e.g., maximum yield), X is the propane dose, E is the dose giving a 50% response between the upper and lower limit (also known as inflection point, I50 or ED50) and B is the slope of the line at the inflection point (also known as a rate of change). Plant injury based on visual ratings and yield loss data were analyzed utilizing the three-parameter log-logistic model where the C term was fixed to zero (Seefeldt et al., 1995):
Y ¼ D=f1 þ exp½Bðlog X log EÞg
(2)
Curve fitting was done by non-linear regression using the least squares method. All statistical analysis and graphs were performed with R program (R Development Core Team, 2006) utilizing the doseeresponse curves (drc) statistical addition package (Knezevic et al., 2007b). The values of VI5 (effective dose that provides a 5% visual crop injury) or YR5 (effective dose that provides a 5% yield reduction) and VI10 or YR10 (10% visual crop injury or 10% yield reduction) were determined from the curves and used as measures of the level of crop damage by flaming treatments. A yield reduction of 5% was arbitrarily assigned as the value above which the loss was considered unacceptable (Knezevic et al., 1998). Based on our previous research, a propane dose of 60 kg ha1 was highly effective in providing 80e90% control of many grasses and broadleaf weeds (Ulloa et al., 2010b,c). Therefore, the predicted yield loss with a propane dose of 60 kg ha1 was also calculated (Table 3). It was not possible to calculate the propane doses that can
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cause a 5% or 10% reduction at each growth stage due to a small effect of flaming treatments on most yield components (Table 2). A lack-of-fit test at the 95% level was not significant for any of the doseeresponse curves (Figs. 1e3) tested indicating that the loglogistic models were appropriate (Knezevic et al., 2007b). Differences between the equation parameters for each combination of growth stage and propane dose were also determined by comparing the standard errors (SE) and t and F tests at the 5% significance level.
3. Results 3.1. Crop injury Visual crop injury symptoms included initial whitening and then browning of leaves. Stunting of growth was especially evident when the plants were flamed with higher propane doses. Most visual crop injuries, however, were transient as plants appeared to be visually recovered within few weeks. Visual crop injury was influenced by the interaction among growth stage, propane dose and time of ratings (Fig. 1). Based on the maximum visual crop injury obtained at 28 DAT (upper limit of the curve, Table 1), the 7leaf was the least affected stage to broadcast flaming. For example, the 7-leaf stage presented the lowest maximum injury (15%) followed by the 2-leaf (21%) and 5-leaf (23%) stages. Similar trends occurred when popcorn injury was compared to other stages across all evaluation dates. In general, the propane dose that caused either a 5% or 10% visual crop injury (VI5 or VI10) did not vary among growth stages. For example, at 28 DAT the amount of propane needed to cause a 5% or 10% injury was similar across the growth stages and stayed around 28 and 45 kg ha1 of propane for VI5 and VI10, respectively (Table 1). Similar trends occurred for other evaluation dates (1, 7 and 14 DAT).
Crop injury decreased from 1 DAT to 28 DAT (upper limit of the curve, Table 1). For example, about 75% injury at the 2-leaf stage was evident with the highest propane dose of 85 kg ha1 at 1 DAT compared to 21% injury at 28 DAT, suggesting that popcorn plants were able to recover over time. Plants flamed at the 5-leaf and 7-leaf stages showed similar trends (Fig. 1).
3.2. Yield and yield components Popcorn yield of the unflamed plots (control) and the plots treated with low propane doses (e.g., 0e24 kg ha1) averaged around 3.9 (0.1) t ha1, irrespective of the growth stage of flaming (Fig. 2). However, popcorn yields were reduced significantly when treated with higher propane doses (e.g., 44 and 85 kg ha1), especially when flaming was conducted at the 2-leaf stage (Table 2). Yields were 2.5 (0.2), 3.6 (0.1) and 3.5 (0.4) t ha1 for the 2-leaf, 5-leaf and 7-leaf stages, respectively, when flamed with the highest propane dose of 85 kg ha1 (lower limit of the curve, Table 2). These results showed that the 5-leaf and 7-leaf showed higher tolerance to propane flaming than the 2-leaf stage. Most yield components of popcorn were negatively affected by increasing propane dose with the exception of ears plant1 (data not shown). Number of plants m2 and 100-kernel weight were the most affected by broadcast flaming followed by kernels cob1 (Fig. 2). Plots flamed at the 2-leaf stage resulted in the highest reduction in the number of plants m2 (Table 2). The average number of plants m2 of the unflamed plots was 5.3 compared to 4.3 for the 2-leaf and 5.0 for the 5-leaf and 7-leaf stages with the highest propane dose suggesting that the 2-leaf stage was the most susceptible stage to broadcast flaming. Number of kernels cob1 was significantly reduced with increasing propane dose across all flaming stages (Fig. 2). Kernels cob1 averaged about 573 in the unflamed plots, compared
1 DAT 100
2-leaf 5-leaf 7-leaf
80 I njury (% )
80 I njury (% )
7 DAT 100
60 40
60 40
20
20
0
0 0
20
40
60
80
100
0
20
Propane dose (kg ha )
80
80
60 40
40 20
0
0 60
100
60
20
40
80
28 DAT 100
I njury (% )
I njury (% )
14 DAT
20
60
Propane dose (kg ha )
100
0
40
−1
−1
80 −1
Propane dose (kg ha )
100
0
20
40
60
80
100
−1
Propane dose (kg ha )
Fig. 1. Popcorn injury at 1, 7, 14 and 28 days after treatment (DAT) as influenced by growth stage at the time of flaming. The regression lines were plotted using equation (2), and the parameter values are reported in Table 1.
4.0
6
3.5
5 Plant s m
3.0 2-leaf 5-leaf 7-leaf
2.5
4 3
2.0
2 0
20
40
60
80
100
0
−1
20
40
60
80
100
−1
Propane dose (kg ha )
Propane dose (kg ha )
600 100-k ernel weight (g)
14.0
−1
580 Kernels c ob
1499
−2
−1
Yield (t ha )
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501
560 540 520 500
13.5 13.0 12.5 12.0 11.5 11.0
0
20
40
60
80
100
0
−1
40
60
80
100
−1
Propane dose (kg ha ) 2
20
Propane dose (kg ha )
1
Fig. 2. Popcorn yield and its components (plants m , kernels cob and 100-kernel weight) as influenced by growth stage at the time of flaming. The regression lines were plotted using equation (1), and the parameter values are presented in Table 2.
to significantly lower kernel number of 513, 547 and 531 in plants flamed with the highest propane dose (85 kg ha1) at the 2-leaf, 5leaf and 7-leaf stages, respectively (lower limit of the curve, Table 2). The weight of 100-kernel was negatively affected with increasing propane dose at all flaming stages (Fig. 2). Again, the 2leaf stage presented the largest 100-kernel weight reduction (Table 2). The average 100-kernel weight was 13.2 g in the unflamed plots compared to significantly lower 100-kernel weight of 11.0, 12.6 and 12.4 g from plant treated with the highest propane dose (85 kg ha1) at the 2-leaf, 5-leaf and 7-leaf stages, respectively.
35 2-leaf 5-leaf 7-leaf
30
In general, yield loss increased with increase in propane dose regardless of the growth stage (Fig. 3). Overall, plants flamed at the 5-leaf stage exhibited the lowest yield reduction followed by the 7-leaf and 2-leaf stages. The maximum yield reductions with the highest propane dose of 85 kg ha1 were 9%, 16% and 45% for the 5-leaf, 7-leaf and 2-leaf stages, respectively (upper limit of the curve, Table 3). In addition, propane doses that resulted in a 5% yield reduction were 23 kg ha1 for the 2-leaf and 7-leaf and 30 kg ha1 for the 5-leaf stage indicating that popcorn flamed at the 5-leaf stage can tolerate higher dose of propane for the same yield reduction compared to other growth stages. It is important to mention that none of the propane doses tested in these experiments caused a 10% yield reduction when popcorn was flamed at the 5-leaf stage. These Table 1 Propane doses (kg ha1) that resulted in 5% and 10% visual crop injury [VI5 and VI10 (SE)] at three growth stages of popcorn at 1, 7, 14 and 28 days after treatment (DAT) (Fig. 1). Regression parameters are estimated using equation (2).
25 Yield loss (%)
3.3. Yield loss
20
40
60
80
100
-1
Fig. 3. Popcorn yield loss (%) as a function of propane dose and growth stage at the time of flaming. The regression lines were plotted using equation (2), and the parameter values are reported in Table 3.
(4) (6) (8) (5)
14 27 33 42
(5) (6) (8) (4)
29 41 40 45
(4) (5) (5) (7)
6 17 21 28
(2) (5) (6) (9)
10 24 30 41
(3) (6) (6) (8)
27 31 23 36
(3) (5) (4) (5)
9 14 13 25
(3) (5) (3) (4)
14 25 30 52
(3) (5) (5) (7)
1 7 14 28
2.1 3.1 2.7 3.7
(0.6) (1.0) (1.1) (1.2)
75 55 33 21
(8) (4) (3) (2)
35 44 45 44
5-leaf
1 7 14 28
1.6 2.3 2.4 2.8
(0.4) (0.7) (0.8) (1.6)
70 43 30 23
(5) (3) (3) (3)
7-leaf
1 7 14 28
1.9 1.8 1.5 1.9
(0.4) (0.6) (0.4) (0.5)
43 25 23 15
(3) (3) (3) (1)
0
Propane dose (kg ha )
10 21 24 31
2-leaf 10
20
(5) (4) (6) (4)
Regression parameters (SE) B
0
VI10 (SE)
DAT
15
5
VI5 (SE)
Growth stage
D
I50
B: the slope of the line at the inflection point; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit.
1500
S.M. Ulloa et al. / Crop Protection 29 (2010) 1496e1501
Table 2 Regression parameters for popcorn yield and its components (plants m2, kernels cob1 and 100-kernel weight) as influenced by propane dose and growth stage of flaming (Fig. 2). Regression parameters are estimated using equation (1). Variable
Growth stage Regression parameters (SE) B
Yield (t ha1)
Plants m2
Kernels cob1
C
D
I50
2-leaf 5-leaf 7-leaf
6.6 (8.0) 2.0 (0.6) 1.3 (0.9)
2.5 (0.2) 3.6 (0.1) 3.5 (0.4)
3.9 (0.1) 50 (7) 4.0 (0.1) 27 (6) 4.0 (0.1) 29 (20)
2-leaf 5-leaf 7-leaf
1.6 (0.6) 18.8 (2.3) 18.1 (2.3)
4.3 (0.6) 5.0 (0.1) 5.0 (0.1)
5.3 (0.1) 19 (5) 5.3 (0.1) 33 (1) 5.3 (0.1) 18 (1)
2-leaf 5-leaf 7-leaf
1.9 (1.3) 1.4 (0.6) 3.1 (0.4)
513 (15) 547 (7) 531 (13)
571 (8) 24 (10) 576 (12) 22 (5) 572 (14) 16 (2)
100-kernel weight (g) 2-leaf 5-leaf 7-leaf
2.1 (0.9) 11.0 (0.4) 13.1 (0.3) 32 (10) 9.6 (19.2) 12.6 (0.5) 13.3 (0.1) 25 (4) 3.7 (1.8) 12.4 (0.5) 13.2 (0.1) 32 (8)
B: the slope of the line at the inflection point; C: the lower limit; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit.
results showed that the 5-leaf was the most tolerant while the 2-leaf was the most susceptible stage to broadcast flaming.
4. Discussion These experiments showed that growth stage and propane dose influenced popcorn sensitivity to broadcast flaming. These findings support previous findings that have shown that plant susceptibility to flaming varied among species and their growth stages (Ascard, 1994; Knezevic et al., 2009a,b; Ulloa et al., 2010a,b,c,d). In general, broadleaf species were more susceptible to flaming than grasses regardless of growth stage (Knezevic et al., 2009a,b; Ulloa et al., 2010b,c). In our study, popcorn plants showed maximum crop injury of 21% and 23% at the 2-leaf and 5-leaf stages, respectively, compared to much lower injury of 15% at the 7-leaf stage at 28 DAT and recovered over time after flaming treatments. Our results are similar to those of Wszelaki et al. (2007) and Ulloa et al. (2010a,b,c,d), who also reported greater crop injury in flamed plants at early evaluation dates compared to later rating dates. Similar results were also reported in field maize and sweet maize (Z. mays L. var. rugosa) (Teixeira et al., 2008; Ulloa et al., 2010d). The susceptibility of plants to flaming largely depends on their heat avoidance, heat tolerance, or both (Ascard, 1995). The extent to which the heat from the flames affects plants depend on the flaming technique and leaf surface moisture (Parish, 1990). The tolerance of different plant parts to heat injury can be influenced by several other factors, including the presence of protective layers of hair, wax, lignification, external and internal water status of the plant and the species regrowth potential (Ascard, 1995). The effect of broadcast flaming on different yield components varied with crop growth stages at the time of flaming. For example, plants flamed at the 2-leaf stage had the highest yield loss and the lowest yield components. This might be explained by the fact that the ear and tassel tissues are not differentiated at the 2-leaf stage (McWilliams et al., 1999), thus exposing the plants to the stress from heat can result in potentially shorter cobs. In comparison, flaming popcorn plants at later growth stages (e.g., 5-leaf or 7-leaf), had less effects on cob size as the ear and tassel tissues start to differentiate at the 5-leaf stage and by the 7-leaf stage, cob and tassel sizes are already pre-determined (McWilliams et al., 1999). Response of popcorn to broadcast flaming from our study is similar to the previous findings of Ascard (1994) and Ulloa et al. (2010a,b,c,d), who reported that plant size at the time of flaming had an important influence on plant sensitivity, with smaller plants
Table 3 Regression parameters and propane doses (kg ha1) that caused 5% and 10% yield reduction [YR5 and YR10 (SE)] for each growth stage of popcorn (Fig. 3). Regression parameters are estimated using equation (2). The predicted yield loss (%) with a propane dose of 60 kg ha1 is represented by PR60 for each growth stage. Growth stage
2-leaf 5-leaf 7-leaf
Regression parameters (SE) B
D
I50
2.0 (1.1) 1.9 (0.4) 1.2 (0.6)
45.1 (4.3) 9.4 (3.9) 16.3 (2.2)
65 (8) 28 (3) 45 (5)
YR5 (SE)
YR10 (SE)
PR60b
23 (3) 30 (3) 23 (2)
35 (5) NAa 67 (8)
20.7 7.7 9.4
B: the slope of the line at the inflection point; D: the upper limit; I50: the dose giving a 50% response between the upper and lower limit. a Plants flamed at the 5-leaf stage did not reach 10% yield reduction level. b Based on previous research, a propane dose of 60 kg ha1 was highly effective in providing 80e90% control of many grasses and broadleaf weeds (Ulloa et al., 2010b,c).
being more susceptible than larger ones. Smaller plants are more heat sensitive because they have thinner leaves, lower biomass and the meristems are fully exposed (Ascard, 1995). In contrast, larger plants have larger and thicker leaves, the meristems are well protected by the surrounding leaves and they possess larger amounts of reserve food (soluble sugars, proteins and lipids) in the stems and the roots providing the plant with the increased capacity for regrowth (Ascard, 1995). Based on our previous studies conducted to determine the response of various weed species to broadcast flaming, a propane dose of 60 kg ha1 provided up to 80e90% control of many broadleaf and grass species (Ulloa et al., 2010b,c). From the practical point of view, such propane dose resulted in an 8% and 9% yield reduction at the 5-leaf and 7-leaf stages, respectively, which could not be acceptable by organic farmers. These yield reductions are the result of our intentional flaming under weed-free conditions where torches were positioned directly over the crop. However, we believe that positioning flames below the popcorn canopy would reduce the exposure time to the heat and therefore should reduce popcorn yield losses. Future studies are needed to test such hypotheses. Acknowledgments We thank the Propane Education and Research Council (PERC) and the Nebraska Propane Association for the partial financial support of this work. We are also grateful for the help provided by Jon Scott, Goran Malidza, Marco Fontanelli, Nicoletta Nasso and Patrick Gunther. References Anonymous, 2007. Organic Crop Production. University of Kentucky Cooperative Extension Service. Web page: http://www.uky.edu/Ag/NewCrops/introsheets/ organicproduction.pdf (accessed 07.04.10). Ascard, J., 1994. Doseeresponse models for flame weeding in relation to plant size and density. Weed Res. 34, 377e385. Ascard, J., 1995. Effects of flame weeding on weed species at different developmental stages. Weed Res. 35, 397e411. Bond, W., Grundy, A.C., 2001. Non-chemical weed management in organic farming systems. Weed Res. 41, 383e405. Flame Engineering, 2007. Red Dragon Liquid Torches. Web page:. Flame Engineering Inc., LaCrosse, KS, USA http://www.flameengineering.com/Liquid_Burners.html (accessed 08.04.10). Hansen, R., 2009. Popcorn Profile. Agricultural Marketing Resource Center (AgMRC). Web page: http://www.agmrc.org/commodities__products/grains__ oilseeds/corn/popcorn_profile.cfm (accessed 07.04.10). Heiniger, R.W., 1998. Controlling weeds in organic crops through the use of flame weeders. Web page: http://ofrf.org/funded/reports/heiniger_94-43.pdf (accessed 30.03.10.). Knezevic, S.Z., Sikkema, P.H., Tardif, F., Hamill, A.S., Chandler, K., Swanton, C.J., 1998. Biologically effective dose and selectivity of RPA 201772 (isoxaflutole) for preemergence weed control in corn (Zea mays). Weed Technol. 12, 670e676.
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