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Late-Season Weed Control

ROCKVILLE, Md. (DTN) — Those waterhemp escapes towering over your soybeans are more than just a mar on the landscape — they are the source of weeds for years to come.

“Pigweed species can produce anywhere from a couple hundred thousand to a million seeds per plant,” said Purdue University weed scientist Bill Johnson. “So just allowing a few weeds in an 80-acre field to go to seed can result in an almost catastrophic situation the next year.”

Weed control failures become quite visible in farm country this time of year, particularly in soybean fields where the plants loom above the canopy.

In Indiana, waterhemp, giant ragweed, Palmer amaranth, marestail and even lambsquarters and velvetleaf have made their presence known in crop fields, ditches, field edges and fencerows, Johnson said.

Weed scientists are urging growers not to give up on these mature weed escapes.

“The potential to spread this problem at harvest via the combine is great, so anything that can be done to control the pigweeds prior to crop harvest is imperative,” Pennsylvania State University weed scientist Bill Curran warned growers in a university newsletter.

Right now, Indiana growers are facing the results of poor waterhemp control in 2015, Johnson said. During the soggy months of May and June last year, many farmers had to abandon crop fields that were too wet to plant, spray or till.

Waterhemp moved in and thrived and left an enormous seedbank that will haunt farmers for many years to come, Johnson said.

“We’re talking about years up to decades of survival, depending on the weed species,” he said. “Waterhemp tends to survive longer than Palmer amaranth, but a lot more of the Palmer seed will germinate right away next year.”

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Waterhemp seeds will still germinate, but a larger percentage will stay dormant, waiting for another year to sprout and rob yields from you, he added.

For more information on different weed species and their seed survival, consult weed guides from your local land-grant universities, such as this one from Michigan State University: http://bit.ly/….

KNOW YOUR STRATEGY

For the most part, chemical options are limited and unhelpful at this time of year, Johnson said. Most postemergence herbicides are only labeled for use in soybeans up to the R2 growth stage. After weeds breach 6 inches in size, herbicide control becomes highly variable and unreliable, as well.

To add to the problem, many weed species are resistant to a number of herbicides, and the list grows every year.

In Indiana, glyphosate and ALS-resistance is extremely common in waterhemp, ragweed and Palmer amaranth populations. Many waterhemp populations are also resistant to PPO herbicides. Down south, Palmer amaranth has developed resistance to this class of chemistries as well, Johnson said.

“There are some postemergence grass herbicides that can go on pretty late and will reduce seed set, but for broadleaves, once you can no longer spray the beans, the only resort is to remove them by hand,” he said.

How you approach hand removal also depends on the weed species.

Waterhemp and Palmer amaranth plants are already producing seeds at this time of year. “They set seed over a long period of time,” Johnson said. “You could go out now and grab some waterhemp and Palmer amaranth plants over the next six to eight weeks, and be able to rub viable seeds out of the seed heads.”

Marestail produces seed a little later in the season, typically in mid-to-late August, he added. Giant ragweed seed likewise isn’t viable until later in the season, when it turns brown.

As a result, you could safely lay most marestail and ragweed plants down in the field after pulling them and move on. But for Palmer amaranth and waterhemp, most weed scientists recommend a “bag and burn” approach, where you ensure that the weeds and the seeds they carry are physically removed and destroyed.

With funding from the Pennsylvania Soybean Board, Penn State weed scientists are handing out 40-gallon paper bags to farmers in an effort to stem the Palmer amaranth infestations racing through the state’s fields. The bags have instructions printed on them directing farmers to bag, burn or bury mature pigweed plants.

If an infestation is too severe for hand-weeding, Curran recommends more extreme steps.

“With small, severe infestations, you may consider destroying the crop and the weeds by mowing and/or herbicide application,” he wrote. “Dicamba plus or minus 2,4-D are probably the preferred products. On dairy farms, perhaps the soybeans (or corn) could be harvested for silage, which may occur prior to Palmer amaranth seed production. Harvesting all plant material and ensiling should also kill some of the weed seeds that could be present as we move into the fall.”

Don’t forget to grab a couple samples from those plants before you destroy them, Johnson added. Some land-grant universities offer plant diagnostic services that include molecular tests for herbicide-resistance.

“For the common types of resistance, some of those molecular assays can be turned around pretty quickly,” he said.

Seed destruction of weeds in southern US crops using heat and narrow-windrow burning

Narrow-windrow burning has been a successful form of harvest weed seed control in Australian cropping systems, but little is known about the efficacy of narrow-windrow burning on weed seeds infesting U.S. cropping systems. An experiment was conducted using a high-fire kiln that exposed various grass and broadleaf weed seeds to temperatures of 200, 300, 400, 500, and 600 C for 20, 40, 60, and 80 s to determine the temperature and time needed to kill weed seeds. Weeds evaluated included Italian ryegrass, barnyardgrass, johnsongrass, sicklepod, Palmer amaranth, prickly sida, velvetleaf, pitted morningglory, and hemp sesbania. Two field experiments were also conducted over consecutive growing seasons, with the first experiment aimed at determining the amount of heat produced during burning of narrow windrows of soybean harvest residues (chaff and straw) and the effect of this heat on weed seed mortality. The second field experiment aimed to determine the effect of wind speed on the duration and intensity of burning narrow windrows of soybean harvest residues. Following exposure to the highest temperature and longest duration in the kiln, only sicklepod showed any survival (<1% average); however, in most cases, the seeds were completely destroyed (ash). A heat index of only 22,600 was needed to kill all seeds of Palmer amaranth, barnyardgrass, and Italian ryegrass. In the field, all seeds of the evaluated weed species were completely destroyed by narrow-windrow burning of 1.08 to 1.95 kg m −2 of soybean residues. The burn duration of the soybean harvest residues declined as wind speed increased. Findings from the kiln and field experiments show that complete kill is likely for weed seeds concentrated into narrow windrows of burned soybean residues. Given the low cost of implementation of narrow-windrow burning and the seed kill efficacy on various weed species, this strategy may be an attractive option for destroying weed seed.

Keywords

Introduction

Chemical weed control options have been steadily decreasing over the last two decades because of increasing herbicide resistance in dominant weed species. There is a need to shift weed control programs toward strategies that involve the use of nonchemical approaches in conjunction with current herbicide programs if herbicides are to continue as a sustainable and effective option for growers. Slowing selection for herbicide resistance involves implementing several different techniques, some of which may include tillage, rotating and mixing herbicide sites of action, cover crops, and implementing a weed control technique known as harvest weed seed control (HWSC) (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett 2012). Harvest weed seed control systems target weed seeds collected during crop harvest with the aim of preventing their input into the soil seedbank (Walsh et al. Reference Walsh, Newman and Powles 2013), thereby reducing selection for herbicide resistance.

Harvest weed seed control strategies are currently being investigated to determine their potential fit for weed management programs in U.S. crop production systems. Harvest weed seed control, more specifically narrow-windrow burning, is a widely adopted practice for destroying rigid ryegrass (Lolium rigidum Gaudin) seed and decreasing the soil seedbank when growing wheat (Triticum aestivum L.), canola (Brassica napus L.), and lupin (Lupinus angustifolius L.) in Australia (Walsh et al. Reference Walsh, Newman and Powles 2013). In southern U.S. soybean production systems, there is an opportunity to use narrow-windrow burning of chaff and straw residues in an effort to destroy weed seed that escaped a weed management program and are harvested with the crop (Norsworthy et al. Reference Norsworthy, Korres, Walsh and Powles 2016).

Weeds that have escaped chemical control methods and are allowed to continue to grow and produce seed become major contributors to the soil seedbank. Palmer amaranth has been found to retain more than 97% of its total seed production for the growing season at soybean maturity (Schwartz et al. Reference Schwartz, Norsworthy, Young, Bradley, Kruger, Davis, Steckel and Walsh 2016). Weed seed that is retained and enters the combine during harvest is normally redistributed across fields, thereby helping to replenish the soil seedbank each year (Shirtliffe and Entz Reference Shirtliffe and Entz 2005; Walsh and Powles Reference Walsh and Powles 2007). Seed of weeds, such as Palmer amaranth and common cocklebur (Xanthium strumarium L.), collected by the combine during soybean harvest, predominantly exit in the chaff and straw fractions (Green Reference Green 2019). Capturing and destroying these seed through HWSC practices to prevent seedbank inputs is paramount to the management of these major weed species.

In Australia, HWSC is widely used, with narrow-windrow burning being the most commonly used option (Walsh et al. Reference Walsh, Ouzman, Newman, Powles and Llewellyn 2017). This adoption was facilitated by research comparing the efficacy of burning narrow windrows as opposed to burning standing wheat stubble on the seed survival of problematic species such as rigid ryegrass and wild radish (Raphanus raphanistrum L.). For the narrow-windrow burns, temperatures at the soil surface were sufficiently high for a long enough duration to destroy the seeds of rigid ryegrass and wild radish; however, burning standing stubble remaining after harvest did not produce the required duration of high temperatures (Walsh and Newman Reference Walsh and Newman 2007; Walsh et al. Reference Walsh, Newman and Powles 2013). The low cutting height and high amount of biomass that enters the combine during harvest make soybean a favorable candidate to potentially burn and destroy seed from weed escapes in the field.

In southern U.S. crop production systems, two of the most troublesome weeds are Palmer amaranth and barnyardgrass (Riar et al. Reference Riar, Norsworthy, Steckel, Stephenson, Eubank and Scott 2013; Schwartz-Lazaro et al. Reference Schwartz-Lazaro, Norsworthy, Steckel, Stephenson, Bradley and Bond 2018; WSSA 2017). Palmer amaranth has documented resistance to herbicides that inhibit microtubule assembly, very long chain fatty acid elongase, acetolactate synthase, 5-enolpyruvylshikimate-3-phosphate synthase, photosystem II, 4-hydroxyphenylpyruvate dioxygenase, and protoporphyrinogen oxidase (Brabham et al. Reference Brabham, Norsworthy, Houston, Varanasi and Barber 2019; Heap Reference Heap 2019; Varanasi et al. Reference Varanasi, Brabham and Norsworthy 2018). Barnyardgrass is the most problematic weed of rice (Oryza sativa L.), a crop that is routinely grown in rotation with soybean in the southern United States. Additionally, jungle rice (Echinochloa colona L.), a close relative of barnyardgrass, has recently evolved resistance to glyphosate in the southern United States, further limiting control options (Nandula et al. Reference Nandula, Montgomery, Vennapusa, Jugulam, Giacomini, Ray, Bond, Steckel and Tranel 2018). As resistance continues to increase and become more widespread, effective herbicide options decrease. Stewardship of remaining effective herbicide options must be a priority for successful weed management (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett 2012), requiring growers to diversify weed management tactics. Previous research has shown that narrow-windrow burning can be successful in reducing the population of Palmer amaranth (Norsworthy et al. Reference Norsworthy, Korres, Walsh and Powles 2016).

Understanding the efficacy of narrow-windrow burning in soybean requires that multiple weed seeds, ranging from small to large, be evaluated for their response to combinations of burning temperatures and durations. Other notable weeds of concern would be species such as barnyardgrass (small-seeded grass), johnsongrass (large-seeded grass), and pitted morningglory (large-seeded broadleaf). Like Palmer amaranth, barnyardgrass has been shown to be resistant to multiple herbicide sites of action (Heap Reference Heap 2019). Johnsongrass is considered the most troublesome weed in grain sorghum [Sorghum bicolor (L.) Moench] and corn (Zea mays L.) (SWSS 2012). Johnsongrass has been shown to be resistant to glyphosate in the state of Arkansas (Heap Reference Heap 2019) and can cause substantial yield loss if left untreated in a field. Pitted morningglory is also ranked in the top 10 most troublesome weeds of multiple crops including soybean, corn, and grain sorghum (SWSS 2012, 2013). Pitted morningglory can cause significant yield reduction in soybean (Howe and Oliver Reference Howe and Oliver 1987; Norsworthy and Oliver Reference Norsworthy and Oliver 2002), interfere with harvest, and persist for long periods in the soil seedbank (Egley and Chandler Reference Egley and Chandler 1983).

Harvest weed seed control can be implemented by using various tactics, including narrow-windrow burning, chaff carts, the bale-direct system, or impact mills such as the integrated Harrington Seed Destructor (Walsh et al. Reference Walsh, Newman and Powles 2013). The low cost of implementing narrow-windrow burning makes this strategy an attractive option; however, the efficacy of narrow-windrow burning on various weed seeds that may pass through the combine at harvest is unknown and expected to be different for weed species that differ in size. In previous research by Walsh and Newman ( Reference Walsh and Newman 2007), the destruction of rigid ryegrass and wild radish differed with temperature and duration of temperature. The objective of this research was to examine the specific temperature and duration requirements needed to kill the seed of problematic weeds of southern U.S. cropping systems. This research is crucial for estimating the potential efficacy of narrow-windrow burning on weeds common to soybean production systems. Additionally, the efficacy of narrow-windrow burning following soybean grain harvest on Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory was evaluated to assess the effectiveness of the tactic in killing seed of these weeds prior to entry into the soil seedbank. It was hypothesized that narrow-windrow burning of soybean harvest residues produced during the harvest of a typical irrigated soybean crop will be successful in destroying seed of major weed species of southern U.S. crops.

Materials and Methods

An experiment was conducted at the Altheimer Laboratory (35.0948 N, 94.1733 W; 384 m elev) located in Fayetteville, AR, to determine the temperature and duration needed to kill the seed of Palmer amaranth, barnyardgrass, johnsongrass, pitted morningglory, hemp sesbania, prickly sida, sicklepod, velvetleaf, and Italian ryegrass. These small- and large-seeded grasses and broadleaves, as well as the weed species with hard seed coats (i.e., pitted morningglory, hemp sesbania, sicklepod, and velvetleaf), were evaluated because they are weeds that frequently occur in southern United States soybean fields.

Viability was initially determined for the seed of each weed species using tetrazolium chloride (Wharton Reference Wharton 1955). Once viability was determined, 100 seeds of each species, with the exception of barnyardgrass, were counted into separate packets. For barnyardgrass, samples of 200 seeds were used because of the lower viability of the available seed lot. The seed samples were then emptied into porcelain crucibles measuring 4 cm in height and 5 cm at the top outside diameter (Cole-Parmer, Vernon Hills, IL), placed inside a high-fire kiln (Paragon Industries, L. P. Mesquite, TX), and subjected to 20 combinations of temperature (200, 300, 400, 500, and 600 C) durations (20, 40, 60, 80 s). For the kiln used in this experiment, a burn was considered acceptable if the temperature inside the kiln varied no more than ±10 C of each experimental temperature. Viability of the seeds evaluated prior to burning were accounted for when calculating survival percentage.

The specified temperatures and times for burning seed in this experiment allowed for a calculation of heat index (HI). Heat index is calculated by summing the temperature achieved above ambient for each second duration of heat exposure. The ambient temperature at the time of this experiment was 23.9 C. The experiment was conducted in two runs with two replications per run. After heat treatment, seeds of pitted morningglory, hemp sesbania, sicklepod, and velvetleaf were scarified or sliced with a razor blade and placed between two filter papers soaked with a 1% w/v tetrazolium chloride solution for approximately 24 h before checking for germination and staining. Seeds of Palmer amaranth, barnyardgrass, johnsongrass, prickly sida, and Italian ryegrass were soaked between two filter papers soaked with the same tetrazolium chloride solution for approximately 48 h before being sliced to assess staining. A seed was considered viable if the seed had germinated or if 10% of the internal seed structure was stained pink to red. Results for live seed were then converted into a percentage of survivors based on the viability of the unburned controls so that a seed kill rate (mortality) could be determined for each weed species (Equation 1).

Effectiveness of Narrow-windrow Burning Soybean Harvest Residues on Weed Seed Kill

A field experiment was conducted at the University of Arkansas Northeast Research and Extension Center (35.6720 N, 90.0844 W; 70 m elev) in Keiser, AR, in 2014 and 2015 in a production field of Credenz 4950LL (Bayer CropScience, St. Louis, MO) soybean grown under irrigated conditions to assess the heat intensity and efficacy of killing the seeds of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory. Because the amount of soybean residue will probably affect the heat intensity of burning, narrow windrows with increasing levels of residue were created by harvesting increasingly wider soybean plots (4.8 to 9.6 m) with a Case 2388 combine (Case IH, Mount Pleasant, WI) fitted with a 9.1-m-wide header. This range in plot widths was equivalent to 5 to 10 soybean rows, where one soybean row was added (0.96 m width) for each increase in plot width. The 5 rows harvested represented a low-yielding environment, and the 10 rows represented a normal yield for a typical irrigated, high-yielding soybean, which was approximately 4,700 kg ha −1 each year. The length of row was in excess of 10 m for each narrow-windrow burn that was evaluated. After harvest, 1 m of row was collected from each narrow-windrow treatment near the end of the 10-m row. Samples were weighed in the field and were returned to the Altheimer Laboratory in Fayetteville to be dried. Just prior to burning, 100 seeds each of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory were placed beneath the windrow on the soil surface in separate 5-cm-diam aluminum tins to assess weed seed kill of the burn treatments. The temperature at the location of the weed seed was recorded every second throughout the burn using an Omega Engineering Type K thermocouple and data logger (Omega® Engineering Inc., Stamford, CT).

The data logger allowed for a calculation of HI and effective burn time (EBT). Effective burn time is similar to HI but is only the number of seconds that a burn is above a specified temperature. For example, in this experiment, EBT 200 is the designation used for the number of seconds that a burn was above 200 C. Immediately after burning, the weed seed–containing aluminum tins were collected and returned to the Altheimer Laboratory for germination and viability assessments. Seeds that were recovered from the burns were completely ash, with the exception of pitted morningglory. To ensure that no seed was missed in the ash, germination tests were conducted in an incubator set at 40 C with a 16-h day and 8-h night for each weed species for 14 d. In preparation for the germination test, the ash from the tins was placed in Petri dishes lined with filter paper and moistened with a 1% v/v Captan solution (Captan 4 Flowable; Drexel, Memphis, TN) as needed. At the end of the 14-d period, Petri dishes were examined for any germinated or nongerminated seed. For pitted morningglory, seeds were additionally stained using 1% w/v tetrazolium chloride to test for viability.

Effect of Wind Speed on Narrow-windrow Burning

In 2014 and 2015, the impact of wind speed on HI and EBT was assessed using a Stihl BG 55 leaf blower (Stihl Holding AG & Co. KG, Waiblingen, Germany). For this experiment, an anemometer was placed within 10 cm of the windrow, and a leaf blower was positioned to create a predetermined wind speed parallel to the row. An Omega Engineering data logger was placed under the narrow windrow at the time of burning, and temperatures were recorded every 1 s until temperatures peaked. Heat index and EBT calculations were based on the data logger readings in the same manner as described in the previous field experiment.

Statistical Analyses

Data from the kiln experiment averaged across runs were subjected to regression analysis using Equation 2:

where y = percent survival, B 0 = intercept, B 1 = temperature (C), B 2 = time (s), and B 3 = the coefficient of the product term for x 1 x x 2. A linear model (Equation 3) was also used to determine the relationship between HI and percent survival:

where y = percent survival, B 0 = intercept, B 1 = slope estimate for HI.

A determination of lowest HI where no survival was observed was chosen once 0% survival was reached, and no data points after were > 0%.

Data from the field experiment evaluating the influence of soybean biomass (residues) on narrow-windrow burning were fit using Equation 4 with data combined across site-years:

where y = response (HI, EBT 200), B 0 = intercept, B 1 = regression coefficient for soybean residues (kg m −2 ), and B 2 = regression coefficient for wind speed (m s −1 ).

Data for the field experiment evaluating the impact of wind speed on narrow-windrow burning were fit to a linear model where y = response (HI and EBT 200), B 0 = intercept, and B 1 = slope estimate for wind speed (m s −1 ).

For all experiments, data were fit in the FIT MODEL platform in JMP Pro 13 (SAS Institute Inc., Cary, NC).

Results and Discussion

Heat Effects on Weed Seed Survival

Palmer amaranth, barnyardgrass, hemp sesbania, sicklepod, velvetleaf, and Italian ryegrass seed survival were regressed using temperature and duration of exposure as explanatory variables (Table 1). It is well established that the duration of exposure needed to kill weed seed is lessened as temperature increases to a level typically observed when burning crop residues (Hoyle and McElroy Reference Hoyle and McElroy 2012; Thompson et al. Reference Thompson, Jones and Blair 1997; Walsh and Newman Reference Walsh and Newman 2007; White and Boyd Reference White and Boyd 2016). Likewise, our study found that lengthening the exposure period reduced the temperature needed to achieve mortality of weed seeds evaluated, a result similar to findings in other studies (Egley Reference Egley 1990; Hoyle and McElroy Reference Hoyle and McElroy 2012; Walsh and Newman Reference Walsh and Newman 2007). Seeds of each species tested in this experiment, with the exception of sicklepod, were killed at or before the highest temperature and longest exposure tested, which was 600 C for 80 s (HI = 46,088) (Figures 1 and 2). Using Figure 1, one can estimate the minimum temperature and duration combinations for each weed species necessary to achieve complete mortality, excluding sicklepod. Regardless of species, no seed kill was completely achieved at 200 C for any time tested. These temperature requirements for complete seed kill differ from those for common weeds of turf, where only 5 s at 200 C or 20 s at 150 C was needed to achieve 100% mortality of Virginia buttonweed (Diodia virginiana L.), cock’s comb killinga (Kyllinga squamulata Thonn. ex Vahl), and large crabgrass [Digitaria sanguinalis (L.) Scop] (Hoyle and McElroy Reference Hoyle and McElroy 2012).

Table 1. Parameter estimates and P values from a multiple-regression model a for a high-fire kiln experiment conducted on nine species at the Altheimer Laboratory in Fayetteville, AR.

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The widespread evolution of multiple herbicide resistance in the most serious annual weeds infesting Australian cropping fields has forced the development of alternative, non-chemical weed control strategies, especially new techniques at grain harvest. Harvest weed seed control (HWSC) systems target weed seed during commercial grain harvest operations and act to minimize fresh seed inputs to the seedbank. These systems exploit two key biological weaknesses of targeted annual weed species: seed retention at maturity and a short-lived seedbank. HWSC systems, including chaff carts, narrow windrow burning, bale direct, and the Harrington Seed Destructor, target the weed seed bearing chaff material during commercial grain harvest. The destruction of these weed seeds at or after grain harvest facilitates weed seedbank decline, and when combined with conventional herbicide use, can drive weed populations to very low levels. Very low weed populations are key to sustainability of weed control practices. Here we introduce HWSC as a new paradigm for global agriculture and discuss how these techniques have aided Australian grain cropping and their potential utility in global agriculture. La ampliamente diseminada evolución de resistencia a múltiples herbicidas en las malezas anuales más serias infestando los sistemas de cultivos australianos ha forzado el desarrollo de estrategias de control de malezas alternativas, especialmente nuevas técnicas al momento de la cosecha de granos. Los sistemas de control de semillas de malezas en cosecha (HWSC) se enfocan en las semillas de malezas durante las operaciones de cosecha comercial de granos y actúan para minimizar el suministro de semillas frescas al banco de semillas. Estos sistemas explotan dos debilidades biológicas clave de las especies de malezas anuales de interes: retención de semilla al momento de la madurez y un banco de semillas de corta vida. Los sistemas HWSC, incluyendo las carretas de descarga de grano, la quema de lineas angostas de residuos después de la cosecha, el embalado directo, y el Destructor de Semilla Harrington, se enfocan en los residuos de cosecha que contienen semillas de maleza durante la cosecha comercial de grano. La destrucción de estas semillas de malezas durante o después de la cosecha del grano facilitan la reducción del banco de semillas de malezas, y cuando se combinan con el uso convencional de herbicidas, pueden llevar las poblaciones de malezas a niveles muy bajos. Tener poblaciones muy bajas de malezas es clave para la sostenibilidad de las prácticas de control de malezas. Aquí, nosotros introducimos HWSC como un nuevo paradigma para la agricultura global y discutimos como estas técnicas han ayudado a la producción australiana de granos y su utilidad potencial en la agricultura global.

Weed Technology publishes original research and scholarship focused on understanding “how” weeds are managed. As such, it is focused on more applied aspects concerning the management of weeds.

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