POTENTIAL NEW TOOL FOR WEED CONTROL IN ORGANICALLY GROWN AGRONOMIC CROPS

Field experiments were conducted during summer 2007 to determine a baseline information on crop and weed tolerance to broadcast flaming utilizing different rates of propane. The species evaluated were: maize (Zea mays), sorghum (Sorghum halepense), soybean (Glycine max), sunflower (Helianthus annuus), barnyardgrass (Echinocloa crus-galli), green foxtail (Setaria viridis), velvetleaf (Abutilon theophrasti) and redroot pigweed (Amaranthus retroflexus). The propane rates applied were 0, 12.1, 30.9, 49.7, 68.5 and 87.22 kg/ha. The response of the plants to propane rates were described by log-logistic models. Plant response to flame varied depending on the species, growth stage and propane rate. Broadleaf weeds were more susceptible to flames than grasses. Field maize and sorghum were less susceptible, while soybean and sunflower were severely injured. Of all crops tested, broadcast flaming has the most potential for use in field maize.


I n t r o d u c t i o n
There is an increased interest in organic production among farmers and industries in the United States.This interest is based on the increasing consumer demand for organic foods (J o h n s o n 2005).Organic producers rank weeds as their number one problem (W a l z , 1999).Hand weeding and cultivation are the most popular physical methods for weed control utilized by organic growers.However, hand weeding had become cost prohibitive, while repeated cultivation increases the chance of soil erosion and promotes emergence of new weed flushes (W s z e l a k i et al,.2007; R e i m e n s et, al. 2007).In addition, there are only few organic herbicides approved for use in organic production, and they are cost prohibitive.Therefore, there is a need to evaluate various alternative and integrated methods of weed control.The use of propane for flame weeding could be one of the alternative control methods for weed control in organically grown field crops.
During flaming process, the heat from the flame is transferred to the plant tissues increasing the thermal energy of the plant cells (L a g u e et al.,.2000) and resulting in coagulation of cell proteins if the temperature is above 50C (Parish 1990).Furthermore, exposing plant tissue to a temperature of about 100C for a split second (eg.0.1 second) can result in cell membrane rupture (P e l l e t i e r et al., 1995; M o r e l l e 1993), resulting in loss of water and plant death (R i f a i et al., 1996).
In order to optimize use of flame as a weed control tool, the biologically effective dose (ED) of propane for control of major weed species and tolerance of major crops must be determined.Depending on the desired level of weed control, or tolerable crop injury level, a propane dose could be selected to either control the weed, or reduce its growth thereby offsetting its competitive ability against the crop.Goal of our experiments in 2007 was to develop baseline information on crop and weed tolerance to broadcast flaming.The specific objective was to develop dose response curves for propane on several major weeds and field crops in Nebraska.

Materials and Methods
Field trials were conducted in 2007 at the Haskell Ag Lab in north east Nebraska, and will be repeated in 2008 and 2009.Field site was cultivated on June 15.Seeds of 17 crop and weed species were planted on June 29, as a single row for each species, using push-type planters.In this manuscript we are reporting data for four crops and four weed species, including: field maize (Zea mays), sorghum (Sorghum halepense), soybean (Glycine max), sunflower (Helianthus annuus), barnyardgrass (Echinocloa crus-galli), green foxtail (Setaria viridis), velvetleaf (Abutilon theophrasti) and redroot pigweed (Amaranthus retroflexus).Volunteer weeds were controlled by hand weeding.Field was irrigated as needed to obtain species emergence.
All species were treated with broadcast flaming.Flaming was conducted utilizing a custom built flamer mounted on a four-wheeler, which was driven across all 18 rows of test species.Flamer provided open flames using propane as source of combustion.There were four burners (LT 2x8) (F l a m e E n g i n e e r i n g , 2007) mounted 30 cm apart.Burners were positioned 18 cm above soil surface and angled at 30˚ to the soil.Flamer was calibrated on June 19 th , 2007 at 24C and 59% relative humidity.Flaming treatments were applied using a constant speed of about 6 km/h.Varying propane pressures included: 0, 68.9, 206.8, 344.7, 482.6 and 620.2 kPa, corresponding to 0, 10, 30, 50, 70 and 90 PSI, or the rates of propane of 0, 12.1, 30.9, 49.7, 68.5 and 87.22 kg/ha, respectively.
Experiment was set-up in a randomized complete block design with six treatments (propane rates) and three replications.Each plot was 7.6 by 2.1 m, with 17 different species planted in each plot in 38cm row spacing.Visual ratings of percent weed control, or crop injury, was conducted at approximately 3 hours, 1 day after treatment (DAT), 3DAT, 7DAT and 14DAT, and it was based on a scale from 0 to 100 (where 0 = no injury and 100 = plant death).In this manuscript, however, only data collected 1DAT and 14DAT were presented.Dry matter data was also collected at 14 DAT, but not reported in this manuscript.
Whole experiment was repeated by following the same procedures and planting the exact set of plant species on July 6 th .There was no significant difference between the two identical experiments, based on data analysis in SAS, thus data was combined and the means further utilized in the regression analysis.
Data were fit to a log-logistic function with four-parameter (K n e z e v i c et al. 2007): Where c is the lower limit, d is the upper limit, b is the slope and e is the value of x also known as ED50.Curve fitting was done by non-linear regression using the least square method.All statistical analysis and graphs were performed with R program utilizing the Dose Response Curves (drc) statistical addition package (K n e z e v i c et al., 2007).The values of ED50, ED80 (80% control) and ED90 (90% control) were determined from the curves and utilized as a measure of the level of weed control by flaming.The ED90 is a standard parameter widely utilized to describe weed response to herbicides (K n e z e v i c et al. 2007).Nevertheless, in organic cropping systems a goal of 90% control might be challenging to achieve.It should be noted that physical control methods (eg.flaming, cultivating) may not be as efficient as conventional chemical methods.For this reason, ED50 and ED80 rates were obtained (Table 1).Crop tolerance to flaming was compared among species utilizing values for ED5 (5% injury), ED10 (10% injury) and ED20 (injury) for each species.

Weed control by flaming:
In general, the level of weed control was influenced by the species, their growth stage at the time of flaming, and propane rate.Overall, grasses were harder to control than broadleaf species.Barnyardgrass and green foxtail required higher rates of propane than velvetleaf and pigweed (Figure 1).About 182 kg/ha and 153 kg/ha was needed to obtain 90% control (ED 90) in barnyardgrass and green foxtail, respectively.In contrary, only 53 kg/ha and 67 kg/ha of propane was needed to reach ED90 in velvetleaf and pigweed, respectively (Table 1).Similarly, W s z e l a k i et al., (2006) reported that grasses were more tolerant to flaming than broadleaf species.Such difference is likely a result of the physical position of the species' growing point at the time of flaming.Growing point in broadleaf species was above the ground, thus exposed to the flame.In contrary, growing point in grassy species during early growth stages was below soil surface, thus protected from the flame (A s c a r d , 1995).Growing point positioning at the time of flaming also impacts the way plant responded during the period of 2 weeks after flaming.In grassy species that were flamed at earlier growth stages (eg.growing point was below soil surface) there was significant difference between visual control ratings at 1 DAT and 14 DAT, indicating that grassy species can recover from flaming injury and continue to grow pass the 14 DAT rating.That was the case for both barnyard grass (Figure 1a) and green foxtail (Figure 1b), resulting in significantly different curves for 1 DAT compared to 14 DAT (Figure 1a and 1b).Both grassy species were able to recover at about 30DAT (data not shown).In broadleaf species that were flamed at early stage, there was no much difference between visual control ratings at 1 DAT and 14 DAT, indicating that velvetleaf (Figure 1c) and pigweed (Figure 1d) did not recover after flaming, resulting in their complete control.Furthermore, plant height can cause differential plant response after flaming, especially in grassy species.The level of control in 83 cm tall barnyardgrass (flowering stage) did not differ between 1DAT and 14DAT, resulting is similar curves (Figure 1a), suggesting that barnyardgrass might be sensitive to flaming at the flowering stage.On the other hand, the level of control of 51 cm tall green foxtail (flowering stage) was lower at the 1DAT than 14 DAT, resulting in significantly different curves (Figure 1b).Green foxtail leaves remained green for the first 3 DAT, then turned brown and died.Despite the fact that the treated leaves died, there was much new leaf growth from tillers and the plant continued to grow.Such species response to flaming is not unique.We observed similar response in wheat that was flamed at the flowering stage, resulting in severe stunting, but the stem remained green and tillers produced new leaves to support plant growth (Knezevic and Ulloa, unpublished data).
However, it is also important to notice that the plant height did not significantly impact the level of propane needed to obtain 80%, or 90%, of weed control in two out of four weed species.For example, both, the 6 cm and 83 cm tall barnyardgrass was controlled at the 80% level with about 110 kg/ha (Table 1).Similarly, both the 4 cm and 20 cm tall pigweed was controlled at the 80% level with about 45 kg/ha (Table 1).However, higher rate of propane was needed to control taller velvetleaf (Table 1).In contrary, lower rate of propane was needed to control taller green foxtail, which may indicate that green foxtail is more sensitive to flaming at the flowering stage.Such information can be useful for developing strategies for timing flaming operations to control green foxtail.It is important to notice large difference between the ED80 and ED90 values, especially for grassy weeds at both growth stages of flaming.For example, 102 kg of propane per ha was needed to obtain 80% control of barnyardgrass compared to 182 kg/ha needed for 90% control.It is indicating that 80% more propane was required to get only 10% increase in barnyard control.Similar response was observed for green foxtail, and to some extent in velvetleaf and pigweed.

Crop tolerance to flaming:
Several studies have investigated the effect of flaming on weeds (A s c a r d , 1994, 1995; L a g u e , et al., 1997).We believe that our study is one of the first that evaluated the effects of broadcast flaming in agronomic crops.In our study, crops were less susceptible to flaming than weeds.It is likely a result of the crop size (eg.plant height) at the time of flaming (Table 2).Most crops were much larger than weeds, as crops emerged earlier and grew faster, particularly during the first 3-4 weeks of growth (data no shown).Field maize and sorghum were more tolerant to flames than soybean and sunflower.The ED20 for maize was 57 while ED20 for sunflower was 46 kg/ha (Table 2).About 20% injury in soybean was achieved with 44 kg/ha.Sunflower was the most susceptible crop, a rate of 25 kg/ha was sufficient to reach ED 20 (Table 2).Soybean and sunflower, flamed at early stages (VC and VE), did not survive propane rates above 68 kg/ha, both died.Crop species also differed in their response after flaming.Field maize was the only crop that had more injury at 1DAT than 14DAT in both growth stages (V2 and V7), resulting in different curves for 1DAT and 14DAT (Figure 2a).This indicates that field maize was able to recover after flaming.In fact, maize was not killed by any of the propane rates tested in the study (data not shown).Similar response was observed in sorghum (Figure 2b).Soybean had more injuries at 1DAT than at 14 DAT but only at early stage of growth (VC), resulting in different curves between the 1DAT and 14DAT (Figure 2c), which may indicate that the VC soybean was able to recover from injuries.However, closer examination of the curves showed that soybean recovered only in the low rates of propane, while the rates above 60 kg/ha killed the crop.There was no difference in curves at the later stages in soybean indicating that soybean was not able to recover from flaming injuries.There was no difference in sunflower injuries between 1 DAT and 14 DAT when flamed at VE stage (Figure 1d), resulting in crop kill.However, when sunflower was flamed at V9 there was more injury at 1DAT than at 14 DAT, indicating that later merging sunflower was able to survive flaming treatments.Level of crop injury was impacted by the plant height in all species except in field maize.Both, 6cm and 33 cm field maize had 20% injury with about 60 kg/ha (Table 2).However, 3 cm tall sunflower had 20% injury with 25 kg of propane per hectare compared to 50kg/ha for 23 cm tall plants, indicating that taller sunflower had more tolerance to flaming.Reason for such response might be that the growing point of a taller sunflower (23cm) was positioned above the flames, as the flaming torches were placed at 18cm above soil surface.This allowed 23 cm tall sunflower to survive flaming operation, while the 3 cm tall sunflower was completely killed.
In contrary, there was a reduction in ED5, ED10 and ED20 values for sorghum and soybean with the increase in their size, indicating that taller sorghum and soybean were less tolerant to flaming.It is possible that the injuries in taller soybean resulted in much water loss, and plants were not able to recover.Such hypothesis needs to be tested.

C o n c l u s i o n s
Based on the first year of data, broadleaf weeds and broadleaf crops were more susceptible to flaming than grassy species.About 42kg/ha and 50 kg/ha was needed obtain 80% control of velvetleaf and pigweed, respectively.Higher rates of propane were needed to obtain 80% control of barnyardgrass and green foxtail at 14DAT.Despite the fact that there was 80% control of grassy species at 14DAT, there was no complete kill of barnyardgrass and green foxtail as the plants were able to continue growing several weeks after flaming.From practical standpoint, flaming has a potential to be utilized as a tool to control velvetleaf and pigweed species.Eventhough, there was no complete kill of barnyardgrass and green foxtail, flaming provided early season control of both species by severely reduce their growth, and thereby offsetting their competitive ability against the crop.
It is important to notice large difference in propane rates needed to obtain 80% compared to 90% weed control levels, especially for grassy weeds.It is suggesting that organic producers need to take into account not only the level of weed control but also the economics of control when making weed management decisions.
Crop susceptibility to propane rates varied among the specie and growth stages.Maize and sorghum were less susceptible at the very early stages likely because their growing point was below soil surface.Soybean was more tolerant to flaming at the VC stage, likely due to the fact that cotyledons may have enough food reserve to overcome loss of some surface area from flaming.Such hypothesis needs to be tested.In contrast, sunflower was less tolerant at the V2 stage, than V9 stage.Of all crops tested, broadcast flaming has the most potential for use in field maize.Temporary injury of as much as 20% in field maize was obtained with about 60 kg of propane per ha, which was sufficient to provide control of velvetleaf and pigweed.Further studies are needed to determine the level of yield reduction in maize due to 20% injury levels.Plant response to flaming for each the species evaluated in this study was unique.Plants from the same family and with similar morphological characteristics presented different response (e.g.maize and sorghum).Therefore, the ED values obtained in this experiment for field maize may not be applicable to other types of maize, including pop-corn, seed maize or silage maize.All crops we tested had various levels of susceptibility to flaming.Therefore, the only way to control weeds without causing any crop injury is to minimize crops exposure to the flames.Results of this study suggested that there is a potential for utilizing flaming as one of the tools for weed control, especially in the grassy type crops (eg.maize).However, more research is needed to evaluate flaming procedures (eg.positioning of the flame) in other grass-type crops, and various broadleaf crops.For example, flaming inter-row space, or positioning flames below the crop canopy, and away from crop's growing point, might be much safer in broadleaf crops.Studies are needed to test such hypothesis.

Fig. 1 .
Fig. 1. -Propane dose response curves for control of four weed species based on visual control ratings at 1DAT and 14DAT after flaming.Each data point represent a mean of 6 replications n=6.Data was fitted to log-logistic equations with four parameters.

Fig. 2 .
Fig. 2. -Propane dose response curves for four field crops based on visual injury ratings at 1DAT and 14DAT after flaming.Each point represent the mean of 6 replications n=6.Data were fitted using log-logistic equations with four parameters.
SPALJIVANJE PLAMENOM: POTENCIJALNO NOVO ORUĐE U KONTROLI KOROVA U ORGANSKOJ RATARSKOJ PROIZVODNJI S. Knezevic 1 i S. Ulloa 2 T a b. 2 -Dose of propane that resulted in 5%, 10% and 20% crop injury, as indicated by the respective ED values (standard errors), based on visual ratings at 14 DAT and a function of crop growth stage and plant height.