Photo: Christy Sprague / Flowering & Senescing Horseweed Plant
Editor’s Note:Ten years ago, in the February 2005 issue of The Sugarbeet Grower, I asked weed scientists in several sugarbeet-producing states to select a single weed species that either (1) had become an increasingly serious problem for sugarbeet growers in their state/region within recent years, or (2) had the potential, in their opinion, to become one in the near future. I then also requested basic information on the chosen weed’s biology and recommended control measures. The article, which proved to be a popular one, addressed the following weeds: kochia (by Robert Wilson, University of Nebraska-Scottsbluff), lanceleaf sage (by Abdel Mesbah and Stephen Miller, University of Wyoming), common lambsquarters (by Alan Dexter, North Dakota State University/ University of Minnesota), velvetleaf (by Don Morishita, University of Idaho), yellow nutsedge (by Corey Ransom, Oregon State University) and Powell amaranth (by Christy Sprague, Michigan State University). Read our entire issue and back issues. Click here. Now, with the passage of a decade . . . and with the ensuing introduction and near-total adaptation of Roundup Ready® sugarbeets . . . and with the expansion of tolerance or resistance to glyphosate and certain other herbicides . . . , I thought it timely and appropriate to again ask sugarbeet weed scientists to weigh in on this subject. Specifically, I asked the specialists in this group to pick one weed species that they consider to be among the “most vexing” challenges for their area’s sugarbeet producers. I then asked them to briefly address the extent of that weed’s presence and its impact on their state or region’s beet producers; to discuss how growers have been managing it to date; to outline glyphosate or other resistance issues with this weed (if applicable); and finally, to provide a brief overview of their recommendations for control of this weed. Here, presented in no particular order, are the submitted commentaries. Given that most of these weeds are challenges in multiple states, each commentary should hold value for beet growers throughout the U.S. and Canada. Our sincere thanks to each of these sugarbeet weed specialists for their valued cooperation!
Kochia has always been listed as one of the most problematic weeds in sugarbeets grown in western states. One kochia plant per 16 feet of sugarbeet row has the potential to reduce sugarbeet yield by 12%. The introduction of Up-Beet in the mid 1990s was hailed as the solution to kochia problems in sugarbeets. Now we find most of the kochia growing in western states is resistant to UpBeet. The approval of Roundup Ready sugarbeet in 2008 was also proclaimed to be the end of the kochia problems in sugarbeets. In 2012 and 2013, Western Sugar Cooperative ag staff in the states of Montana, Wyoming, Nebraska and Colorado collected seeds from kochia plants suspected of having elevated tolerance to glyphosate. In all, 71 populations of kochia were screened for tolerance to glyphosate. The effective dose of glyphosate required to control 90% of the plants in each population was determined by testing kochia plants in the greenhouse with different doses of glyphosate. Roundup PowerMAX applied at rates up to 33 ounces per acre controlled 49% of the kochia populations, while 46% of the populations required from 34 to 99 ounces of Roundup for control. One population from Wyoming and two from Colorado required over 100 ounces of Roundup for kochia suppression. The results of this survey point out that many kochia populations have developed increased tolerance, and several have developed resistance to glyphosate. The future of kochia control in all crops depends on grower stewardship of the herbicides we presently have available. In sugarbeets, kochia control can be improved by adding Nortron at planting or tank mixing UpBeet with glyphosate; but, the most effective approach for kochia control is to suppress kochia in the crop preceding sugarbeet — which in the West is usually corn or small grains. Several experiments have shown the benefit of controlling kochia in corn on the following sugarbeet crop. If your corn weed control program allows one (1) kochia plant per 100 feet of row to survive, you can anticipate having 200 kochia plants per 100 feet of row the next year in sugarbeets. Therefore, making the investment in a robust corn weed control program (pre-emergence herbicide at planting followed by a mixture of postemergence herbicides, all having efficacy on kochia) is by far the best kochia management approach. Since kochia seed has a short life span in the soil, preventing seed production for two years can dramatically reduce kochia seed from the soil seed bank. Sugarbeets are always grown in rotation with other crops such as corn, small grains, dry beans or potatoes that allow producers to utilize herbicides other than glyphosate and to implement weed control programs that prevent kochia seed production. If you’re waiting on a new herbicide or a new genetic trait to help with kochia control, there doesn’t seem to be any “silver bullet” in the near future. Therefore, suppressing glyphosate-resistant kochia is going to require a integrated approach that relies on crop rotation, an understanding of kochia biology, and utilizing combinations of herbicides other than glyphosate that provide kochia control.
Waterhemp has become an important weed in crop production in many regions of the country, including fields rotated to sugarbeet in Minnesota and eastern North Dakota. Waterhemp is a summer annual weed that germinates much later than other pigweed species, i.e., through mid-to late June and into July in fields in North Dakota and Minnesota. Waterhemp can germinate and emerge from the soil surface to one-half inch deep in the soil and can remain viable for at least four years in soil. A unique feature about waterhemp is that male and female flowers are on separate plants (dioecious). That is, male plants produce pollen and female plants make seed. This unique biology creates tremendous genetic diversity in populations and results in plants that are biologically and morphologically unique. It also has contributed to development of biotypes that are resistant to several families of herbicides, including ALS, triazine, PPO and glyphosate. Waterhemp’s competitive advantage is in its ability to produce tremendous quantities of seed that potentially germinate and emerge after a farmer has completed postemergence herbicide applications. A few weed escapes in “year one” can lead to a severe weed problem in a field by “year three.” The diversity of biotypes has led to populations that have differential glyphosate tolerance. Control of susceptible biotypes and failure to control more-tolerant biotypes can very quickly lead to weed shifts that will result in the Roundup Ready system being less effective or ineffective in fields planted to sugarbeet. Sugarbeet growers in Minnesota and North Dakota have relied on glyphosate for waterhemp control — not only in sugarbeet, but in other crops in the sequence. However, there now are many fields with multiple biotypes of waterhemp, ranging from susceptible biotypes to those with moderate to full resistance to glyphosate. These observations are supported by findings from our annual survey of sugarbeet growers. Growers indicating waterhemp as their worst weed has increased from 11% and 13% in 2011 and 2013, respectively, to 36% in 2014. The year 2014 marked the first time since 2009 that a single weed species was named “worst weed” by more respondents than “no weed control problem in sugarbeet.” More than 75% of the survey respondents in Traverse, Chippewa and Renville counties in Minnesota indicated waterhemp as their worst weed, highlighting its severity in southern Minnesota and the southern counties in the Red River Valley. Three waterhemp control experiments were conducted in 2014 in a field near Herman, Minn., with a uniform population of, on average, 430 waterhemp plants per square meter. The first experiment evaluated the use of a “postemergenceonly” system. Roundup PowerMAX (glyphosate) at 28 fl oz/A plus NIS and AMS controlled 77%, or 325, two- to three-inch waterhemp per square meter, suggesting there were many susceptible biotypes in the field. However, a second and third application of Roundup PowerMAX at 28 and 22 fl oz/A, respectively, did not control the remaining, partially or fully resistant waterhemp biotypes. Tank mixing Nortron (ethofumesate), Betamix (desmedipham & phenmedipham) or UpBeet (triflusulfuron) with Roundup PowerMAX improved waterhemp control; but all herbicide treatments were commercially unacceptable at harvest, indicating the need for a different strategy. The second waterhemp control experiment considered either Dual Magnum (s-metolachlor) at 1 pt/A, Outlook (dimethenamid-P) at 12 fl oz/A or Warrant at 3 pt/A in combination with Roundup PowerMAX and Nortron at 28 and 4 fl oz/A plus Destiny HC and AMS. The objective was to control the emerged one- to two-inch waterhemp with Roundup PowerMAX and Nortron and Dual Magnum, Outlook or Warrant to provide residual weed control, “lay-by.” Achieving residual control requires precipitation to activate the lay-by herbicide. This control strategy gave 73 to 92% waterhemp control at harvest, depending on herbicide, as compared to 30% waterhemp control from three applications of Roundup PowerMAX. The third experiment considered Ro-Neet SB, Norton or Dual-Magnum applied pre-emergence at planting, followed by glyphosate postemergence. Ro-Neet SB at 5.3 pt/A or Dual Magnum at 0.75 to 1 pt/A, followed by three applications of Roundup PowerMAX, provided 89 to 94% waterhemp control, compared to 70% control from Nortron at 6 pt/A followed by Roundup PowerMAX. All soil-applied herbicides followed by glyphosate provided greater waterhemp control than did three glyphosate applications. Sugarbeet injury was minor from Ro-Neet SB, Dual Magnum at 0.5 to 0.75 pt/A and Nortron. However, Dual Magnum at 1 to 2 pt/A caused either observed sugarbeet growth reduction injury and/or stand loss. Based on results from these and other waterhemp experiments in 2014, our waterhemp control strategy for sugarbeet growers is: 1. For waterhemp control on low-level infestations of biotypes that are mostly susceptible to glyphosate, apply Roundup PowerMAX at 28 fl oz/A plus Nortron at 4 fl oz/A plus Destiny HC and AMS for control of waterhemp one to two inches tall. Make repeat applications at 14- to 21-day intervals as needed, or if new weeds emerge. 2. For waterhemp control in moderate-level infestations and in fields with glyphosate-resistant biotypes, apply Dual Magnum, Warrant or Outlook in combination with Roundup PowerMAX plus Nortron plus Destiny HC and AMS when waterhemp are one to two inches tall for postemergence and lay-by (residual) control. Sugarbeet must be two-leaf or larger at application as required by the herbicide labels. 3. For waterhemp control in moderate- to heavy-level infestations, or in fields with glyphosate-resistant biotypes, apply Dual Magnum at 0.5 to 0.75 pt/A preemergence at planting (use 0.75 pt/A on higher-organic matter soils), followed by Roundup PowerMAX plus Nortron plus Destiny HC and AMS. Make repeat applications at 14- to 21-day intervals as needed, or if new weeds emerge.
Common lambsquarters is a ubiquitous annual weed found throughout the U.S. in irrigated and non-irrigated agriculture. It has long been a challenge to control in sugarbeets, even before the introduction of Roundup Ready beets in 2008. It also has long been known that common lambsquarters is difficult to control with glyphosate. Although it does not require the timeliness of the old Betamix and Betamix Progress applications (which started at the cotyledon stage), it is often poorly controlled with glyphosate because applicators wait too long to spray it. Since Roundup Ready beets became available, many growers have relied solely on glyphosate for controlling common lambsquarters. There are some who have used tankmix partners with glyphosate as a means of getting more-consistent control. A few years ago, when there were some legal issues that threatened the planting of Roundup Ready sugarbeets, grower-cooperatives across the country tried to prepare themselves for planting conventional beets again. As part of the planning, several thousand gallons of Betamix were purchased, and some growers have since been using it with glyphosate. Other growers — employing another mechanism of action — have used Dual Magnum, ethofumesate (Nortron SC, Ethotron, Ethofumesate SC), Eptam, Outlook, Treflan and Warrant. Of these, ethofumesate, Treflan and Warrant will most effectively control common lambsquarters. It’s important to remember, however, that these soil-active herbicides will not control emerged lambsquarters (with the exception of ethofumesate, which has some post emergence activity). Common lambsquarters and kochia have been the two weeds in Idaho of greatest concern to become resistant to glyphosate. Our studies have shown common lambsquarters is best controlled with glyphosate before this weed is more than two inches tall. We have recommended, since 2008, that growers consider tank mixing another herbicide mechanism of action with glyphosate as a resistance management strategy. However, in order to achieve the most with these tank mixtures, the herbicide partner should effectively control target weeds like common lambsquarters. This past summer, we confirmed glyphosate-resistant kochia in eastern Oregon and western Idaho. In addition, at least two common lambsquarters populations in sugarbeets are suspected as possibly resistant due to the lack of control of this species and the control of all other weeds in the same fields. Further tests are currently being conducted to determine whether these populations are resistant or at least have increased tolerance. As we have recommended in previous years to growers, there is a definite need to implement resistance management strategies for controlling common lambsquarters as well as other weeds found in sugarbeets. This means not only using other herbicides with a different mechanism of action than glyphosate; it also means using mechanical and cultural practices. Tillage to control early emerging common lambsquarters and cultivation to control lambsquarters that escaped chemical weed control are just two mechanical options. Cultural practices include rotating to crops where common lambsquarters is less competitive (such as small grains). If corn is in the rotation, there are many other herbicide choices that will effectively control this weed. Yellow Nutsedge |
Horseweed (a.k.a. Marestail) | 
Photo: Christy Sprague / Horseweed Rosette
|
Horseweed (a.k.a. marestail) is a weed that was not on the radar for most sugarbeet growers 10 to 15 years ago. However, in the last five years this weed has been a major concern for Michigan sugarbeet growers. Extended emergence patterns and the development of resistance to glyphosate and other herbicides have made horseweed a management challenge in several crops, including Roundup Ready sugarbeet.
Historically, horseweed was classified as a winter annual since the majority of seedlings (68-95%) emerged in the fall. However, more-recent studies have shown higher numbers of horseweed plants emerging in the early spring, and often emergence continues throughout the growing season. Up to 91% of horseweed that emerges in the fall can survive the winter. Most fall-germinating horseweed overwinters as a rosette and starts to bolt in the spring. The larger the rosette prior to winter, the greater the survival in the spring. Horseweed that germinates in late July and August often remains as a rosette until spring. Spring-emerging horseweed is in the rosette stage for a relatively short period of time before bolting. Once horseweed starts to bolt, it becomes more difficult to control with herbicides and tillage. Both fall- and spring-emerging horseweed tend to flower in late June/early July and begin to senescence and disperse seed in August.
Horseweed produces approximately 200,000 seeds per plant. This small wind-blown seed has a feather-like attachment, like dandelion, known as a pappus. This attachment allows horseweed seed to be dispersed by wind to great distances. In fact, there have been studies that suggest horseweed seed can enter the Planetary Boundary Layer. Other studies have found that as many as 125 horseweed plants per square yard have been found 400 feet from the seed source. This dispersal mechanism makes it difficult to contain a horseweed problem once it occurs.
One of the greater challenges with managing horseweed is the development of herbicide resistance. Many populations of horseweed are resistant to ALS-inhibiting herbicides (Group 2), glyphosate (Group 9) or a combination of both. This makes horseweed management extremely difficult, since there are very few alternative herbicide options available for control in several crops. For example, if a horseweed population has multiple resistance to ALS-inhibiting herbicides and glyphosate, there currently are no post emergence herbicide options for control in soybean, unless a grower chooses to plant Liberty Link soybean.
As mentioned earlier, horseweed generally emerges in the fall and early spring, so tillage can often be an option for management prior to planting. The tillage pass or passes need to create uniform disturbance in the top few inches. Fall tillage alone and many of the new vertical tillage units may not be completely effective. If horseweed is bolting and gets larger size, control can also be difficult with tillage. Due to early spring emergence, we often see horseweed as more of a problem in sugarbeet planted into stale seedbed fields. However, even with spring tillage there is the possibility of horseweed emergence after sugarbeet planting. While there are no perfect options for control of this weed, horseweed is the one glyphosate-resistant weed species that we may have a better chance managing in sugarbeet. At Michigan State University, we have conducted research looking for the best methods of control for glyphosate-resistant (Group 9) horseweed in Roundup Ready sugarbeet. From this work we know that we can have good control of glyphosate-resistant horseweed with Stinger (Group 4). However, control with Stinger is rateand horseweed size-dependent.
From our work, Stinger applied at 2 fl oz/A was not very effective, even if it was applied twice, two weeks apart. From the rates that we examined, 3 to 4 fl oz/A or more of Stinger applied at least twice was needed to control glyphosate-resistant horseweed. However, our best management strategy for season-long control was Stinger applied three times at 2 fl oz/A, followed by 4 fl oz/A, followed by 4 fl oz/A at the typical glyphosate application timings (two-leaf beets, six- to eight-leaf beets, and at canopy). Stinger can be tank mixed with glyphosate + AMS or applied alone.
We are continuing to examine different strategies to manage this weed.
Historically, horseweed was classified as a winter annual since the majority of seedlings (68-95%) emerged in the fall. However, more-recent studies have shown higher numbers of horseweed plants emerging in the early spring, and often emergence continues throughout the growing season. Up to 91% of horseweed that emerges in the fall can survive the winter. Most fall-germinating horseweed overwinters as a rosette and starts to bolt in the spring. The larger the rosette prior to winter, the greater the survival in the spring. Horseweed that germinates in late July and August often remains as a rosette until spring. Spring-emerging horseweed is in the rosette stage for a relatively short period of time before bolting. Once horseweed starts to bolt, it becomes more difficult to control with herbicides and tillage. Both fall- and spring-emerging horseweed tend to flower in late June/early July and begin to senescence and disperse seed in August.
Horseweed produces approximately 200,000 seeds per plant. This small wind-blown seed has a feather-like attachment, like dandelion, known as a pappus. This attachment allows horseweed seed to be dispersed by wind to great distances. In fact, there have been studies that suggest horseweed seed can enter the Planetary Boundary Layer. Other studies have found that as many as 125 horseweed plants per square yard have been found 400 feet from the seed source. This dispersal mechanism makes it difficult to contain a horseweed problem once it occurs.
One of the greater challenges with managing horseweed is the development of herbicide resistance. Many populations of horseweed are resistant to ALS-inhibiting herbicides (Group 2), glyphosate (Group 9) or a combination of both. This makes horseweed management extremely difficult, since there are very few alternative herbicide options available for control in several crops. For example, if a horseweed population has multiple resistance to ALS-inhibiting herbicides and glyphosate, there currently are no post emergence herbicide options for control in soybean, unless a grower chooses to plant Liberty Link soybean.
As mentioned earlier, horseweed generally emerges in the fall and early spring, so tillage can often be an option for management prior to planting. The tillage pass or passes need to create uniform disturbance in the top few inches. Fall tillage alone and many of the new vertical tillage units may not be completely effective. If horseweed is bolting and gets larger size, control can also be difficult with tillage. Due to early spring emergence, we often see horseweed as more of a problem in sugarbeet planted into stale seedbed fields. However, even with spring tillage there is the possibility of horseweed emergence after sugarbeet planting. While there are no perfect options for control of this weed, horseweed is the one glyphosate-resistant weed species that we may have a better chance managing in sugarbeet. At Michigan State University, we have conducted research looking for the best methods of control for glyphosate-resistant (Group 9) horseweed in Roundup Ready sugarbeet. From this work we know that we can have good control of glyphosate-resistant horseweed with Stinger (Group 4). However, control with Stinger is rateand horseweed size-dependent.
From our work, Stinger applied at 2 fl oz/A was not very effective, even if it was applied twice, two weeks apart. From the rates that we examined, 3 to 4 fl oz/A or more of Stinger applied at least twice was needed to control glyphosate-resistant horseweed. However, our best management strategy for season-long control was Stinger applied three times at 2 fl oz/A, followed by 4 fl oz/A, followed by 4 fl oz/A at the typical glyphosate application timings (two-leaf beets, six- to eight-leaf beets, and at canopy). Stinger can be tank mixed with glyphosate + AMS or applied alone.
We are continuing to examine different strategies to manage this weed.
Redroot Pigweed | 
Photo: Andrew Kniss / Redroot Pigweed
|
Many pigweed species can be problematic in sugarbeet fields. The most common species in western U.S. sugarbeet growing regions (Nebraska, Colorado, Wyoming, Montana and Idaho) is redroot pigweed, though redroot pigweed can be found in nearly all sugarbeet states. Redroot pigweed is characterized by an erect main stem that can grow up to six feet tall, and a taproot that is relatively easy to pull, compared to weeds like common lambsquarters or kochia.
Redroot pigweed is so named because of the red or pink color on the taproot, but several other species of pigweed also share this trait. The flowering sections of redroot pigweed are considerably shorter in length than many other pigweeds, especially Palmer amaranth and waterhemp. There are some reports that a white or dark chevron on the leaves can be used to differentiate between pigweed species, but this is incorrect; nearly all pigweed species can have this mark on the leaves. There are several excellent university extension bulletins available to help identify pigweed species. (See the link to the Kansas bulletin at the end of this commentary.)
Redroot pigweed densities of only eight plants per 100 square feet can reduce sugarbeet yield by nearly 20%, in large part due to competition for sunlight. Redroot pigweed can produce many thousands of seeds, with some reports approaching one million seeds per plant. Seeds can remain viable in the soil for up to 17 years. Optimal soil temperatures for redroot pigweed seed germination can approach 95° F, meaning it tends to emerge later in the season than many other weeds. The late emergence time is one reason redroot pigweed has continued to be problematic since the conversion to Roundup Ready sugarbeet.
There currently are no confirmed cases of glyphosate-resistant redroot pigweed in the U.S. Even so, redroot pigweed has been the most common weed observed late in the season in sugarbeet fields in Wyoming, Nebraska and Colorado. Roundup (and other glyphosate-based herbicides) will provide excellent control of emerged redroot pigweed. But because redroot pigweed can emerge well into June and July, a substantial number of its plants may emerge after the final glyphosate application of the season. To manage this late-season flush of redroot pigweed, there are many residual herbicides that can be applied with glyphosate between the eight-true-leaf stage of sugarbeet and canopy closure. Nortron (ethofumesate) applied postemergence can provide very good control of late-season flushes of redroot pigweed. Warrant (acetochlor) is a recently registered herbicide that can also provide good control of late-emerging redroot pigweed if tank mixed with the last application of glyphosate.
It is also important for growers in the western regions to be vigilant with respect to pigweed identification. Although redroot pigweed is still the dominant pigweed species in the region, Palmer amaranth has been moving westward the last several years and has been found as far west as the Nebraska Panhandle and near Fort Collins, Colo. Palmer amaranth has been confirmed as glyphosate-resistant in most states where it has been found, so continued monitoring for this close relative is necessary.
Here is the Kansas State University pigweed ID bulletin: http://www.ksre.ksu.edu/bookstore/pubs/s80.pdf
Read our entire issue and back issues. Click here.
Redroot pigweed is so named because of the red or pink color on the taproot, but several other species of pigweed also share this trait. The flowering sections of redroot pigweed are considerably shorter in length than many other pigweeds, especially Palmer amaranth and waterhemp. There are some reports that a white or dark chevron on the leaves can be used to differentiate between pigweed species, but this is incorrect; nearly all pigweed species can have this mark on the leaves. There are several excellent university extension bulletins available to help identify pigweed species. (See the link to the Kansas bulletin at the end of this commentary.)
Redroot pigweed densities of only eight plants per 100 square feet can reduce sugarbeet yield by nearly 20%, in large part due to competition for sunlight. Redroot pigweed can produce many thousands of seeds, with some reports approaching one million seeds per plant. Seeds can remain viable in the soil for up to 17 years. Optimal soil temperatures for redroot pigweed seed germination can approach 95° F, meaning it tends to emerge later in the season than many other weeds. The late emergence time is one reason redroot pigweed has continued to be problematic since the conversion to Roundup Ready sugarbeet.
There currently are no confirmed cases of glyphosate-resistant redroot pigweed in the U.S. Even so, redroot pigweed has been the most common weed observed late in the season in sugarbeet fields in Wyoming, Nebraska and Colorado. Roundup (and other glyphosate-based herbicides) will provide excellent control of emerged redroot pigweed. But because redroot pigweed can emerge well into June and July, a substantial number of its plants may emerge after the final glyphosate application of the season. To manage this late-season flush of redroot pigweed, there are many residual herbicides that can be applied with glyphosate between the eight-true-leaf stage of sugarbeet and canopy closure. Nortron (ethofumesate) applied postemergence can provide very good control of late-season flushes of redroot pigweed. Warrant (acetochlor) is a recently registered herbicide that can also provide good control of late-emerging redroot pigweed if tank mixed with the last application of glyphosate.
It is also important for growers in the western regions to be vigilant with respect to pigweed identification. Although redroot pigweed is still the dominant pigweed species in the region, Palmer amaranth has been moving westward the last several years and has been found as far west as the Nebraska Panhandle and near Fort Collins, Colo. Palmer amaranth has been confirmed as glyphosate-resistant in most states where it has been found, so continued monitoring for this close relative is necessary.
Here is the Kansas State University pigweed ID bulletin: http://www.ksre.ksu.edu/bookstore/pubs/s80.pdf
Read our entire issue and back issues. Click here.
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Mike Spieker
Editor & General Manager of The Sugarbeet Grower
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