Foster, S., and Schultz, B. 2016, Russian Knapweed Control Trial 2013-2015, Pershing County, University of Nevada Cooperative Extension, PS-16-04

Introduction

Located in northwestern Nevada, Pershing County has 135 farms and ranches covering more than 244,249 acres, excluding public lands in BLM-administered grazing allotments. The average farm size is 1,809 acres (2011 USDA Agricultural Census).

The local economy is heavily dependent on mining, agriculture and recreation. In addition, several large transportation and utility corridors cross the county. Each of these land uses may facilitate the establishment and spread of noxious and invasive weeds.

Noxious and invasive weeds have the potential to reduce crop yield and the amount of forage available to livestock on both public and private grazing lands (Duncan and Clark 2005). Substantial reductions in crop and livestock production could reduce the income of agricultural producers. The agricultural multiplier data strongly suggest that reduced agricultural income would have a substantial negative impact on the local economy. Furthermore, noxious weeds can significantly reduce land values. A classic study about the Brooks Ranch in North Dakota found that once noxious weeds heavily infested the ranch, its sale took 13 years, and the final price was 60 to 80 percent less per acre than for nearby areas without weeds (Weiser 1997).

In 2006, University of Nevada Cooperative Extension published the results of a statewide needs assessment about the general research and education needs of agricultural producers (Singletary and Smith 2006). Statewide, 83 percent of respondents stated that weed identification and control methods were a high priority. For Pershing County, 74 percent of the respondents identified weed identification and weed control as a moderately high to high priority.

The University of Nevada Cooperative Extension Factsheet, “Needs Assessment for Noxious Weeds in Pershing County: Part 1 of 5 – Problem Weeds and Approaches and Methods of Control,” (Foster 2011) identifies the most problematic weeds in Pershing County, in descending order: Russian knapweed, Rhaponticum repens L, perennial pepperweed, Lepidium latifolium (tall whitetop), downy brome, Bromus tectorum L. (cheatgrass), hoary cress, Cardaria draba (short whitetop) and foxtail barley, Hordeum jubatum L.

Traditional methods of controlling broadleaf weeds have included mowing, burning and the use of 2,4-D (Dimethylamine salt of 2,4-Dichloro-phenoxyacetic acid) herbicide. However, in recent years new herbicides with the active ingredient aminopyralid have been developed to help control deep-rooted, root-sprouting noxious weeds, such as Russian knapweed. Among these heribicides are Milestone™ VM and Opensights®.

Milestone™ VM is a group 4 specialty selective herbicide labeled for use to control broadleaf weeds, including many invasive and noxious weeds on non-cropland and grazed areas, without injuring most perennial grasses. The active ingredient is Triisopropanolammonium salt of 2-pyridine carboxylic acid, 4-amino-3,6-dichlorol- 40.6%, which has an acid equivalent (ae) of aminopyralid (2-pyridine carboxylic acid, 4-amino-3,6-dichloro-) equal to 21.1% or 2 pounds per gallon.

Opensights® is a group 2 and 4 specialty selective herbicide used to control susceptible broadleaf weeds and certain woody plants, including invasive and noxious weeds on rangeland, permanent grass pastures, CRP (Conservation Reserve Program) acres, noncropland sites and grazed areas. It does not injure most perennial grasses when applied according to the label’s directions.This herbicide has a combination of two active ingredients, potassium salt of 2-pyridine carboxylic acid, 4-amino-3,6-dichlorol - 62.13% and metsulfuron methyl (Methyl 2-[[[[(4-methoxy-6-methyl-1,3,5- triazin-2-yl)-amino]carbonyl] amino]sulfonyl]benzoate) - 9.45%. This formulation contains 1.7 ounce acid equivalent of aminopyralid (equal to 7 fluid ounces of Milestone) and 0.36 ounces active ingredient of metsulfuron-methyl (equal to 0.5 ounces of a 60% active ingredient metsulfuron product).

The Pershing County office of University of Nevada Cooperative Extension used grant funds from the Humboldt Watershed Cooperative Weed Management Area to develop a three-year case study with the following objectives:

1. To compare traditional methods of broadleaf weed control (mowing and 2,4-D herbicide, respectively) against more recently developed herbicides (Milestone™ VM or Opensights®, respectively ) for control efficacy of Russian knapweed.

2. To compare economics of traditional methods of broadleaf weed control (mowing and 2,4-D herbicide, respectively) against more recently developed herbicides (Milestone™ VM or Opensights®, respectively) for control of Russian knapweed.

3. To determine if desirable forages (Hycrest crested wheatgrass and Forage kochia) could be established after the herbicide treatments and if they could competitively exclude or retard the re-establishment of Russian knapweed in an unirrigated setting.

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Methods

The trials were located In Lovelock, Nevada on a level site (< 1 percent slope) with predominately silty soils. Most of the site was Humboldt silty clay soil (40-60 percent silt) and a small area was Humboldt silt loam (50-90 percent silt): United States Department Agriculture, Natural Resources Conservation Service Web Soil Survey,Web Soil Survey The study plots had been fallow for over six years, but annual cultivation in the spring was used to control weeds.

The trial was designed as a randomized complete block with three replications of each weed control method (treatments = 4). Each replicated plot was 40 feet by 90 feet 9 inches which represented 0.083 acres with the total area for each treatment equaling 0.25 acres (Figure 1).

The site was not irrigated and only received moisture from annual precipitation. The treated area was a mono-culture of Russian knapweed with no desirable plants present (Image 1).

The perimeter of the study area and the three mowed treatment plots (treatment #1) were treated with a brush hog mechanical mower on August 8, 2013; June 5, 2014; and September 9, 2014.

Hycrest crested wheatgrass (A. cristatum x A. desertorum) was planted with a broadcast spreader and drag at a rate of 9 pounds per acre on August 23, 2013 and March 13, 2014 (Image 2).

Hycrest crested wheatgrass was developed at the Agricultural Research Station in Logan, Utah by crossing fairway and desert crested wheatgrass (A. cristatum x A. desertorum). We selected this species because it has excellent seedling vigor and is easier to establish than either of its parents. Also, it has excellent drought tolerance, establishes well on dry sites, and thrives in sagebrush communities (Ogle 2002).

It is best adapted to 5,000 to 9,000 feet in elevation and does well on shallow to deep, coarse to fine textures, moderately well to well-drained soils. It is not adapted to excessively saline areas.

All herbicide treatments occurred on October 16, 2013 with a boom sprayer mounted on an All Terrain Vehicle (ATV) (Image 2). Plants were becoming dessicated and turning tan/brown as they shut down photosynthesis for the winter.

russian knapweed mono culturebroadcast spreader with drag

The following are the chemical treatments and rates of application:

- Treatment #2 – 2,4-D at a rate of 3.2 pints per acre (38.6% 2,4-D acid or 3.74 pounds per gallon) with 4.8 ounces per acre of the non-ionic surfactant Liberate.

-Treatments #3 – Milestone (active ingredient of aminopyralid at 21.6% acid equivalent) at a rate of 6.0 ounces of product per acre with 4.8 ounces per acre of Liberate surfactant (nonionic surfactant product).

-Treatments #4 – Opensight (active ingredient of aminopyralid at 52.5% acid equivalent and 9.45% metsulfuron methyl) at a rate of 3.3 ounces of product per acre, with 4.8 ounces per acre of Liberate surfactant (non-ionic surfactant product).

All treatment plots were evaluated pre-treatment, on August 9, 2013 and post-treatment on April 17, 2014; September 8, 2014; and May 1, 2015 by randomly selecting three 1-foot-square samples (quadrats) and counting the stems in each sample. Each solitary stem or discrete cluster of stems were considered as individual plants. Means were calculated for each replicated plot, and these values were used to calculate the average number of Russian knapweed plants per square foot, per treatment. In addition, photographs were taken for each random sample from the replicated plots, on the evaluation dates. An analysis of variance (ANOVA) was performed for all evaluation dates, on the Russian knapweed plant population per square foot for each treatment, with mean separation at the P≤0.10 level. In addition, a comparison of the pre-treatment and final post-treatment evaluations of the Russian knapweed plant population per square foot were analyzed with a mean separation at the P≤0.10 level. We applied the standard null hypothesis that the treatments applied would not result in any difference in mean Russian knapweed density among treatments. Thus, for P≤ 0.10 the result would mean that if the mean Russian knapweed density for the treatments being compared truly were not different from one another, there is only a 10 percent or smaller chance that the difference between the two means would be as great as that found by the experiment (i.e., the treatments applied). When a p-value is small, one can reliably conclude that the difference between the means has a large probability (i.e., very high confidence) of being due to the treatments applied.

Forage kochia (Kochia prostrata) was planted with a broadcast spreader and drag at a rate of 6 pounds per acre on September 9, 2014 and March 11, 2015. Forage kochia population checks were conducted on October 10, 2014; March 9, 2015; and May 1, 2015.

Forage kochia is native to the heavily grazed rangeland regions of Central Asia (e.g., Uzbekistan). It is a long-lived, perennial, semievergreen half-shrub that is well adapted to arid rangelands in the Western United States that receive 6 to 16 inches annual precipitation and have a wide range of soil texture, including sands, gravels, clays, silts and loams. Plants do best in medium-textured soils. Forage kochia is highly saline and sodic tolerant (Tilley 2006).

In addition, an economical comparison of the four treatments was calculated in order to determine if they were economically viable alternatives for Russian knapweed control.

atv mounted boom sprayer

Results

The density of Russian knapweed plants was similar among all treatment plots, prior to any treatments being applied (Figure 2). Across the treatment plots, Russian knapweed density ranged from 7.1 to 8.9 plants per square foot. There was a 90 percent or greater probability the difference in Russian knapweed density among plots was due to some factor other than their specific location in the study site.

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The mowing treatment had a steady increase in Russian knapweed density through early May 2015 (Figure 3). There is a high probability (P≤0.10) that the initial increase in density (April 2014) was due to the mowing treatment, and that repeated mowing treatments maintained the high density of Russian knapweed. Annual mowing was associated with a small increase in Russian knapweed density in 2015, but the increase could not be attributed to the mowing treatment with high confidence (P≥0.10).

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The application of 2,4-D to Russian knapweed in October 2013 occurred as the plants were going dormant and resulted in no control (Figure 4). Russian knapweed density on all three subsequent sampling dates was slightly to substantily greater than prior to application. In general, there is low confidence that the difference in means among the sampling dates was due to the herbicide treatment. The exception was in September 2014 when Russian knapweed density spiked at over 15 plants per square foot. In May 2015, however, Russian knapweed density was very similar to the values found in the earliest sampling periods; thus, the larger density count in September 2014 probably was unrelated to the herbicide treatment and an effect of the late sampling date. During the long growing season, additional stems probably emerged from the creeping root system and were counted as new plants in September. These new stems, or at least many of them, may not have persisted the subsequent winter resulting in a smaller density count in May 2015.

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A late-season herbicide treatment with 2,4-D is unlikely to control Russian knapweed for two reasons. First, the plant is shutting down its photosynthetic processes as it prepares for winter dormancy, which would limit the uptake and movement of 2,4-D from the leaves to the site of action at the buds on the root crown and creeping roots. This reduces herbicide efficacy. Second, 2,4-D is not a soil active herbicide so any chemical that reaches the soil would not be picked up by Russian knapweed roots and moved to the site of action.

The Milestone treatment resulted in a very large decline in Russian knapweed during both growing seasons after its application (Figure 5). The small p-values (P≤0.10) suggest there is high confidence that the decline in Russian knapweed was due to the Milestone treatment.

*chart here

The Opensight treatment also resulted in a large decline in Russian knapweed for two growing seasons after treatment (Figure 6). The large and persistent decline in Russian knapweed can be attributed to the herbicide treatment with high probability (P≤0.10). 

A late-season application of Opensight is much more effective at controlling Russian knapweed, than is a late-season application of 2,4-D, but appears less effective than Milestone, despite both chemicals having the same primary active ingredient (aminopryralid). It is possible that the inclusion of metsulfuron-methyl in the herbicide mix slightly reduced the efficacy of aminopyralid. Aminopyralid-based herbicides provide effective late-season (plants entering or in fall/winter dormancy) control of Russian knapweed, at least in part, because aminopyralid is a persistent soil-active herbicide. The buds and roots of Russian knapweed have the ability to extract the active ingredient from the soil during the winter and following spring, which results in plant death when the above ground stems are dormant. Most plants develop their buds for growth the next growing season, in late winter to early spring, but Russian knapweed starts to develop its buds in late fall to early winter, and the buds can increase in size all winter, as long as the surrounding soil is not frozen. Thus, persistent soilactive herbicides known to control Russian knapweed can be effective much of the winter and subsequent spring, provided there is enough unfrozen moisture in the soil to move the active ingredient from the soil into the plant

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Establishment of Competitive Perennial Grasses

Hycrest crested wheatgrass (A. cristatum x A. desertorum) had poor establishment on all treatment plots. This does not appear to be the result of an herbicide influence on seed germination, seedling emergence or seed survival. Even the mowed plots, which did not receive any herbicide application did not have any plants establish until the second growing season after all herbicide treatments occurred (Table 1). This pattern occurred across all treatments. The low population size of Hycrest crested wheatgrass is more likely due to the harshness of the site than the influence of any residual herbicide. The failure of seedings on sites with low annual precipitation and highly variable precipitation across years has been well documented in northern Nevada (Bollinger 2007).

Forage kochia (Kochia prostrata) was planted with a broadcast spreader and drag at a rate of 6 pounds per acre on September 9, 2014 and March 11, 2015. The presence/absence of Forage kochia was evaluated on October 10, 2014; March 9, 2015; and May 1, 2015.

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There were no established Forage kochia plants observed during the study. The lack of normal precipitation during the study (Figure 7) maybe attributed to the poor performance of both the Forage kochia and Hycrest crested wheatgrass, as well as a few residual perennial grasses on the site.

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Treatment Economics

Table 2 shows an economic comparison of the applied treatments on a cost per acre basis. From a cost-benefit perspective, Milestone was the most effective herbicide product and control method. Mowing or fall application of Amine 4 (2,4-D) resulted in no control of Russian knapweed, and in the case of mowing, may have resulted in an increase in population density.

The financial cost of these two treatments was large with no positive outcome. Milestone and Opensight both resulted in large declines in Russian knapweed, but the density of Russian knapweed was slightly less on the Milestone plots, and this was achieved with a lower cost per acre then the Opensight treatment.

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Conclusion

Russian knapweed is a nonnative, noxious and invasive weed that is extremely difficult and expensive to control once it becomes established. It is found in almost every county in Nevada. In addition, it is defined by Nevada state law as a noxious weed in NAC 555. According to NRS 555.150 “…every person owning, controlling, or occupying lands in this state…shall cut, destroy, or eradicate all weeds declared noxious …before such weeds propagate and spread…” All property owners are responsible for controlling noxious weeds on their properties. If they do not, county government is authorized to have the work done and bill the owner for the costs incurred. If the bill is not paid, the county can place a lien on the property.

The results showed that Russian knapweed treated in the late fall with maximum labeled rates of Milestone™ VM and Opensights® was significantly more effective than late fall treatment with 2,4-D or mowing. In addition, Milestone™ VM and Opensights® displayed excellent residual control of Russian knapweed over the duration of the project. The other two treatments had slight to moderate increases in Russian knapweed density.

Establishment of desirable forages on the treated sites was largely unsuccessful. Sowing competitive species is very difficult, and four factors may have contributed to the poor seeding success.

1. Local precipitation was less than average during the planting and germination time period of the study. This can be very important on post-treatment sites that have been reduced to largely bare ground, especially on silty soil. Silty soils readily develop a hard cap on the soil surface when they dry, which can prevent seedling emergence. Sustained moist soil is needed to prevent the establishment of a hard soil cap and permit widespread seed emergence.

2. Russian knapweed is an allelopathic weed (Allelopathy refers to the beneficial or harmful effects of one plant on another plant), which is often too competitive and forms monocultures by eliminating surrounding plants (Graham 2004).

3. Forage kochia was first planted 12 months after the herbicide treatments were applied. This is the minimum plant-back period for a broadleaf species. The second seeding was 18 months after treatment. A soil bioassay, however, was not conducted to determine the concentration of aminopyralid and/or metsulfuron in the soil each year after treatment. If these active ingredients were present at sufficient concentrations, it is possible they could have affected seedling survival, especially for Forage kochia, a broadleaf species.

4. The Forage kochia seed was not Pure Live Seed (PLS). This may have resulted in a lower germination rate; hence, fewer potential seedlings to emerge and establish.

The seeding study found a greater density of Hycrest crested wheatgrass plants in the mowing treatment plots. This may reflect the increased amount of plant residue (litter) on the soil surface, potentially creating a microenvironment that facilitated seedling emergence and survival, compared to the large amount of bare ground in the two herbicide treatments. Also, the mowed plots would not have had any herbicide residue that could have affected seedling survival of Hycrest plants. The seedling growth stage typically is the growth stage most susceptible to most active ingredients.

The economical comparison of the different treatments showed a range of $51.00 to $19.67 cost per acre. The 2,4-D treatmet was least expensive but was completely ineffective in controlling the Russian knapweed. The mowing application cost was $17.00 per acre per treatment, however three separate treatments were conducted, and still the Russian knapweed was not controlled. The Milestone™ VM and Opensights® treatments costs were $27.70 and $33.51 per acre, respectively, and both were signicantly better at controlling the Russian knapweed than the other two treatments.

The use of an herbicide alone, without establishment of competitive vegetation, will not effectively control Russian knapweed. The exception is for sites purposely managed as bare ground (e.g., stackyards, fence lines, etc.). The results suggest that Milestone VM or Opensight could be effective control agents in those types of settings. For locations with established perennial grasses, these and perhaps other aminopyralid-based products could control much of the Russian knapweed with little harm to the established perennial grasses, effectively releasing them from competition from the knapweed and permitting them to increase their density. Several studies have found few, if any, adverse effects of aminopyralid on established perennial grasses (Samuel and Lym 2008; Almquist and Lym 2010).

To be most successful, chemical applications should be combined with mechanical or cultural controls and revegetation with competitive plants, including grasses and forbs. Avoid using herbicides that will injure desirable vegetation needed to provide competition. Remember to always carefully read the label before using herbicides (Graham 2004).

Information herein is offered with no discrimination. Listing a product does not imply endorsement by the authors, University of Nevada Cooperative Extension or its personnel. Likewise criticism of products or equipment not listed is neither implied nor intended. University of Nevada Cooperative Extension and its authorized agents do not assume liability for suggested use(s) of chemical or other pest control measures suggested herein. Pesticides must be applied according to the label directions to be lawfully and effectively applied.

Acknowledgment

The authors thank the following people for their support: Grant provided by the Humboldt Watershed Cooperative Weed Management Area, Pershing County Road Department, Pershing County Water Conservation District and Crop Production Services, Inc. Reno, Nev.

References

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