Due to the current drought conditions in certain areas, post-harvest weed emergence and growth are expected to be accelerated this year. The shorter and thinner wheat crop, combined with recent rain precipitation in drought-stricken regions will provide favorable conditions for weed growth. The increased availability of moisture will encourage the germination and development of weeds in the post-harvest period. Refer to Figure 1 for visual representation of this year's wheat, which is shorter and thinner. Additionally, consult Figure 2 to observe the impact of straw level on the weed population.

Wheat farmers can usually count on heavy wheat residue, especially with wheat harvested with a stripper header, to prevent emergence and slow the growth of weeds after wheat harvest. After harvest, weed growth can accelerate and weed control is not always the priority of farmers working to finish harvest.

The value of controlling weeds in wheat stubble after wheat harvest was reported in the research results of the late Gail Wicks, which concluded up to three inches of water were saved in the soil profile when weeds were controlled in a timely manner after winter wheat harvest. Subsequently, each inch of soil water increases the yield of corn or grain sorghum by approximately 12 bushels the following year.

Figure 1. Shorter and thinner wheat in 2022 — note weeds in the less competitive wheat.

Also, experience with controlling weeds after wheat harvest sometimes resulted in yields of as much as 100 bushels more per acre compared to fields where the weeds were not controlled after harvest. Where does the “as much as 100 bushel” increase in yield occur? That happens when the corn or sorghum without timely weed control after winter wheat harvest has three inches less soil water and is lost to drought, and the crop with timely weed control after winter wheat harvest with three inches more soil water can hang on longer in the drought and receives moisture before it uses up the additional three inches of soil water. The extent of yield increase is contingent upon the quantity of additional moisture available.

When Ecofallow Cropping System (Ecofallow is controlling the weeds during the fallow period — after winter wheat harvest until the following crop is planted the next spring — by using herbicides and/or tillage with minimum disturbance of crop residues and soil) began in the early 1970s, the main herbicides used were paraquat and atrazine. Some did not want the extra cost of paraquat (Gramoxone and other products with the same active ingredient) so they used a sweep blade to kill the weeds. Some farmers even added sprayers to their sweep implement to spray the atrazine at the same time they used the sweep. In many cases, this practice has not been favorable due to the potential loss of soil water through tillage and the absence of a firm seedbed for spring planting. Also, tillage plants weed seeds.

Ecofallow corn

The crop following the Ecofallow period is referred to as Ecofallow corn or Ecofallow sorghum. Crop producers who had problems with weeds in their corn or sorghum found it was usually the result of not controlling the weeds timely or not at all after wheat harvest. This, of course, did not make it Ecofallow corn or sorghum. This permitted the weeds to use soil water and produce a great deal of seed since there was no crop competition in the fallow.

When left uncontrolled after harvest, volunteer wheat can present similar challenges to weeds as it competes for water and nutrients. Particularly, early volunteer wheat that emerges due to factors like hailstorms is a significant contributor to the occurrence of wheat streak mosaic disease, see Why It’s So Important to Control Volunteer Wheat and Weeds in Wheat Stubble After Harvest in the June 23, 2023 issue of CropWatch. Additionally, uncontrolled volunteer wheat after harvest can act as a host for cutworms, leading to issues in Ecofallow corn and sorghum, respectively.

The Ecofallow production system (later called "ecofarming") also includes reduced soil erosion and production costs, and increased weed control, water infiltration, moisture conservation and crop yield. The task of managing weeds became more challenging with the emergence of kochia resistance to triazine herbicides and subsequently to other herbicides as well. However, as with many situations, the problem can escalate further.

A farmer recently expressed his perspective, stating that while dealing with kochia resistant to multiple herbicides was a significant issue, his perception changed when he encountered herbicide-resistant Palmer amaranth. This suggests that the presence of herbicide-resistant Palmer amaranth posed an even greater problem and highlighted the escalating nature of herbicide resistance issues in weed management. It underscores the importance of proactive strategies and diverse approaches to effectively tackle evolving weed resistance challenges in agriculture.

Timing

Weeds under stress are difficult to control with herbicides. Kochia and Russian thistle are day-length sensitive and will begin to flower toward the end of July and into August. In general, for wheat grown as part of a three-year rotation, it is commonly advised to wait up to 30 days after harvest before spraying.

However, it's important to note that specific circumstances may require earlier intervention. In the case of aggressive weeds like Palmer amaranth, it is crucial to control them before they reach a height of three to four inches. If the wheat was planted without an 11- to 14-month fallow period, spray it within 15 days of harvest. Examine each field separately and adjust your treatment schedule accordingly.

Due to the less competitive nature of winter wheat stands in certain fields this year, it may be necessary to apply herbicides earlier than the usual 15– to 30-day timeframe after harvest.

The crucial factor is timely weed control to prevent them from utilizing soil water and producing weed seeds. Ensuring prompt action against weeds helps safeguard the water resources in the soil and prevents the proliferation of weed populations through seed production.

As with all weed control, it is essential to closely watch for weed development and spray at the proper time as noted on the label to achieve maximum control. Most labels state that weeds must be treated before they are six inches tall. If weeds are under severe drought stress, wait for rain and spray after they recover from drought stress.

With many weeds developing resistance to several herbicides, we are back to using paraquat in many of our burndown treatments. The standard rate of paraquat in the 1970s was 16 fl oz/acre. To obtain adequate control at those rates, proper timing and spray coverage was critical. Research by Kansas State University helps increase our knowledge on paraquat and other treatments for Palmer amaranth (view at the end of this article).

Split Treatments

Split treatments have a good history of effectiveness. When using a split treatment, apply the glyphosate products with companion herbicides if glyphosate-resistant weeds are present in the field (adding surfactant, if needed, plus ammonium sulfate) as the first application in July or early August. Glyphosate products differ in active ingredient salt, concentration and surfactant requirements, and labeling for different crops. Always refer to the product label to ensure proper usage and adherence to safety guidelines. Additional information about glyphosate formulations can be found on page 220 of EC130 (2023 Guide for Weed, Disease, and Insect Management in Nebraska).

For all glyphosate brands, add ammonium sulfate (spray grade) at 17 lb per 100 gallons of spray solution. The ammonium sulfate is the first item put into the spray tank after the water. Ammonium sulfate is especially helpful when stress conditions are present. Liquid ammonium sulfate, with or without drift reducing agents, also is available. It's vital to ensure adequate rates of liquid ammonium sulfate to promote weed efficacy. Additionally, considering water analysis — including water hardness — is crucial as it can affect the efficacy of herbicides.

Improve control by increasing the rate of glyphosate for weeds not resistant to glyphosate. Do not exceed the label use rates. Allow at least six hours — and longer with some weeds — for the glyphosate product to become rainfast. Barnyardgrass may require 24 hours without rain for maximum efficacy. With glyphosates, use a spray volume of seven to 10 gallons per acre and don't apply when temperatures reach or exceed 90°F.

Exercise caution when choosing herbicides to ensure they do not pose a risk to susceptible crops or other sensitive vegetation. Sharpen (ai saflufenacil) may help with the control of pigweeds and provide some residual control. To achieve limited or no residual broadleaf weed control, the herbicide Sharpen can be applied at a rate of 1.0 fl oz/acre (on most soil types). This application is aimed at addressing specific broadleaf weeds without leaving long-lasting residual effects in the soil.

Sharpen works best with the addition of methylated seed oil and can provide good burndown on smaller weeds. Therefore, it is important to consider the size of the weeds when using Sharpen herbicide. If the weeds are already large, applying Sharpen may lead to burning of the tops of the plants, but they may eventually resume growth. In such cases, it is advisable to address weeds at an earlier stage of growth to maximize the effectiveness of the herbicide and minimize the chances of regrowth. Sharpen requires complete coverage, so using 15 to 20 gallons/acre spray solution is important.

One herbicide alternative to glyphosate that can work well to control emerged pigweed and kochia is paraquat. Paraquat is a contact herbicide, so spray coverage is critical. Spray volumes of 20 gallons/acre or higher are preferred, especially on larger and thicker weeds. A commercial applicator who has been using a spray volume of 35 gallons/acre reported that herbicide efficacy has improved and spray drift has been reduced with the larger spray particle size they have been using with the increased spray volume. 

Paraquat also needs to be applied with a nonionic surfactant or oil concentrate to enhance surface coverage of the plant foliage. A tank mix with atrazine will enhance control and provide some residual weed control if planning to plant corn or sorghum next spring. Likewise, metribuzin can be tank-mixed with paraquat if rotating to soybean to enhance control and provide some residual.

If planting wheat this fall, adding Sharpen to a tank mix can offer residual weed control. However, the level of residual activity depends on the rate of Sharpen applied. Recent work suggests that applying paraquat as soon as possible following wheat harvest allows for better coverage and more effective control, especially of pigweeds.

Another herbicide that can be added to the burndown treatments for residual broadleaf weed control in wheat stubble is flumioxazin-based herbicides (Valor and others). Flumioxazin has been used commonly as a preplant/preemergence treatment in soybeans for years, but it hasn’t been used much in wheat stubble because of the cost. However, with the recent reduction in flumioxazin prices, it may be cost-effective.

Figure 2. The effect of winter wheat crop residue levels on weeds in corn following winter wheat treated with herbicides post-harvest. In general, one bushel of wheat produces 100 lbs of crop residue or 60 bushels of winter wheat produces 6,000 lbs of crop residue.

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Control with flumioxazin will depend on rainfall for activation, just as with the preplant treatment in soybeans. Tank mixes with glyphosate will help control weeds that are difficult to control with glyphosate alone and will help reduce the chances of developing glyphosate-tolerant weed populations. Always check labels and rotation restrictions.

If temperatures are above 80°F, use the amine formulation of 2,4-D. Do not use 2,4-D or dicamba if sensitive crops or other vegetation may be affected. For additional information, treatments, and rates, go to the ecofarming section of the 2023 Guide for Weed, Disease and Insect Management in Nebraska EC130.

The second part of the split treatment should be applied in September. It should contain at least 0.5 lb per acre of atrazine (see "Weed Challenges" below) and possibly paraquat (add surfactant), depending on the amount and size of volunteer winter wheat, downy brome, jointed goatgrass or other weeds present.

Herbicide weed control can be challenging when spraying into tall wheat stubble because the stubble intercepts the spray droplets. Recent research in the Nebraska Panhandle indicated that 12-inch stubble intercepted over 20% of the spray solution compared to tall stubble, which was close to 40%. Spray droplet penetration can increase by using coarse droplets and by traveling in an angular direction compared to the old wheat rows. Figure 2 illustrates the relationship between crop residue levels and their impact on weed growth and control.

Maximizing Herbicide Efficacy: Nozzle Selection, Carrier Volume and Multiple-Fan Nozzles

The effectiveness of herbicides is greatly influenced by the selection of nozzles, which in turn affects the droplet spectra, as well as the carrier volume used (Figure 3). Choosing the right nozzle that produces different droplet spectra is important for effective weed control because it directly affects the coverage and deposition of herbicides.

Figure 3. Spray coverage using different TeeJet (Glendale Heights, IL) nozzle types (XR, AIXR, and TTI) and carrier volumes (10, 15, and 20 gal/ac).

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Similarly, the volume of carrier used for herbicide application has a significant impact on weed efficacy. Using higher volumes generally improves the coverage and deposition on target weeds, thus increasing the effectiveness of the herbicide treatment.

In addition to nozzle selection and carrier volume, another way to enhance efficacy is by using multiple-fan nozzles. These nozzles have been developed to provide better coverage and uniformity compared to single-fan nozzles. However, it's worth noting that increasing travel speeds can improve the overall average spray deposit but may result in reduced uniformity.

By employing multiple-fan nozzles, it may be possible to target weeds in covered areas, including those within wheat straw residue. These nozzles can enhance overall coverage, making it easier to access hidden weed leaves and ensure thorough leaf burn. However, it's important to consider that increasing the rearward (>30°) plume angle can increase the potential for drift, especially when operating at higher driving speeds. Therefore, in areas where drift is a concern, it may be necessary to use multiple-fan nozzles with larger droplet spectra to minimize drift.

To maintain compliance with labeled guidelines for droplet size, it is essential to adhere to recommended practices, which include selecting the appropriate nozzles and implementing effective drift management strategies. These practices contribute to optimizing the efficacy of herbicide applications. Research is currently being conducted at UNL’s Pesticide Application Technology Laboratory to explore the potential benefits and drawbacks of using multiple-fan nozzles for spray deposition and herbicide effectiveness in three challenging broadleaf species: kochia, Palmer amaranth, and horseweed/marestail.

Weed Challenges

When dealing with difficult-to-control weeds and utilizing nonselective herbicides such as paraquat, there are several options available. To enhance the effectiveness of paraquat, it is recommended to use a minimum of two pints of surfactant or an equivalent surfactant per 100 gallons of solution. However, if using less than 20 gallons of carrier, two quarts should be used per 100 gallons of spray solution.

It's important to note that the active ingredient may vary among different products, so it is crucial to carefully review and follow the instructions on the product labels. Adjusting the rates according to the specific product being used ensures optimal results and proper herbicide application.

The atrazine rate varies with soil and rainfall patterns. In southwest Nebraska, use two quarts of atrazine per acre unless the soil or the following crop limits the rate to a lower amount. In the Panhandle, the maximum allowed in one season is often 0.5 quart per acre.

Volunteer Wheat and Grasses

Split treatments offer the advantage of effectively controlling volunteer winter wheat and other winter annual grasses. By using one quart or less of atrazine before Sept. 10, it becomes possible to plant winter wheat 12 months later in most regions and soil types. This approach allows for flexibility in crop rotation. If there is adequate soil moisture in the following spring, corn can be planted. Alternatively, if moisture is limited, the field can be fallowed, and winter wheat can be planted in the fall. This strategy takes into account soil water availability and provides options for crop selection based on the specific conditions and requirements of the area.

Downy Brome

When dealing with downy brome as a persistent issue in a winter wheat-fallow rotation, tillage is often recommended immediately after harvest to facilitate seed planting and maximize weed germination during the fallow period. However, if the crop residue after harvest is limited, avoid tillage to prevent soil erosion risks from wind and water. Herbicides can be utilized for downy brome control within the growing winter wheat, with early application being most effective.

In cases where jointed goatgrass and/or feral rye are problematic, it is advisable to implement a crop rotation where wheat is not planted for a minimum of three years. This break in planting wheat helps suppress the growth and spread of these weed species.

It is important to consider these management strategies and adapt them according to specific conditions and the recommendations provided on herbicide labels and by local agricultural authorities.

Jointed Goatgrass and Feral Rye

For fields facing challenges with jointed goatgrass or feral rye, herbicide-tolerant winter wheat varieties are a viable option. The Clearfield wheat system utilizes the Beyond™ herbicide, while the CoAXium Wheat Production System incorporates the use of Aggressor herbicide. These systems offer effective weed control options specifically tailored for managing jointed goatgrass or feral rye.

However, it is important to note that when using herbicides that contain either imazamox or quizalofop-ethyl, there may be replanting restrictions for corn and wheat. It is crucial to consult the herbicide labels for specific instructions and guidelines regarding the recommended waiting period before replanting corn or wheat crops after the application of these herbicides. Adhering to these replanting restrictions ensures the safety and efficacy of subsequent crop plantings and helps prevent any potential adverse effects on the new crops.

Efficacy of Late-Season Herbicide Programs for Controlling Palmer Amaranth in Post-harvest Wheat Stubble

Late-season control of Palmer amaranth in post-harvest wheat stubble is a challenge for Kansas producers. The objective of this study was to determine the effectiveness of POST herbicide programs (with multiple modes of actions) for late-season control of Palmer amaranth in post-harvest wheat stubble (Figure 4). The study was conducted at the Kansas State University Agricultural Research Center in Hays, Kansas, in 2019.

Figure 4. (A) Emerged Palmer amaranth seedlings under poor wheat canopy, and (B) Palmer amaranth growth in post-harvest wheat stubble at the time of herbicide applications in 2019. Photos by Vipan Kumar, K-State Research and Extension

The study site had a natural seedbank of Palmer amaranth that emerged immediately after wheat harvest. All selected herbicide programs (Figure 5) were tested three weeks after wheat harvest, when Palmer amaranth plants had attained a height of 2-2.5 feet with inflorescence initiation. Twenty-four herbicide programs comprising Roundup PowerMax, Clarity, 2,4-D, Aatrex, Gramoxone, Sencor, Valor SX, Spartan, Sharpen, Authority Supreme, Kochiavore, Panther MTZ and Huskie applied alone or in tank-mixtures were tested at recommended-use rates. All herbicide treatments were arranged in a randomized complete block design with four replications. Visual Palmer amaranth control was assessed at two, four and eight weeks after treatment (WAT) by using a rating scale of 0–100% (where 0 = no control and 100% = complete plant death). The aboveground Palmer amaranth biomass and seed production were determined by harvesting plants from a 10.7 ft 2 quadrat placed at the center of each plot 8 WAT.

All tested herbicide programs, except Kochiavore and a tank-mixture of Huskie + Aatrex provided > 88% control of Palmer amaranth 8 WAT. In contrast, late-season control of Palmer amaranth did not exceed 71% at 8 WAT with Kochiavore or a tank-mixture of Huskie plus Aatrex treatments. Consistent with visual control (%), many of those tested programs significantly reduced shoot dry weights (>77% reduction) and seed production (>93% reduction) of Palmer amaranth compared to non-treated weedy check.

Overall, these results suggest that several POST herbicide programs exist that growers can utilize for effective late-season control of Palmer amaranth in post-harvest wheat stubble.

Figure 5. Effect of late-season herbicide programs on Palmer amaranth control at two, four and eight weeks after treatment (WAT) in post-harvest wheat stubble (see Table 1 for herbicide treatment number).

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Conclusions and Implications

These preliminary results indicated that several alternatives (other than glyphosate) POST burndown herbicides — including Clarity, 2,4-D, Gramoxone, Sharpen and Liberty — exist which can be utilized in combination with Aatrex, Authority Supreme, Panther MTZ, Sencor, Spartan and Valor for effective late-season control of Palmer amaranth in post-harvest wheat stubble.

Important note: It is highly recommended for growers to carefully read the label of each herbicide product to understand and adhere to any rotational crop restrictions. These restrictions provide important guidelines regarding the time interval that needs to elapse between herbicide application and planting specific crops in the following summer. Following the rotational crop restrictions specified on the herbicide labels ensures compliance with best agricultural practices and helps avoid any potential negative effects on the intended summer crops.

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