Ben Beale, Principal Agriculture Agent| bbeale@umd.edu and Alan Leslie, Agriculture Agent University of Maryland Extension
A common question asked by farmers dealing with herbicide resistant Palmer amaranth is, “What are the most effective burndown options in situations where Palmer amaranth is larger then the ideal 3-4 inch control range?” We often encounter this scenario in fields left fallow for a year, fields with delayed planting due to saturated soils, and double crop fields following wheat. In the summer of 2022, we undertook a study evaluating eight different treatments for control of larger Palmer amaranth in Southern Maryland. The study was completed at a site with a history of Glyphosate and ALS-resistant Palmer amaranth. The field was fallowed through the spring and early summer, and mowed at a 8 inch height in mid July. At the time of herbicide applications on August 4, Palmer amaranth was 5-8 inches tall and present at a high density. Most plants were not clipped (Figure 1). Germination of Palmer amaranth was delayed due to a heavy cover of winter annual weeds. Other weeds present at the site at the time of application included annual foxtail and perennial broomsedge. Enlist soybeans were planted on August 3rd in order to evaluate any potential phytotoxicity or herbicide injury. We evaluated treatments with Roundup, Liberty, Enlist and Gramoxone with some using a non ionic surfactant or crop oil as the adjuvant. We also evaluated combinations of Liberty + Roundup or Liberty + Enlist.
Figure 1. Palmer amaranth plants were 5- 8 inches tall at time of application.
A randomized complete block design with four replications was used. Plot size was 10 ft. by 30 ft., with 5 ft aisles separating plots and running checks on each side of the plots. Treatments were applied using a ATV plot boom sprayer utilizing TeeJet AIXR 11003 flat fan nozzles at 35 psi applying 20 gallons per acre of spray solution. Application was made on August 4th in the mid-afternoon with clear bright sunshine and air temperature of 90°F. Plots were evaluated for percent control of Palmer amaranth and grass weeds 10, 20, and 30 days after treatment.
Results
Palmer Amaranth Control
As depicted in Figure 2, treatments containing Gramoxone either with crop oil or a non-ionic surfactant, and the tank mix treatment of Liberty and Enlist One performed well throughout the study. Liberty applied either with non-ionic surfactant or crop oil, and Liberty with Roundup only achieved around 50 % control of Palmer Amaranth. It is notable that all Liberty treatments saw reduction in the level of control as the season progressed. This was evidenced in the field by Palmer amaranth plants suckering out from the base and re-growing approximately 2 weeks after the application. We did not observe suckering with the Gramoxone treatments or Liberty + Enlist treatments. Surprisingly, Enlist One treatments did not provide acceptable control of larger Palmer Amaranth plants in this study with an average control of around 25%. Enlist One control did gradually increase after the 10 day evaluation most likely due to the systemic nature of the product. As expected, we saw negligible control of Palmer amaranth with Roundup. The evaluation 20 days after treatment provided the clearest assessment of control, with no significant difference between Gramoxone treatments or the Liberty + Enlist One treatments (Figure 3). Liberty + NIS and Liberty + Crop Oil and Liberty + Roundup were not significantly different from each other, but significantly less effective than Gramoxone or Liberty + Enlist one treatments. Enlist treatment was not significantly different than the Roundup treatment. We should also note that new Palmer amaranth seedlings began to emerge just 20 days after the burn down treatments. Palmer amaranth continues to germinate throughout the summer, especially in open areas where sunlight reaches the soil. Controlling this weed takes a season long approach.
Figure 2. Palmer amaranth control at 10, 20, and 30 days after treatment.Figure 3. Palmer amaranth control at 20 days after treatment.
Grass Control
As depicted in Figure 4, all treatments provided a satisfactory level of grass control. While Roundup didn’t have any efficacy on resistant Palmer amaranth, the enduring benefit of this product can clearly be seen in control of other tough weeds, such as grasses with 100% control in our study. We saw no reduction in control of the grasses present when Roundup was tank mixed with Liberty. The Gramoxone +NIS treatment had slightly lower grass control at 84%, and was significantly lower than Roundup treatments. Enlist only has activity on broadleaf and was omitted in the analysis for grass control. An interesting question for further study is the potential efficacy of Gramoxone + Roundup treatments.
Kurt Vollmer, Extension Weed Management Specialist | kvollmer@umd.edu and Alan Leslie, Agriculture Agent University of Maryland Extension
Starting clean, or weed-free, is a critical component of any weed management program. In 2022, growers experienced either a lack of or higher cost of glyphosate-based herbicides that are typically used to control weeds prior to planting soybean. In a study sponsored by the Maryland Soybean board and conducted at the University of Maryland’s Central Maryland and Wye Research and Education Centers, different glyphosate-based and non-glyphosate based herbicide treatments were evaluated for their ability to provide effective preplant weed control (Table 1). These treatments included both single and sequential herbicide applications.
Table. 1. Evaluation of glyphosate-based and non-glyphosate-based programs for preplant weed control in soybean.
Treatmenta
Application A
Group
Rate (fl oz a-1)
Application Bb
Group
Rate (fl oz a-1)
1
Select Max
1
16
Enlist One
4
32
2
Select Max
1
16
Xtendimax
4
22
3
Enlist One
4
32
Select Max
1
16
4
XtendiMaxc
4
22
Select Max
1
16
5
Sharpen + Select Max
14 +1
2
—-
—-
6
Sharpen
14
2
Select Max
1
16
7
Select Max
1
16
Sharpen
14
2
8
Roundup + Enlist One
9 + 4
22 + 32
—-
—-
9
Roundup + XtendiMax
9 + 4
22 + 22
—-
—-
10
Roundup + Liberty
9 + 10
22 + 43
—-
—-
11
Roundup + Gramoxone
9 + 22
22 + 64
—-
—-
12
Roundup
9
22
—-
—-
13
Liberty
10
43
—-
—-
14
Gramoxone
22
64
—-
—-
a Required adjuvants were included with each treatment according to the product label.
b Application B indicates a sequential herbicide application made 1 week after the initial application A.
c Treatments containing XtendiMax were only applied at the Wye REC location.
Our results showed that non-glyphosate-based herbicides can provide excellent weed control. A single application of Liberty or Gramoxone alone provided greater than 92% control of both grasses and broadleaf weeds at both sites (Fig.1). In addition, there was no significant improvement in efficacy with combinations of Roundup + Liberty or Roundup + Gramoxone.
Figure 1. Preplant applications of Liberty (left) and Gramoxone (middle) compared to the untreated check (right) 10 days after application at Wye REC. Images: Kurt Vollmer, Univ. of Maryland.
Results varied when herbicide applications were split. Weed control was better at the Wye REC compared to the CMREC. At CMREC, sequential applications containing Select and Sharpen controlled broadleaves better than sequential applications containing Select and Enlist. A similar result occurred at the Wye REC, however, sequential applications containing Select and Sharpen only provided significantly greater broadleaf control when compared to Select followed by Enlist rather than Select followed by Xtendimax.
Almost no grass control occurred with sequential applications at the CMREC with Select followed by Sharpen only providing 14% control. At the Wye REC, sequential applications only provided up to 69% grass control. Only one major difference was observed for this set of treatments with better grass control observed when Select was applied first followed by either Enlist, Xtendimax, or Sharpen compared to XtendiMax followed by Select.
Roundup, or other generic products containing glyphosate have been a major part of preplant burndown programs because of the broad spectrum of control, systemic activity, and relatively low cost. With the recent price increases and limited availability, farmers may be forced to consider other herbicide(s) for their burndown applications. Our results help to demonstrate some of the products available. Growers should become familiar with the unique characteristics of each individual herbicide and apply them as instructed by the label. Liberty (glufosinate) provides broad-spectrum control of weeds similar to glyphosate, but does not have the systemic activity and tends to provide weaker control of grasses. Gramoxone (paraquat) is similarly non-selective, and provides rapid burndown, but as a contact herbicide, it may not completely kill some perennial weeds. Certain selective herbicides are also potentially useful for burndown programs, though multiple products would be necessary to kill the full spectrum of weeds. Sharpen (saflufenacil) is a broadleaf-selective herbicide that is occasionally incorporated into burndown programs to target glyphosate-resistant weeds, like marestail. Enlist One (2,4-D) and XtendiMax (dicamba) are broadleaf-selective herbicides that would be compatible with soybeans with the respective Enlist or Xtend herbicide tolerance traits. These broadleaf-selective herbicides could be paired with either a broad-spectrum herbicide, or with a grass-selective herbicide, like Select Max (clethodim) for complete preplant burndown. However, antagonism has been known to occur with tank mixes of broadleaf-selective and grass-selective herbicides. So it is suggested that a broadleaf and grass applications be split. Of these herbicides, Select Max is the slowest acting, and would benefit from early application, as grass weeds or grass cover crops green-up in the spring.
Kurt Vollmer, Extension Weed Management Specialist | kvollmer@umd.edu and Alan Leslie, Agriculture Agent University of Maryland Extension
Italian ryegrass is a winter annual weed that often needs to be managed prior to soybean planting. In Maryland, populations of Italian ryegrass have been confirmed to be resistant to group 1 (Axial, Select) and group 2 (Harmony, PowerFlex) herbicides, but recent populations have appeared to develop resistance to glyphosate (Figure 1). With limited herbicide options, preplant control of Italian ryegrass will be challenging for growers. In the fall of 2021 through spring of 2022, a study sponsored by the Maryland Soybean Board was conducted at a farm site in Talbot County, MD to evaluate different herbicides and application timings for controlling suspected herbicide-resistant Italian ryegrass. This study had two objectives: determine the optimum application timing and herbicides to control Italian ryegrass ahead of planting soybeans. Italian ryegrass seed will germinate in both the fall and the spring, therefore herbicide treatments were applied in the fall, spring, and in the fall followed by a second application in the spring. Herbicide selections consisted of glyphosate applied alone and glyphosate plus either clethodim (Select Max), flumioxazin (Valor), or saflufenacil (Sharpen). Valor and Sharpen can be applied preplant in both winter wheat and soybean to control many different weeds and Select Max is an herbicide that is used to control many annual and perennial grasses after they emerge.
Figure 1. Herbicide-susceptible and herbicide-resistant Italian ryegrass following a glyphosate application.
Our results showed application timing and herbicide selection had a significant effect on Italian ryegrass control, density, and height, but there appeared to be no additional benefit when these two factors were combined. In the spring of 2022, Italian ryegrass density and height were lower with sequential herbicide treatments compared to single applications in the spring or fall (Table 1). This was reflected in improved control with sequential compared to single applications. With multiple germination periods, it is likely that two applications will be needed to provide better control of both early and late emerging Italian ryegrass.
Table 1. Italian ryegrass response to three different herbicide application timings.a
Application timing
Control
Density
Height
%
Plants m-2
cm
Fall
44
b
12
a
20
a
Spring
48
b
16
a
25
a
Sequential
81
a
4
b
15
b
a Means with the same letter do not differ significantly according to Fisher’s LSD (α = 0.05).
Italian ryegrass density and height were lower when Roundup was applied with Select Max compared to all other treatments, regardless of application timing (Table 2). This was reflected in a significant improvement in control with this treatment; however, no treatment provided over 95% control. These results indicate this Italian ryegrass population to be glyphosate-resistant (Figure 2). Tank mixing glyphosate and other herbicides did not always improve control. Sharpen is a broadleaf-selective herbicide that is commonly used to help control glyphosate-resistant weeds like marestail, but it will not control Italian ryegrass. Although, Valor can provide residual control of this weed, including it with Roundup did not significantly improve control over Roundup plus Sharpen or Roundup alone. The combination of Roundup plus Select Max appeared to have a synergistic affect; however, previous research at this site showed Select Max alone provided no more than 30% control. Additional research is currently being conducted to evaluate the level of resistance to these different herbicide groups. Moving forward, additional strategies, including non-chemical ones, will likely be needed to manage this weed before planting soybeans and other summer crops in Maryland.
Figure 2. Italian ryegrass response in June of 2022 following a previous glyphosate application.
Table 2. Italian ryegrass response in spring to selected herbicide treatments.a
Herbicide(s)
Group
Rate (fl oz a-1)
Control
Density
Height
%
plants m-2
cm
Roundup
9
36
38
b
16
a
25
a
Roundup + Select Max
9 + 1
36 + 16
87
a
2
b
13
b
Roundup + Sharpen
9 + 14
36 + 2
49
b
16
a
23
a
Roundup + Valor
9 +14
36 + 2
58
b
12
a
23
a
a Means with the same letter do not differ significantly according to Fisher’s LSD (α = 0.05).
Registration is now open for the Mid-Atlantic Crop Management School, which will be held in-person at the Princess Royale in Ocean City, MD from November 15 – 17, 2022. The school offers a 2 ½ day format with a variety of breakout sessions. Emphasis is placed on new and advanced information with group discussion and interaction encouraged. Individuals needing training in soil and water, nutrient management, crop management, and pest management can create their own schedule by choosing from 5 program options offered each hour.
New this year for CCAs: we are offering specialty CCA certification credits in sustainability and precision agriculture. Specialty credits count toward recertification for the associated specialty certification or can be applied toward your overall CCA recertification credit requirements (for individuals not holding a specialty certification). We also anticipate offering state nutrient management credits for DE, MD, PA, VA, and WV and state pesticide credits for DE, MD, NJ, PA, VA, and WV.
Online registration will close at 11:59 p.m. EST on Monday, November 7, 2022. Registration Fees are $325 by October 15, and $375 from October 16 through November 7. We look forward to seeing you there.
Reports are for crop conditions up to September 2, 2022.
Western Maryland
As the old saying goes you are only two weeks away from a drought. We have been 11 days without rain until August 30, when we received a welcome soaking. The high heat coupled with even more sporadic showers led us to the cusp of drought stress. Corn was curling and beans were cupping, the shower alleviated the situation and will help the double crop beans. On the upside, the dry weather has allowed for corn silage harvest to shift into high gear. Yields are very good leading us to hope corn grain yields will be as well. Cover crop planting will commence shortly with the winter annual forages behind corn silage being the first. Combines will hit the fields later in the month to start on the early corn followed by the early beans. Thus once in full swing harvest will last into late October or early November depending on precipitation. Pastures and hay fields are looking great for this time of rain thanks to the showers of late July and early August.—Jeff Semler, Washington Co.
Central Maryland
The last month has been pretty dry in our region. Corn is quickly drying down, and silage harvest has begun. Double crop beans are filling pods. Hopefully we get a bit more rain to help finish out the season. —Kelly Nichols, Montgomery Co.
Northern Maryland
End of July and first couple of weeks of August have been without much moisture. While most of our soils hold a decent amount of moisture, but I believe we’ve come up a little shy on water for corn and full season soybeans to have reached their full yield potential. Corn silage harvest started last week and it will not be very long before combines hit the fields. Some full season soybeans have started to turn, which is a bit earlier than usual, likely due to the lack of August rains. We did have a period two weeks ago of cooler, wet weather, which brought on the first occurrence of tar spot in corn to our county, which is now a hot topic of discussion.—Andy Kness, Harford Co.
Upper & Mid Shore
While half of the region has received adequate rains recently, the other half is in drought conditions. 99% of corn is made and harvest is beginning. Early maturity beans are starting to turn, but later maturity groups are suffering in the droughty areas. Troublesome weeds are popping up above bean canopy. A few areas have podworms and stink bug levels above thresholds.—Jim Lewis, Caroline Co.
Lower Eastern Shore
Weather has been extremely dry in most of the region. Farmers have been irrigating when possible; however, the majority of our rowcrop land is unirrigated. Corn is drying down. Soybean is on average in early reproductive stages. Cover crops are beginning to be aerially seeded via airplane. However, there are concerns of a poor cover crop stand due to the dry weather. Herbicide-resistant Palmer amaranth is apparent in many fields. Now is the time to walk the fields to pull mature Palmer plants. Herds of deer continue to be sited grazing in soybean fields.—Sarah Hirsh, Somerset Co.
Southern Maryland
We have experienced a return to drier conditions in the majority of the region. Harvest of shorter maturity corn has started with reports of very good yields. Corn is drying down quickly with harvest expected to hit full swing in the next week or so. Soybeans have suffered over the last few weeks due to limited rainfall. We are finding podworms and podworm feeding injury in many double-crop soybeans throughout the area with many fields above threshold levels. If you have not already scouted fields for worm activity, I encourage you to do so soon. We are also observing Palmer amaranth and common waterhemp becoming more evident in the drier conditions.—Ben Beale, St. Mary’s Co.
Mark Townsend, Agriculture Agent Associate | Ben Beal, Principal Agriculture Agent | Kurt Vollmer, Weed Management Specialist University of Maryland Extension
Harvest is fast approaching for grain producers; some of which have already begun shelling corn in recent days. Though this time of year often marks the end of the season, some decisions we make at harvest have lasting effects on subsequent growing seasons; one of which is weed control. If weeds are allowed to mature and set-seed, harvest equipment can be highly effective at retaining, and transporting weed seeds from field to field, as well as dispersing weed seeds across the field, further increasing the weed seed bank.
In a previous Agronomy News article, we offered some cultural Palmer amaranth control options at harvest. This article will expand on these strategies and provide additional context on the impacts of weed seed movement.
Before the combine ever reaches the field, determining the order in which fields will be harvested based on weed pressure can reduce weed seed movement. Determining the relative weed pressure and species composition of fields offer some guidance to a harvest order. Infested areas should not be harvested before clean areas, as weed seeds can easily be transported to the clean field. If possible, save the worst for last: highest weed pressure fields should be combined last to minimize spread to the other less pressured fields. Finally, combine operator judgment should not be understated: it may be prudent not to harvest some portions of the field heavily infested with difficult weeds and a poor crop stand. What you do not harvest in crop, may be repaid in weed seeds unscattered. This is especially true in operations where only a small percent of the fields have palmer amaranth present. Our experience suggests the most common and costly mistakes when dealing with early infestations of Palmer amaranth is harvesting an infested field and then driving directly to clean fields.
Once out of the field, cleaning the combine before moving to another field can significantly reduce the potential for weed seed transport. Following harvest of a field, combines can retain as much as 150 lbs of biomaterial on and in combine components. This additional material protects and holds weed seeds that can be later dislodged at other locations. An efficient, yet thorough, cleanout takes only 20-30 minutes with a leaf blower and air compressor:
With the header removed, and after running the unloading auger for at least a minute, open the clean grain door, tailings door, rock trap, and unloading auger sump.
Run the separator, adjust the shoe-fan and rotor to full speed while electronically opening and closing the sieves and concaves.
Run in this configuration for at least two minutes. Operators may also choose to drive in this state to dislodge more material during this operation.
Wearing personal protective equipment (dust mask, eye protection, ear protection, and gloves), blow off material from the external components of the combine–remember to work from top to bottom, and to focus on high-chaff regions like the feeder-house, and straw-chopper. Video of the process may be found at: https://www.youtube.com/watch?v=kHON9cnCIdo&ab_channel=AgricultureandNaturalResources
At the end of the harvest season or between highly infested and lower pressure fields, a more thorough combine clean out should occur following the above steps with the addition of the straw bale method before the blower clean-out procedure.
With the combine components at normal operating speeds, begin feeding flakes from small straw bales into the raised header (between 2-3 bales required depending on combine size).
Allow straw to work through the combine and flow out of the machine onto an area appropriate to receive the straw and additional harvest chaff (tarps, concrete pad, etc).
To clean the grain tank, mix 25 lbs of wood pellets with half a small straw or hay bale and place the mixture at the grain tank auger. Run the auger to remove the mixture along with any additional harvest residue from the grain tank. Video of the process may be found at: https://growiwm.org/the-straw-bale-methodology-for-cleaning-weed-seeds-out-of-a-combine/
Additionally, given that some weeds are especially difficult to control in certain crops (ex. Palmer Amaranth in a soybean crop), observe the planned crop rotation when determining a harvest order. Ensure that fields containing potentially difficult to manage weed species will be rotated to a crop in which the weed species may be more manageable.
Photo: Kondinin Group Research Report www.farmingahead.au
New technology is being developed that can further reduce seed viability during harvest. While in the field, combine add-ons have shown efficacy in reducing weed seed spread. Seed impact mills and chaff lining devices are two common options of combine modifications. Seed impact mills were devised and first implemented by Australian growers, and have received significant attention world-wide in recent years. Seed impact mills pulverize weed seeds that enter the combine with an aggressive chopping and hammer-mill-like system powered by a drive belt and fits behind the combine’s straw-chopper. There is a growing number of brands; however, the documented research focuses on the Harrington Seed Destructor (now iHID) and the Reddekopp Seed Control Unit. Recent research from Virginia Tech with trailing the Reddekopp system indicates a 99% kill rate for Palmer amaranth and common ragweed seeds, with a subsequent 42% reduction in common ragweed density the following year. Similar research from the University of Missouri documents a 98% reduction in viable waterhemp weed seeds. These systems, though effective, come at a significant capital expense. Additionally, these systems place an additional load on the combine engine increasing fuel consumption, but does not significantly impact combine performance, as measured by harvested acres per hour.
Chaff Lining devices funnel the chaff exiting the straw-chopper into wind-rows directly behind the combine, or in the compacted wheel tracks while the straw is distributed evenly behind the combine. Chaff lining does not directly destroy weed seeds, instead the device concentrates the weed seed containing chaff to less than 10% of the field. The layer of debris slowly decomposes and exposes the weed seeds to decay and a less suitable environment for germination. Ongoing research from Iowa State finds that 95% of weed seeds can be concentrated behind wheel tram-lines, reducing the distribution of waterhemp and aboveground biomass of germinated seeds. Currently, these systems are the most inexpensive harvest weed seed management device available, with DIY plans available online and other relatively inexpensive commercial options for sale as well. However, these systems are more well-suited for species that retain a majority of their seed at harvest, such as Palmer amaranth and waterhemp harvest of small-grain and other relatively low-biomass producing crops.
Locally-designed chaff line chute working in Delaware. Photo: Claudio Rubione, GROW.
These harvest weed seed management methods will require multiple seasons of good stewardship to effectively reduce weed populations. They will not substitute for preventative or in-season control treatments, but are a core component of effective integrated weed management programs.
A series of field days will be held on June 28th and 29th for farmers, ag industry, and others interested in seeing the latest results from university weed management trials. These will include tours of herbicide demonstration plots, updates on integrated weed management trials, and updates on UAVs (drones) for herbicide applications.
First stop
June 28 at the Virginia Tech Eastern Shore AREC, 33446 Research Drive, Painter, VA from 8:00 AM to 11:00 AM.
Second stop
June 29 at the University of Delaware Carvel Research and Education Center, 16483 County Seat Highway, Georgetown, DE from 8:00 AM to 10:00 AM.
Final stop
June 29 at the University of Maryland Wye Research and Education Center, 211 Farm Lane, Queenstown, MD from 4:00 PM to 6:00 PM.
Pesticide credits will be available for MD and DE.
These events are free and open to the public. For more information, contact Kurt Vollmer at kvollmer@umd.edu, Mark VanGessel at mjv@udel.edu, or Vijay Singh at v.singh@vt.edu.
Cerruti R2 Hooks$ and Dwayne Joseph* $Professor and Extension Specialist, *Post-Doctoral Fellow, CMNS, Department of Entomology
Introduction
Mechanical cultivation and hand weeding are organic producers most preferred choices for weed control. However, repeated cultivation has a negative influence on soil structure and organic matter content, and can make conditions more susceptible to soil erosion. In addition, repeated cultivation promotes new weed flushes. On the other hand, hand weeding, which may require a ready supply of field workers, can be expensive, especially if conducted over large areas and in less competitive crops that require multiple hand weeding tasks. For commercial producers who mostly rely on herbicides, concerns may arise regarding herbicide-resistant weeds, the potential risk to the groundwater supply, and their effect on food quality. This suggests that alternative weed management tools should be considered. Multiple studies have examined effects of flaming on annual weeds; and successful application of propane flaming to manage weeds has been reported in crops such as cotton, field corn, cabbage, carrots, sweet corn and onions. This suggests that weed flaming may be a formidable tactic for incorporation into an integrated weed management (IWM) program. Its successful integration could result in reduced usage of cultivation, hand weeding and herbicide sprays.
Flame weeding or flame cultivation involves using propane burners to create intense heat to manage weeds in the interrow region of row crops or across the whole field in fallow situations. Flame-weeding systems range from handheld flamers for small-scale production to tractor-mounted systems for large-scale, row-crop flaming (Fig. 1). Covers or reflectors used to contain the heat energy close to targeted weeds improve energy efficiency by reducing heat loss. Very brief periods [e.g., 125 millisecond (ms)] of exposures to high temperatures interfere with plant cellular processes such as photosynthesis. Practicing flaming to manage weeds was developed for field crops, fruits and vegetables during the 1940s. However, as interest in low priced herbicides became more prevalent, flaming popularity took a hit. After its almost complete disappearance in the 1970s, flame weeding has regained interest, especially in organic production.
Fig. 1. Tractor equipped with propane tank and burners. Attribute: Ian Abbott (CC).
How does flaming impact weeds?
Flaming differs from burning in that plant tissues are not incinerated, but rapidly heated causing denaturation and aggregation of cellular proteins. The extent to which heat from the flames penetrates plants depends on the flaming technique and environmental factors such as leaf surface moisture. Denaturation of the plant cells starts at roughly 40 °C, depending on exposure time. An increase of temperature above 50 °C inside the plant cells can result in the coagulation of membrane proteins leading to loss of membrane integrity. Lethal temperatures are reported to range from 55 to 94 °C. Plant cells comprise 95% water; and if plant tissue reaches temperatures above 100°C for a split second, it causes water boiling and cell membrane rupture, resulting in loss of water and plant death. Exposure times in the range of 65–130 ms are sufficient to kill many annuals. Plants may survive flaming by avoidance or heat tolerance. To optimize propane flaming as a weed control tool, the effective dose (ED) of propane required to control targeted weed species must be known. Depending on the desired level of weed control, a propane dose can be used to kill weeds or reduce their competitiveness with crops. For fast-growing crops, pre-emergence flame weeding can provide sufficient weed suppression to allow the formation of full crop canopy, which impedes later weed emergence. In addition to open flame units that target emerged weeds, flames may be directed to the soil surface. Direct heat to the soil surface increases soil temperatures to the weed seed thermal death point. The thermal death point is the temperature at which a seed will not germinate after heat application. Many soil attributes influence the efficacy of direct heat and contribute to variations in treatment efficacy. These include planting depth, soil texture, soil moisture thermal conductivity, soil chemical properties and soil porosity. The efficacy of flaming is determined by the amount of heat transferred from the burner and the duration of time that weeds are exposed to the heat. The amount of heat transferred by the flamer to weeds is determined by the number of burners for a given working width, the nozzle size, and gas pressure; as well as exposure time which is determined by application speed.
What influences the flamer?
Timing. The efficacy of flame weeding is influenced by several factors, including the plant’s growth stage, the physical location of its growing point during flaming, the presence of protective layers of hair or wax and lignification, and the time of day. Multiple studies have shown that plants are more heat sensitive when flamed in the afternoon than early morning. One study compared the effectiveness of flaming at 8 a.m., 12 p.m., 4 p.m., and 8 p.m., and reported better weed control at the noon and 4 p.m., treatment periods. It was suggested that differences in weed control among flaming times could not be explained by differences in temperature, relative humidity or the presence/absence of dew. Thus, in general, it is believed that flaming will be more effective if conducted during the afternoon period. Though, afternoon flaming may result in better weed control; it is important to note that crops are also more vulnerable to injury during this period. Adjusting the angles of torches and positioning the flames below the crop canopy may reduce risk of crop injury during flaming.
Plant type. Weeds susceptibility to flaming varies among species and in general, dicots are more effectively controlled with flaming than monocots. Further, annual weeds are more vulnerable to flame weeding than biennials and perennials. Annual broadleaf and grass species also differ in their response to flaming. Leaves of annual broadleaf species may turn brown and die within a few days after flaming, resulting in no regrowth whereas leaves of grass species may turn white shortly after flaming, leaving an appearance of a dead plant. However, within a week, grass species begin to recover with the growth of new leaves. These varying responses to flaming between broadleaf and grass species are due to the physical positioning of their growing point at the time of flaming. The growing point in grass species during early growth stages is typically below the soil surface and as such, are protected from flames. In contrast, the growing point of broadleaf species is above the ground where it is exposed to the flame. Grasses also have a sheath that protects their growing point. Weeds with unprotected growing points, such as lambsquarters, have been found to be more sensitive than those with protected growing points, such as shepherd’s purse (Capsella bursa-pastoris) and that barnyardgrass (Echinochloa crus-galli) and green foxtail (Setaria viridis) are more tolerant to flaming than velvetleaf (Abutilon theophrasti; Fig. 2) and ivyleaf morningglory (Ipomea hederacea). Broadleaf weeds in their vegetative growth stages require propane doses ranging from 30 to 60 kg ha-1, whereas a vegetative grass such as barnyardgrass can require up to 79 kg ha_1 to achieve the same level of control. Relative to this, crops in the grass family such as maize and sorghum exhibit higher tolerance to flaming than broadleaf crops such as soybean and sunflower when treated at early growth stages.
Plant growth stage and size. A plant’s tolerance to flaming varies according to its maturity/growth; and developmental stage is probably the most important factor. The growth stage of weeds at the time of flaming helps determine their sensitivity to heat. The growth stage establishes the type and degree of protective layers, the lignification level and location of growing points. Relative to this, flaming is more effective on most weeds at an early growth stage. Smaller plants typically have thinner leaves, lower biomass and fully exposed meristems (not protected by surrounding leaves). In contrast, older seedlings or bigger plants have larger and thicker leaves and greater surface area and biomass, which requires higher temperature and longer exposure to achieve control. Further, they possess larger amounts of food reserves (soluble sugars, proteins and lipids) in stems and roots, providing them with the increased capacity for regrowth. Another critical part of young plants that determines their heat tolerance is the growing point in the shoot apex. In older plants, the shoot apex is often protected by surrounding leaves. The tolerance of different plant parts to flaming can also be influenced by protective layers of hair and/or wax, lignification level and their overall plant water status.
Relative to the growth stage, four weed species [green foxtail (Setaria viridis, Fig. 3), yellow foxtail (Setaria pumila), redroot pigweed (Amaranthus retroflexus) and common waterhemp (Amaranthus rudis)] exhibited more tolerance when flamed at the flowering stage compared with earlier vegetative stages; and popcorn plants flamed at the 2-leaf stage had the highest yield loss. In comparison, flaming popcorn plants at the 5- or 7-leaf stage had less of an effect on ear size. One study found that plant size had a greater influence upon sensitivity to flaming than plant density, with small weeds being more sensitive than large weeds. For example, 42 kg ha−1 of propane killed 95% of individuals within a solid stand of white mustard plants with 0-2 leaves. However, 74 kg ha−1 was required to kill 95% containing 2-4 leaves.
Fig. 3. Green foxtail (Setaria viridis). Attribute: Dick T. Johnson (CC).
Advantage. Flaming can be a viable weed management alternative to hand weeding and cultivation. From an economic standpoint, labor cost associated with hand weeding is more expensive, time-consuming and labor intensive (Fig. 4). Further, flaming can be used when the soil is too moist or stony for hand or mechanical weeding. In comparison with cultivation, flame weeding does not disturb the soil surface or bring buried weed seeds to the surface which makes it compatible with the stale seedbed technique. Further, in some instances, flaming can be as effective or better than cultivation. A study conducted to compare flaming to mechanical cultivation on weed control in popcorn reported that weed control was better with flaming than cultivation. Moreover, flaming helps reduce concerns regarding direct effects of weed suppression tactics on soil, water, and food quality. For instance, flaming does not leave chemical residues on plants, soil, air or water, and produces no hazardous drifts or chemical carry-over to the next season. Flaming does not contribute to herbicide-tolerant or resistant weeds and weeds are less likely to become resistant to flaming. As such, flaming in some situations is a more feasible alternative to using herbicides or mechanical cultivation.
Fig. 4. Wheat plots being hand-weeded. Attribute: Fred Miller, Arkansas Agri Media (CC).
Disadvantage. The main disadvantages of flame weeding are the lack of residual weed control, the lack of selectivity for crop safety, low speed of application, increased application costs, and applicator safety. The efficacy of flaming may be reduced when environmental conditions such as dew is present. It should also be noted that flaming is not as efficient as chemical control. However, it can be repeated as needed during the growing season and more importantly integrated with other weed management tactics. In addition, postemergence flaming can damage the cash crop. For heat-resistant crops such as cotton, corn, and sugarcane, flames can be directed to the plant’s base during certain growth stages. This technique, is called selective flaming and controls intra-row weeds. For heat-sensitive crops, postemergence flaming can be applied using a covered flamer to help protect crops from the heat. This technique, also known as parallel flaming, controls weeds between the crop rows. Flaming may also be incompatible for conservation tillage and cover cropping as the heavy plant residue that remains on the soil surface may ignite during flaming. Another drawback is that most flame weeding systems are designed to treat a lower number of rows per pass compared to chemical treatments which makes the process slower.
Summary
Flame weeding uses propane burners to generate combustion temperatures of up to 1,900 degrees Celsius, which raises the temperature of exposed weed leaves very rapidly and kills them without burning. Flaming in the absence of killing weeds can severely reduce their growth, thereby making them less competitive with crops. Weeds and other plants’ susceptibility to flaming will vary according to species, plant size and growth stage during flaming. Broadleaves are generally more sensitive than grasses, and older and larger plants require higher energy rates for control than younger and/or smaller plants with fewer leaves. Flaming before crop emergence has been the predominant thermal weed control method in slow-germinating row crops such as onion, leek, carrot and corn. However, preemergence flaming may be of limited value in fast emerging crops because the crop may easily emerge before most weeds. As such, preemergence flaming would only control a fraction of the weeds that will emerge during the cropping season. Still, flaming has shown good results after weed emergence and before crop emergence in crops such as potato, sugar beet, carrot and cayenne pepper. When conducted postemergence, vulnerable crops such as soybean and sunflower must be protected from flaming. Other crops such as corn and sorghum have some tolerance to flaming. Flaming similar to other weed management tactics should not be viewed as a stand-alone tactic. However, it can be successfully incorporated into an IWM plan and thus be used in concert with other tools. For example, weeds in corn were controlled through integration of tillage and flaming.
Financial support for the publication of this article is via USDA NIFA EIPM grant award numbers 2021-70006-35384 and NESARE – Research for Novel Approaches (LNE20-406R).
The program will start at the seed building and proceed to the fields.
Dr. Vijay Tiwari will discuss the small grain variety trials and his wheat breeding program. Dr. Nidi Rawat will then discuss fusarium research and prevention in wheat and barley. Next, we will have Dr. Alyssa Koeler touching on other small grain pathogens and Dr. Kurt Vollmer will bring us up to date on weed control in wheat. Finally, we will hear from Dr. Kelly Hamby on insect threats and control.
We also have a commercial variety strip trial organized by the Maryland Crop Improvement Association (MCIA) and industry reps will be on hand to discuss their entries.
Dinner will be served at 6:30, sponsored by Nagel Farm Service.
Erratic weather patterns continue to dominate and challenge us here in Washington County. While we can still use precipitation, it has recently been coupled with wild temperature swings. In the 70s or low 80s and then back to the low fifties with nighttime temperatures on several occasions hovering near freezing. Triticale harvest is over half finished but very little corn has gone in the ground. First cutting alfalfa will begin early next week barring rain showers. First cutting hay of any kind looks like it will be below normal. Forever the optimist, second cutting will be better.—Jeff Semler, Washington Co.
Central Maryland
Small grain silage harvest is in full swing. Manure is being hauled and corn is being planted. Some soybeans have been planted. First cutting of hay may also start soon. Over the past month, most of the area has been at or above normal rainfall, according to the National Weather Service. Field work may slow down depending on much rain we get this weekend, but next week is forecasted to be drier and in the 70s. —Kelly Nichols, Montgomery Co.
Northern Maryland
The trend for April has been a continuation of March—cool temperatures that rarely want to move out of the 60s. As a result, the start of planting was about a 5-10 days later than usual for this region. Most planting kicked off the week of Easter. In general, small grains look good. Barley is headed out and wheat is a couple of weeks away. Some rye has been chopped for feed. Pastures and hay fields are enjoying the cooler temperatures.—Andy Kness, Harford Co.
Upper & Mid Shore
Soil moisture has been perfect for spring crops, field work, and planting. Soil temperatures have been a little cool for this time of year. Many acres of corn and beans have went in over the past week. Barley is fully headed and wheat will be in a few days. Both look good. Hay and pastures look good, but the cool weather has them a little behind normal.—Jim Lewis, Caroline Co.
Lower Eastern Shore
Wheat is starting to pollinate. It’s been a relatively dry spring, making for an average to above average wheat crop, with minimal disease pressure. Cover crops have been mostly terminated. However, where still standing, cover crops are looking great. These late-terminated cover crops should bring additional benefits to the fields, such as providing substantial organic matter to the soil, and in some cases releasing nitrogen on the soil surface. Growers have spread poultry manure. Weather has been relatively favorable these last couple of weeks for planting. Corn has started to be planted. Much more corn acreage is expected to be planted in the next weeks, to be followed by soybean planting.—Sarah Hirsh, Somerset Co.
Southern Maryland
The region has experienced good planting conditions over the last week. Corn planting progress is still a bit behind schedule with cooler soils limiting the number of early-planted acres. Most farmers are finishing up with corn planting and have started with soybean planting. Recent showers were welcome with soil conditions becoming slightly dry during the last couple of weeks. Wheat is in the early heading stage and many fields will be flowering by the time you read this. Farmers will be evaluating the need for a head scab fungicide in the next few days. Many wheat fields are exhibiting yellowing in the upper canopy that is not readily attributable to any disease. We are also seeing a lot of powdery mildew in the lower canopy, but very little in the upper canopy. Insect and disease pressure has been light so far. Forage crops look great this spring. Alfalfa weevil are very active this year and many fields required treatment. Pyrethroid resistance continues to be an issue for our growers. First cutting of cool season grasses is underway.—Ben Beale, St. Mary’s Co.
