Long-term success for weed control in both the PVWCD ... forage and/or cover for many wild and domestic ..... disappointing results when the same chemical is ...
COOPERATIVE EXTENSION Bringing the University to You
Special Publication-06-02
Paradise Valley Weed Control Demonstration Plots: Leafy Spurge Brad Schultz, Extension Educator, Humboldt County University of Nevada Cooperative Extension INTRODUCTION Noxious weeds are widespread across the western United States, including all of Nevada. Forty-five plant species in Nevada have been legally designated as noxious weeds by the Nevada Department of Agriculture. Noxious weeds are plant species the Nevada Department of Agriculture has legally classified as such. They include species that are, or are likely to be, detrimental or destructive and difficult to control or eradicate. In Nevada, noxious weeds are prohibited in commerce, and state law mandates their control or management. Landowners are responsible for controlling weeds on their property. Paradise Valley is a large agricultural center in north-central Humboldt County. The area winters about 10,000 beef cows every year and produces grass hay and/or alfalfa hay on about 40,000 acres. Most of the hay is fed in the valley, but some is exported to other cattle producing areas. The meadows, riparian areas and brush fields throughout the valley provide habitat for many species of wildlife, including mule deer, pheasant, wild turkey, ducks, geese, dove, shorebirds, small
mammals and predators. The agricultural fields and adjacent wildlands provide both year-long and seasonal habitat. In addition to being an agricultural center, Paradise Valley is surrounded on three sides by public land administered by the Bureau of Land Management and the United States Forest Service. These lands have many valuable resources that support wildlife, recreation, livestock production and other land uses. All of these public lands, but particularly the meadows and riparian areas, provide important habitat for many wildlife species, including sage grouse, mule deer and pronghorn antelope. All are susceptible to invasion by one or more noxious weeds. Noxious weeds have been an issue in Paradise Valley since before formation of the Paradise Valley Weed Control District (PVWCD) in the late 1940s. The PVWCD was initially formed to address a noted increase in hoary cress (Cardaria spp.) or short whitetop. Since creation of the PVWCD, the scale of weed control efforts in Paradise Valley has varied widely across time. Since 2002, control efforts have expanded substantially for numerous reasons. In late
2002, the PVWCD appointed a permanent coordinator, initiated regular interaction with the University of Nevada Cooperative Extension, and developed a comprehensive weed management plan. These developments occurred because hoary cress remains uncontrolled, and numerous other species with more significant ecological impact have become widespread. These include Canada thistle (Cirsium arvense), leafy spurge (Euphorbia esula), mayweed chamomile (Anthema cotula), musk thistle (Carduus nutans), medusahead (Taeniatherum caputmedusae), perennial pepperweed (Lepidium latifolium), poison hemlock (Conium maculatum), Russian knapweed (Centaurea repens), saltcedar (Tamarix ramosissima), Scotch thistle (Onopordum acanthium) and water hemlock (Cicuta douglasii). Each of these species covers tens of acres to thousands of acres in and/or near the PVWCD. The weed management plan developed by the PVWCD acknowledges the need for a large scale treatment program to: 1) prevent the loss of additional acreage to noxious weeds; and 2) prevent the establishment of additional noxious weed species in the district. Long-term success for weed control in both the PVWCD and other areas of Humboldt County will require that all residents understand that noxious weeds can be effectively controlled. Control is defined as reduction to a manageable level with minimal economic impact and subsequent prevention of new infestations. Many landowners are hesitant to spend a substantial amount of money and/or time for chemical treatment of noxious weeds. They are unsure about the
effectiveness of existing herbicides, many of which are expensive. Also, they lack knowledge about when (i.e., timing) specific herbicide applications should occur and/or new chemicals that only recently have become available. Finally, treatment often does not occur because the window of opportunity for some species is short, and other time critical tasks are considered a higher priority. Since 2003, the Humboldt County office of the University of Nevada Cooperative Extension, in cooperation with the PVWCD, has established a series of demonstration plots to increase local awareness and knowledge about the control of four noxious weed species: hoary cress (plots established 2003), leafy spurge (plots established 2003 and 2004), Russian knapweed (plots established fall 2004) and perennial pepperweed (plot established 2005). Each demonstration plot (or group of plots) was developed to address one or more of the following issues: 1) a weed the PVWCD had previously put little control effort toward; 2) a recently released herbicide was available but had no history of local use; and/or 3) to investigate whether an alternative time of year for treatment existed. This special publication addresses the use of Tordon® (Picloram) and Plateau® (Imazapic) on leafy spurge. Other special publications address the use of Plateau® on both hoary cress and Russian knapweed. LEAFY SPURGE Leafy spurge is a long-lived (perennial), deep rooted, broadleaf forb. Initial colonization of a site usually begins from plants that germinate from seed; however, root 2
fragments may be transported to uninfested areas and start new plants. Within four months of germination, seedlings can have roots that are three feet deep, with a lateral spread of up to 40 inches. Subsequent spread of the infestation is from creeping horizontal roots that create ever expanding clonal patches. Clonal patches expand on two fronts: 1) an internal increase in density (number of stems/unit area) and 2) lateral spread to previously unoccupied locations. Individual patches eventually merge into a homogeneous infestation that excludes almost all other species. The long-term result is the displacement of desired plants that provide forage and/or cover for many wild and domestic species, across large areas. Vertical roots can grow to 26 feet deep, and depths of 8 to 15 feet are common if a restrictive layer in the soil is absent. Lateral roots from a single plant can spread outward up to 16 feet per year. Small patches are estimated to expand at rates up to 500 times faster than large patches. This strongly suggests that new patches need to be eliminated the first year they appear. Typically, each 1/2 inch segment of root that is at least 1/8 inch in diameter will have at least one viable bud capable of producing a new plant or stem. These small root segments have produced new shoots from depths up to 12 inches, within three weeks after burial. Mature root systems have a biomass many times greater than all of the above ground stems. The carbohydrate reserves in these extensive root systems provide the energy for regrowth following chemical or mechanical treatment and sustain plants during prolonged
periods when growth potential is sub-optimal (e.g., drought). Also, this makes control of large root systems (i.e., old plants and clonal patches) very difficult. The extremely deep and extensive root system provides leafy spurge access to water and nutrients through most of the growing season. Photosynthesis is common all summer until freezing temperatures occur or dry soil results in desiccation and mortality of the stems. Each flowering stem produces an average of 140 seeds, with total seed production per acre ranging from 24 to 4,000 pounds. Seed can remain viable for up to eight years and perhaps longer. Long-lived seed allows the development of large seed-banks. Even low germination rates can result in many new seedlings annually for many years following elimination of existing plants. Until recently, the recommended treatments for leafy spurge included dicamba (e.g., Banvel®), picloram (Tordon®), 2,4-D ester, and/or glyphosate (RoundUp®). These products have limitations that prevent its use on one or more of the many sites on which leafy spurge grows. Dicamba has proven to be cost prohibitive for the level of control afforded. 2,4-D ester provides relatively low cost, short-term reductions in leafy spurge, but only short-term control. Glyphosate is a nonselective herbicide and cannot be used for release of desired species. Picloram is a restricted use chemical and should not be used in diversified crop settings or locations where air- or water-borne drift could move the chemical off-site to locations that produce sensitive broadleaf plants. Also, each chemical has a limitation on the maximum 3
amount that can be applied to any site within one growing season. Retreatment within the same growing season will often require use of two or more herbicides. METHODS Demonstration plots were established in two areas: Cottonwood Creek and Stonehouse. The Cottonwood Creek site is located on very deep valley bottom soils along the historic floodplain of Cottonweed Creek. This area is near the lowest point of Paradise Valley and has very deep soils with a shallow water table (0-10 feet, depending on time of year). The demonstration plot is in an established pasture that receives supplemental irrigation when water from spring run-off is available. The area was flooded extensively in the early 1980s, and this left a layer of course soil material on the floodplain near Cottonwood Creek. It is this elevated sand sheet that has been extensively colonized by leafy spurge. Cottonwood Creek Plots At Cottonwood Creek, an infestation of about five acres was initially treated with a tank mix of Tordon® (Picloram) and 2,4-D at peak flowering in early July 2003 (Figure 1a). By mid-July almost 100 percent of the leafy spurge appeared to have died (Figure 1b). However, in early August 2003 there was substantial regrowth from the shallow roots (Figures 2a-2c). Two follow-up treatments were applied in two separate areas. Tordon® was used at one location the first week of September 2003, and Plateau® at the second site on September 16, 2003. All treatments were within the annual recommended label
rate. Both follow-up treatments occurred before leafy spurge showed signs of entering fall dormancy. Plants were green, unwilted, and had an abundance of white, milky sap. Photographs were taken before the July 2003 treatment, several weeks after the July 2003 treatment to document regrowth, and on June 4, 2004, the approximate middle of the following growing season. The area was exceptionally dry in 2003 and received only average to slightly above average precipitation in 2004. Irrigation water was abundant in the spring of 2004, with the last irrigation occurring after the site was photographed on June 4. Stonehouse Plot The Stonehouse site is located on the lower end of an alluvial slope that drains the east side of the Santa Rosa Mountain Range. Soils are moderately deep to deep, but lack the summer long shallow water table found at the Cottonwood Creek site. Shallow ground water is limited to the spring months and typically only in wetter years. The site is a grass-hay pasture that receives supplemental irrigation in the spring. The amount depends on the annual snow pack, and may range from almost none (e.g., 2003) to several acre-feet or more (e.g., 2005). The Stonehouse site received several treatments. First, one-half the area (Tordon® Plot) was treated with Tordon® very early in September 2003, prior to plants entering fall dormancy. Second, one-half of the area (Plateau® Plot) was treated with Plateau® approximately two weeks later, on September 16, 2003. The Plateau® treatment occurred 4
after plants showed signs of entering dormancy. Stems had varying degrees of wilting, were turning brown, and the milky sap was less abundant than during the earlier period of vigorous growth. The Plateau® plot was retreated in the third week of August 2004, prior to any plants entering fall dormancy. All treatments were made according to label rates. Photographs of all treatments were taken at least one growing season after treatment to document the shortterm success or failure of the respective treatment.
all were less than three inches tall. Most did not have a vigorous appearance. A similar response was observed on the two acres where Plateau® was applied to regrowth on September 16, 2003 (Figures 4a and 4b). Annual spot spraying treatments (Tordon®) at Cottonwood Creek by the landowner has kept leafy spurge from reoccupying this site. New plants are found every year, and presumably are from recently germinated seed. Annual treatment, prior to the establishment of deep and widespread root systems by seedlings is preventing a serious reinfestation of the area.
RESULTS
Stonehouse Plots
Cottonwood Creek Plots
Figures 5a through 7b show the Stonehouse treatment site. Treatment results at this site generally were less successful than at Cottonwood Creek. The September 2, 2003 Tordon® treatment appeared successful one growing season after treatment (Figures 5a5b). The effect of this herbicide was evident on all plants two weeks after treatment (Figure 5a), and few plants were found in the treated areas the following spring (Figure 5b). The initial control was estimated at 75-85 percent or more. Two growing seasons after treatment (July 2005), however, there were many leafy spurge plants (Figure 5c). The infestation ranged from solitary plants with single stems to relatively large clonal patches. This area received no follow-up treatment after the initial control effort in early September 2003. The area treated with Plateau® on September 16, 2003 showed the least success, whether treated once (Figures 6a-6c) or on two consecutive years (Figures 7a-7b). The lack of further follow-up treatments has
The mid-term response of the two sites has been very different. The Cottonwood Creek site (Figures 1a-4b) had an initial kill of at least 95 percent of the top growth (Figure 1b). Regrowth, however was widespread and abundant (Figures 2a-2c). The rapid regrowth during the three to six weeks after the initial treatment with Tordon® suggests that the early July treatment had little or no effect on leafy spurge’s shallow or deep roots. This interpretation is based on other studies that documented leafy spurge having new stems from shallow roots (12 inches depth), within three weeks of burial and/or other disturbance. A second application of Tordon® was made to this regrowth on September 2, 2003 and appears to have had significant impact on both shallow and deep root systems (Figures 3a3c). In early June 2004, leafy spurge in nearby areas of Paradise Valley was entering the flowering stage. In the treatment plot fewer than six leafy spurge plants were observed and
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allowed the spurge population to remain at or above pre-treatment levels. Other weed species were not present (perennial pepperweed) or not common (thistles) on the site in 2003, but were abundant in July 2005 (Figure 6c). DISCUSSION This case study has documented that two sites infested with leafy spurge can have very different responses to treatment with herbicides, even when they lie only a few miles apart. Because this study did not have multiple study plots (i.e., replication), including good control plots, interpretation of the results requires careful consideration. Knowledge about the biology of leafy spurge and the site specific conditions of each treatment area, however, provides insight about the observed results. Cottonwood Creek Plots The Cottonwood Creek site has very deep soil and a water table within 6-10 feet of the soil surface throughout the late summer. During spring runoff, groundwater may reach the soil surface and remain within 40 inches of the surface through mid-summer. Leafy spurge has roots that can easily reach depths much deeper than 10 feet when restrictive layers are not present, but a comparatively small plant surface area above ground. Tordon® is readily absorbed by the roots and less so through the leaves and stems. July applications of Tordon® typically occur after irrigation has ceased and when rainfall is insufficient to move the chemical into the root zone. The small amount of annual above-
ground growth, compared to the annual biomass of the root system, along with less reliable herbicide uptake from the leaves and stems (compared to roots), probably resulted in enough herbicide uptake to kill most of the foliage, but few roots. The followup Tordon® treatment in September appears effective for several reasons. First, the biomass of the root system was reduced to some extent by the diversion of carbohydrates from the roots to create new stems and leaves. Herbicide entering the plant through recent regrowth should have less total root mass to kill. Second, leafy spurge began to enter dormancy shortly after the September 2 Tordon® application. The normal seasonal redistribution of nutrients from leaves and stems to roots probably facilitated transport of more herbicide to deeper roots, resulting in better weed control the following spring. Third, eventual uptake of Tordon® from the soil by the roots (the most effective mechanism of uptake for this herbicide) was probably higher after the fall application, than after the July application. Tordon® has a soil half-life of about 100 days, longer in arid conditions and when soils are dry. Degradation from sunlight or photodegradation and perhaps soil microbial agents is less likely to occur in the fall than in July and August. From September onward, day length decreases, sun angles become lower and average temperatures decline. Also, soils in the fall are the driest of the year, at least until the first significant precipitation event. Shorter days coupled with lower sun angles should reduce the rate of photodegradation. Microbial degradation is 6
highest when soils are moist and warm. In the fall, prior to the first significant precipitation event, soils are at their driest and are cooling. These conditions reduce soil microbial activity. The interval between a fall herbicide application and the first precipitation event large enough to transport chemical into the root zone usually is much shorter than it would be in early July. Summer precipitation is usually too sporadic and insufficient to transport herbicides into the root zone, due largely to the small amount received and the high evaporation rate. When abundant fall or winter precipitation eventually occurs, onehalf or more of the Tordon® applied in early July probably has been degraded, resulting in limited chemical available for root uptake (from the soil). When rainfall finally arrives, most of the fall application, together with any remaining herbicide from the summer application, will be transported into the root zone. At Stonehouse, the apparent one-year success of the September 2, 2003 Tordon® treatment (Figures 5-5c) when compared with the early July application of Tordon® at Cottonwood Creek (Figures 1a-2c), is likely due to the fall timing of the application. The fall treatment probably enabled more chemical to be transported into the root zone than did the early July treatment at Cottonwood Creek. The second application of Tordon® at the Cottonwood Creek site was probably more successful for three reasons. First, the plants were weakened by the first treatment and were more susceptible to a second herbicide application in the same growing season. Second, in September, more of the foliar
applied herbicide was absorbed by the leaves due to cooler temperatures that reduced evaporative losses. Also, more chemical may have been transported to the root system during the seasonal redistribution nutrients from leaves to roots. Third, there was substantially less degradation of Tordon® that fell onto the soil surface because the fall has less potential for photodegradation and lower rates of soil microbial activity. The positive result from the application of Plateau® on leafy spurge regrowth at Cottonwood Creek (Figures 4a4b) is probably due to the same mechanisms discussed above for Tordon®. Plateau® has good foliar and root uptake and a soil half-life of about 120 days. Fall application on plants with good sap flow and growing in moist soil should result in good leaf uptake of Plateau®, and subsequent translocation deep into the plant’s root system. Plateau’s® long persistence in the soil allows much of the herbicide that falls onto bare ground to persist and eventually be transported into the root zone during fall and/or winter precipitation events. This would adversely affect both seedlings and sprouting root stock the following spring. Microbial decomposition of Plateau® is reduced in the fall and winter for two reasons. Prior to the precipitation the soil is too dry, and by late fall or early winter, soils are too cold for significant microbial activity to occur. Stonehouse Plots The mid-term result at the Stonehouse site can be explained largely by the different soils at this site, compared with those at 7
Cottonwood Creek, and also by the characteristics of the herbicides. Soils at Stonehouse are drier than soils at Cottonwood Creek. This should result in reduced photosynthesis during late summer and/or fall, when all herbicide applications occurred. Foliar uptake of Tordon® applied on September 2 probably was less than in the July application at Cottonwood Creek, but still sufficient to kill much of the top growth (Figure 5a). Root uptake would not have occurred until the following spring, after fall and winter precipitation worked the herbicide into the root zone. The combined effect of foliar uptake in the fall and root uptake the following spring probably was sufficient to kill most shallow roots and perhaps some or many deeper roots, as evidenced by the lack of growth the following spring (Figure 5b). One fall application on a site with dry soil, however, does not appear sufficient to kill deep roots. The apparent sharp decline for leafy spurge in the spring of 2004 (Figure 5b), but the abundance of vigorous mature plants and clonal patches within two years (Figure 5c) of the last application strongly suggests that one treatment does not eliminate most of the deep roots. Multiple follow-up treatments when plants are in a weakened state are necessary for effective control. Figures 6a-6c clearly show that leafy spurge cannot be effectively treated with Plateau® once the flow of the milky sap has declined. There was apparently little if any absorption of Plateau® when it was applied at this phenological stage. Residual chemical in the soil should have been abundant given
Plateau’s® long soil half-life, but it appears to have had little effect. The reason is unknown. Application of Plateau® at Stonehouse prior to the reduction of the milky sap also showed poor results (Figures 7a-7b). Label information about Plateau® states it should be applied when soils are moist and there is abundant flow of the milky sap. Both applications of Plateau® at Stonehouse occurred when soils were dry and the seasonal water table had dissipated. Despite the green appearance of spurge plants in August 2004, they may have had little photosynthetic activity. Under this condition, any herbicide deposited on the stems and leaves would have had very poor chemical uptake. Cattle graze the Stonehouse site in the fall and winter and usually receive supplemental hay. It is quite likely that leafy spurge plants were grazed before fall and/or winter precipitation washed the residual herbicide from the leaves and stems into the soil. This situation, combined with poor uptake due to low photosynthetic activity, would result in little of the Plateau® being available for eventual uptake through the roots. Plateau® is primarily degraded by microbial decomposition, and any chemical ingested by livestock when it is on the leaves of the forage plants would be exposed to a large microbial population in the animal’s rumen. This scenario is speculative, but its possibility should be considered for all sites that receive late summer or fall applications of persistent herbicides that may not be adequately translocated into the plant through photosynthesis or root absorption.
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Figure 1a. Leafy spurge on the Cottonwood Creek plot in Paradise Valley. This photo was taken in late June 2003, prior to the area being treated with Tordon® in early July 2003.
Figure 1b. This photo is an expanded view of area in Figure 1a, approximately two weeks after treatment with Tordon®. The dark brown areas are clonal patches of leafy spurge whose top growth was killed with one application of Tordon®.
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Figure 2a.
Figure 2b.
Figure 2c.
Figures 2a-2c. Regrowth of leafy spurge at the Cottonwood Creek plot, four to six weeks after the initial treatment with Tordon® in early July, 2003. The quick return of leafy spurge indicates that most of the buds on the shallow roots were not affected and quickly produced new stems and leaves. Figures 2a and 1a show the same area.
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Figure 3a.
Figure 3b.
Figure 3c.
Figures 3a-3c. The Cottonwood Creek Tordon® treatment plots on June 4, 2004. All three photos are in the area that received the Tordon® applications in early July 2003 and on the regrowth on September 2, 2003. Figures 3a and 3b show the same general area seen in Figures 1a, 1b and 2a. Figure 3c is the same ditch shown in Figure 2c. Fewer than six seedlings were found in the entire treated area, and all were less than three inches tall. In contrast, leafy spurge plants throughout Paradise Valley in untreated areas had begun to flower. All of the vegetative growth in Figures 3a-b is from pasture grasses and grass-like plants. They have had good recovery and are occupying ground previously inhabited with leafy spurge. The light tan stems are dead leafy spurge plants. The follow-up treatment in early September 2003 resulted in a nearly complete kill of leafy spurge in this area by June 2004.
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®
Tordon only applied in early July 2003
Figure 4a.
®
Tordon only applied in early July 2003
Figure 4b.
Figures 4a and 4b. Cottonwood Creek plot. The area between the white lines in Figure 4a, and to the left of the white line in Figure 4b was treated with Tordon® in early July 2003 and retreated with Plateau® on September 16, 2003. The area in both photos to the right of the white lines was treated with only Tordon® in early July. The photos show substantially less spurge (dark green patches) in the areas treated with both Tordon® and Plateau®, compared to the area treated with only Tordon®. 12
Figure 5a.
Figure 5b.
Figure 5c.
Figures 5a-5c. Stonehouse demonstration plot. In Figure 5a, the area between the pipe and the fence on the right was treated with Tordon® on September 2, 2003. The photo shows the effect of Tordon® on the leafy spurge (reddish brown plants) on September 16, 2003. Figure 5b shows the same area on June 4, 2004. Very few live spurge plants are present, with none visible in the photo. Figure 5c is the same area on July 19, 2005. The white outline shows a large clonal patch of leafy spurge that was not evident in June 2004. Individual plants of leafy spurge and smaller clonal patches are in the area but cannot be seen due to the tall grass.
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Figure 6a.
Figure 6b.
Figure 6c.
Figure 6a-6c. Stonehouse treatment plot. Figure 6a was taken on September 16, 2003, immediately prior to the ® application of Plateau . Some spurge plants are showing sign (slight brown tint) of entering fall dormancy. Figure 6b is the same location on June 4, 2004. Leafy spurge is very abundant and was unaffected by the application of Plateau® on September 16, 2003. Figure 6c is the same area in July 2004. No treatment occurred beyond the September 2003 Plateau® application. The leafy spurge population has expanded and Scotch thistle is now widespread.
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Figure 7a.
Leafy spurge Figure 7b.
Figures 7a and 7b. The Stonehouse treatment area. In Figure 7a, the area in front of the wheel line was treated with Plateau® on September 16, 2003 and again in late August 2004. The photo shows the extent of the leafy spurge infestation at the time of initial treatment on September 16, 2003. All of the taller vegetation, regardless of color, is leafy spurge. Most plants were beginning to enter fall dormancy. Figure 7b shows the same area on July 19, 2005. The tall dark green plants are leafy spurge. There has been little, if any, reduction of leafy spurge following the two late summer applications of Plateau®.
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CONCLUSIONS •
Tordon® is an effective herbicide for control of leafy spurge in Paradise Valley, but one application is insufficient to achieve long-term control.
•
Plateau® is effective as a follow-up treatment on leafy spurge when good top kill is achieved from Tordon®. Additional work is needed to determine if the same result can be achieved with other herbicides applied in sequential ® combination with Plateau . Plateau® appears ineffective on leafy spurge when applied to plants growing in dry soil in late summer or early fall. Fall application of Plateau® needs to be tested when soils are moist throughout the root zone.
•
•
•
Leafy spurge is a tenacious plant with a deep and widespread root system, but a comparatively small surface area of stems and leaves. Successful control will require several sequential and well-timed herbicide treatments. Follow-up treatments should occur on re-growth when plants are in a weakened state due to reduced carbohydrate root reserves. Herbicide characteristics (e.g., persistence, mobility, etc.) must be well understood and matched with the bio-physical characteristics of the site and vegetation treated. Sites having sub-optimal characteristics are likely to exhibit disappointing results when the same chemical is applied to each location. Multiple applications of the same herbicide within the same growing season
may violate the maximum permitted use stated on the herbicide label. This would be a violation of Federal law; therefore, multiple treatments in one growing season should be made using chemicals with different active ingredients. Additional research is needed to determine the best mix of herbicides, their best order of use, and how different bio-physical constraints may affect the mix and order of use. •
Applications of herbicides with high root uptake potential must be timed to facilitate movement of the herbicide into the root zone. Proper timing will reduce the potential for loss of chemical from photo or microbial degradation.
•
Herbicide applicators should consider how the effects of livestock grazing and the bio-physical conditions of the site may affect the movement of herbicides from the plant to the root zone.
BIBLIOGRAPHY Anderson, W.P. 1999. Perennial Weeds: Characteristics and Identification of Selected Herbaceous Species. Iowa State University Press. Ames, Iowa. 228 pp. BASF Corporation. Plateau Specimen Label. Available at http://www.cdms.net/ldat/ld2LP000.pdf EXTOXNET. 2005. Picloram (Tordon). Available at: http://extoxnet.orst.edu/cgiwebglimpse/webglimpse/services/data/inf
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o/extoxnet?lines=jump&query=Picloram& case=on Johnson, D.W. and B.W. Schultz (1999). Response of carbon and nitrogen cycles to disturbance in forests and rangelands. Pp. 545-569. In: Lawrence W. Walker (ed). Ecosystems of the World 16. Ecosystems of Disturbed Ground. Elsevier Science. New York. Lajaunesse, S.R. Sheley, C. Duncan, and R. Lym. 1999. Leafy spurge. Pages 249-260. In: R.L. Sheley and J.K Petroff (eds). Biology and Management of Noxious Rangeland Weeds. Oregon State University Press. Corvallis, Oregon. 438 pp.
Lym. R.G. 2005. Leafy Spurge. Euphorbia esula L. pp. 99-118. In: Celiste L. Duncan and J.K. Clark (eds). Invasive Plants of Range and Wildlands and Their Environmental, Economic, and Societal Impacts. Weed Science Society of America. Lawrence, Kansas. 222 pp. United States Department of Agriculture, Natural Resources Conservation Service. 2002. Soil Survey of Humboldt County Nevada, East Part. Vencill, W.K. (ed). 2002. Herbicide Handbook. Eighth Edition. Weed Science Society of America. Lawrence, Kansas. 493 pp.
Information herein is offered with no discrimination. Listing a product does not imply endorsement by the authors, University of Nevada Cooperative Extension (UNCE) or its personnel. Likewise criticism of products or equipment not listed is neither implied nor intended. UNCE and its authorized agents do not assume liability for suggested use(s) of chemical or other pest control measures recommended herein. Pesticides must be applied according to the label directions to be lawfully and effectively applied
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