2007 Annual Report

Grassland Section

Dickinson Research Extension Center
1041 State Avenue
Dickinson, ND 58601

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Chemical Management of Silver Sagebrush

Llewellyn L. Manske PhD
Range Scientist
North Dakota State University
Dickinson Research Extension Center

Introduction        

Silver sagebrush is a shrub that grows on highly productive prairie soils with the potential to produce considerable grass herbage. The shrub, which is taller than the grasses and forbs in the plant community, shades the understory from sunlight, severely reducing herbaceous plant density and herbage biomass production; diminished carrying capacity of the grasslands results. Traditional grazing practices that include early or late season grazing or heavy utilization of the grass component increase the abundance of silver sagebrush. Numerous grazinglands in the Northern Plains have problem levels of silver sagebrush infestations and require brush management.

The goal of silver sagebrush management should be reduction rather than elimination, because the shrub is assumed to have value for some wildlife and a few subspecies or ecotypes are palatable as livestock browse. This report summarizes available biological information about silver sagebrush and evaluates chemical management treatments using tebuthiuron to reduce silver sagebrush plants.

Management of silver sagebrush is not simple. Biologically effective grazing management practices improve the health and competitive abilities of the native grass plants and enhance the biogeochemical processes within the ecosystem but do not reduce the size and density of preexisting silver sagebrush plants. Silver sagebrush topgrowth is relatively easy to kill with a heavy duty brush mower, prescribed burning, or 2,4-D applied at 2.0 lb ae/ac when rapidly growing new twigs are 3 to 4 inches long (Vallentine 1974). However, management practices that kill only the aboveground plant parts produce merely short-term setbacks.

Silver sagebrush has two well developed recovery mechanisms, and after the tops of the shrubs have been killed by management treatments, the plants usually grow back at densities greater than pretreatment levels. Silver sagebrush reproduces by both seedlings and vegetative sprouts. Seed viability is usually quite high, and germination is generally around 60% to 85% (Hou and Romo 1998a). Vegetative spouts grow from belowground vertical stems and horizontal rhizomes, with about 3 to 14 sprouts for each plant with topgrowth killed (White and Currie 1983).

Names

The scientific name of silver sagebrush is Artemisia cana Pursh, and the plant belongs to the Anthemideae tribe in the aster family, Asteraceae, which is also known as the sunflower family, Compositae (Great Plains Flora Association 1986, Johnson and Larson 1999). The “silver” portion of the shrub’s common name comes from the impression given by sunlight reflecting off the pubescent leaves. The plant’s other common names are dwarf sagebrush and hoary (white) sagebrush.

Distribution        

Silver sagebrush is widely distributed throughout most of west central North America. The primary range includes western North Dakota, western South Dakota, northwestern Nebraska, northern Colorado, Wyoming, Montana, southern Alberta, southern Saskatchewan, and southwestern Manitoba. The shrub has scattered distribution in eastern North Dakota, central South Dakota, southern British Columbia, Washington, Oregon, Idaho, Nevada, Utah, Arizona, and California (Great Plains Flora Association 1986, Johnson and Larson 1999). The land area occupied by silver sagebrush is second in size only to the area occupied by big sagebrush (Artemisia tridentata Nutt.) (White and Currie 1983).

Habitat

Silver sagebrush grows in dry prairie on fertile, relatively deep clayey, loamy, and sandy soils. It is tolerant of alkali conditions (Johnson and Larson 1999) and occasionally supersaturated soils (Barbour and Billings 2000). Silver sagebrush is not known to grow in North Dakota on soils that developed from glacial till (Cosby 1964). Silver sagebrush grows in soils developed from alluvial deposits on floodplain terraces along streams, in soils developed in the unglaciated regions from sedimentary deposits, and in soils developed in the glaciated regions from sedimentary deposits that were exposed after the overlaying glacial till had been eroded away (Stevens 1950, Cosby 1964).

Associated herbaceous plants found growing with silver sagebrush in the Northern Plains are yarrow, white sage, white prairie aster, long-headed coneflower, western wheatgrass, slender wheatgrass, green needlegrass, and Kentucky bluegrass.

Plant Description

Silver sagebrush is a long-lived native deciduous warm-season shrub that can form extensive colonies. The aromatic shrub produces a characteristic strong odor that sometimes reaches irritating levels during the growing season. Mature shrubs are usually between 2 and 3 feet (0.61-0.91 m) tall; under some conditions, plants can reach a height of about 5 feet (1.52 m). Multiple large woody basal stems grow horizontal for a short distance belowground (the rhizome portion of the stem) before turning vertical (the upright portion of the stem). The horizontal rhizome portions do not appear to be progressive unless the upright portion of the stem is killed or removed. Rhizomes of young single stemmed plants are about 2 to 4 inches (5.08-10.16 cm) long, and rhizomes of mature plants have a length of about 2 to 4 feet (60.96-121.92 cm). The numerous twisted branches of aboveground stems form an elliptical aerial crown that is about as wide as the plant is tall. Plants with a history of damage from unfavorable environmental conditions or from management treatments are usually wider than they are tall. Older branches have grayish-brown shredding bark that is shed in irregular thin, long, narrow strips. Younger branches have tan bark. Current year’s stems are silvery and covered with long, soft hairs. The long, narrow leaves are simple, alternate, linear, and pointed at both ends. Typical leaves are 0.75 to 3.25 inches (1.91-8.26 cm) long and up to 0.25 inch (0.64 cm) wide. Both top and bottom leaf surfaces are pubescent; the short, soft hairs covering the surface give the leaves a grayish or silvery hue. Leaf edges are usually entire (not toothed), but occasionally 1 or 2 lobes may develop (Great Plains Flora Association 1986, Johnson and Larson 1999).

Sexual Reproduction

Numerous small yellowish flowers develop from the upper leaf axils in August and September. Flowers are borne on tight branching stems that form small leafy panicles. Florets on each cluster mature from the bottom upwards. Each floret contains both male and female organs and produces one small fruit, an achene. Fruits containing a single seed within a brownish, dry, hard, oblong covering that has 4 or 5 ribs ripen during September, October, and November (Great Plains Flora Association 1986, Johnson and Larson 1999). Seed (achene) dispersal occurs by wind. The seeds can be transported greater distances during late fall and winter when they are carried along with drifting snow (Barbour and Billings 2000).

Hou and Romo (1998a) investigated the relationships among seed size, time of germination, and seedling growth in silver sagebrush. Generally, large vigorous seedlings that develop from large seeds show a greater advantage for successful establishment and survival than small seedlings from small seeds (Booth and Haferkamp 1995).

The silver sagebrush seeds (achenes) collected in late September by Hou and Romo (1998a) in Saskatchewan were separated into 7 weight classes, with weights ranging from 0.53 to 0.83 mg per seed. Germination rate was determined for 4 replicates of 50 seeds in each of the 7 weight classes. Length of seedling root (radicle) was measured 5 days after germination (Hou and Romo 1998a).

The larger silver sagebrush seeds produced seedlings with greater root length. Seedling length increased 3 mm per 0.1 mg increase in seed weight. However, the heavier seeds had lower germination than the smaller seeds. Seeds with greater weight than about 0.57 mg had reduced percent total germination (Hou and Romo 1998a).

Seedling root length was greater for seeds that germinated early (within the first 3 days of incubation) than for late germinating seeds. Seedlings that germinated on days 6 to 12 were about 15% shorter than seedlings that germinated on the first day of incubation (Hou and Romo 1998a). The germination rate of silver sagebrush seeds was similar among weight classes (Hou and Romo 1998a), and seed size did not influence how early or late seedlings germinated. Silver sagebrush seedling success seems to be affected more by early seed germination than by size of the seed.

Hou and Romo (1998b) conducted a study that evaluated freezing tolerance in silver sagebrush seedlings to determine if freezing temperatures limit establishment in the Northern Plains. Hou and Romo (1998b) evaluated mortality of 1, 2, 4, and 6 week old silver sagebrush seedlings grown in a growth chamber and exposed to freezing temperatures of 28.4°, 24.8°, 21.2°, 17.6°, 14.0°, 10.4°, and 6.8° F (-2, -4, -6, -8, -10, -12, and -14° C, respectively). Mortality was evaluated for 8, 10, 12, and 14 week old seedlings grown in a greenhouse, acclimated to decreasing temperatures in a growth chamber for 2 weeks, and exposed to freezing temperatures of 21.2°, 17.6°, 14.0°, 10.4°, 6.8°, 3.2°, -0.4°, -4.0°, and -7.6° F (-6, -8, -10, -12, -14, -16, -18, -20, and -22° C, respectively). Silver sagebrush seedlings grown in a field for one growing season were evaluated for mortality in November, March, and April, at 7, 11, and 12 months of age, respectively, and exposed to freezing temperatures of 15.8°, 5.0°, -5.8°, -27.4°, -38.2°, and -49.0° F (-9, -15, -21, -33, -39, and -45° C, respectively) (Hou and Romo 1998b).

Freezing tolerance in silver sagebrush increased with age of the seedlings. Silver sagebrush seedlings 1 to 6 weeks old had 50% mortality at 18.1° F (-7.7° C) and 95% mortality at 12.0° F (-11.1° C). Cold acclimated seedlings 8 weeks old had 54.7% mortality at 6.8° F (-14° C); 10 and 12 week old seedlings had only 1.3% and 0.7% mortality, respectively, and 14 week old seedlings had no mortality at 6.8° F (-14° C). Silver sagebrush seedlings grown in a field for a full growing season had no mortality at -27.4° F (-33° C) in November and March. At -38.2° F (-39° C), only 6.9% of the seedlings died in November, and after exposure to -38.2° F (-39° C) and -49.0° F (-45° C), an average of only 5.6% of the seedlings died in March. The cold-hardiness of silver sagebrush seedlings decreased between March and April, during early spring, as temperatures rose and day length increased. There was no mortality of seedlings at 15.8° F (-9° C) in April, but 100% mortality at -5.8° F (-21° C) (Hou and Romo 1998b). Freeze tolerance in silver sagebrush is adequate for most seedlings to survive their first winter in the Northern Plains.

Vegetative Reproduction

Silver sagebrush can reproduce vegetatively by sucker sprout development from the belowground vertical stem base or from horizontal rhizomes when topgrowth is destroyed by unfavorable environmental conditions or by management treatments.

Phenology

Phenology is the relationship between plant growth stages and calendar date. Because the calendar is based on the solar year, phenological information shows relationships of plant growth to seasonal changes and to changes in length of daylight. Perennial plants growing in temperate zones use daylight length, or photoperiod, to program their growth stages and biological activities appropriately with the seasonal conditions.

White and Currie (1984) tested a numeric phenological scoring system to predict phenological development in silver sagebrush. Three study areas located west of Miles City, MT, had three degrees of silver sagebrush infestation: light, with 5 to 10 meters between plants; moderate, with 2 to 5 meters between plants; and heavy, with less than 2 meters between plants. From early April to late October 1981, fifteen plants on each study area were evaluated biweekly for described morphological characteristics and assigned a corresponding numeric phenological score of 0.0 to 13.0.

White and Currie (1984) found that during the growing season, changes in the selected morphological attributes in silver sagebrush were readily described by linear regression when phenological development was examined as a function of calendar date. Phenological development of silver sagebrush on the three sites with different degrees of infestation showed only small differences of less than half of a scoring unit on each evaluation period. Generally, the plants on the heavily infested site were slightly more advanced than the plants on the moderately and lightly infested sites (White and Currie 1984).

White and Currie (1984) observed the progression of phenological growth stages in silver sagebrush: buds swell in early April, leaf expansion occurs during late April and May, rapid twig growth occurs from mid May to late June, floral development occurs during mid to late July and August, flowering (anthesis) occurs during August and September, and seed development occurs during mid to late September and October. Seed dispersal is by wind during late fall and winter.

Burning Management Research

White and Currie (1983) compared the effects of spring and fall prescribed burning of silver sagebrush on two mixed grass prairie sites near Miles City, MT. The spring burn was conducted in mid April 1977, when the soil moisture was near field capacity. The fall burn was conducted in early October 1979, after a summer drought had substantially reduced soil moisture and placed plants under considerable water stress. The spring burn was a headfire, and the fall burn was a backfire. Fuel amounts were comparable for both burns, and climatic conditions were similar, with temperature near 69.8° F (21° C) and wind speed less than 5.0 mi/hr (8 km/hr). Fire intensity was rated by progressive degree of plant parts consumed: foliage consumed, twigs consumed, and stems consumed. Measurements for survival/mortality, number of sprouts, height of sprouts, and a canopy cover index were taken along 10 line transects with 210 individual plants per site during July following each burn (White and Currie 1983).

Topgrowth of silver sagebrush was completely killed by both spring and fall burns at all three fire intensity ratings. Survival/mortality of belowground plant parts depended upon fire intensity and time of year. Plant survival was greater on the spring burn. On the spring burn, plants with foliage consumed and twigs consumed sustained less than 10% mortality, and plants with stems consumed to the stump sustained about 33% mortality. On the fall burn, plants with foliage consumed and twigs consumed sustained about 40% mortality and plants with stems consumed sustained about 75% mortality (White and Currie 1983).

Plants with only foliage consumed tended to resprout sooner than plants burned at greater fire intensity. The point of origin of sprouts was not different among fire intensity classes. However, fire intensity changed the rate of sprouting; as burning intensity increased, regrowth decreased. Spring burning under favorable soil moisture conditions resulted in low mortality and high survival of silver sagebrush plants. On the spring burn, surviving plants with foliage consumed and twigs consumed had 4.2 sprouts per plant, sprout height of 12.3 inches (31.4 cm), and a canopy index of 16.1 inches (40.9 cm). Surviving plants with stems consumed had 3.3 sprouts per plant, sprout height of 8.5 inches (21.6 cm), and a canopy index of 14.1 inches (35.8 cm). On the fall burn, surviving plants with foliage consumed and twigs consumed had 11.8 sprouts per plant, sprout height of 7.4 inches (18.8 cm), and a canopy index of 12.2 inches (31.0 cm). Surviving plants with stems consumed had 4.2 sprouts per plant, sprout height of 2.2 inches (5.6 cm), and a canopy index of 3.8 inches (9.6 cm) (White and Currie 1983). The spring burn treatment resulted in fewer sprouts per surviving plant than the fall burn treatment. However, with greater plant survival and more robust sprouts, the spring burn had greater restored brush cover than the fall burn. Fall burning under dry soil moisture conditions resulted in low survival and high mortality of silver sagebrush plants, and fall burning was more effective in reducing shrub canopy cover for a longer period.

Chemical Management Research

Whitson and Alley (1984) evaluated effects of tebuthiuron on silver sagebrush near Wheatland, WY. Tebuthiuron was applied as both a 10% and a 20% pelleted formulation at the rates of 0.27, 0.54, 0.71, and 0.98 lb ai/ac (0.3, 0.6, 0.8, and 1.1 kg ai/ha, respectively) on 12 June 1980 on plots arranged in a randomized complete block design with three replications. The study control treatment had no herbicide applied. Percent live canopy cover data were collected 15 July 1980, 28 July 1981, and 7 July 1982 (Whitson and Alley 1984).

The decreases in percent live canopy cover of silver sagebrush were not significant after one year of treatment. Two years after treatment, significant decreases in percent live canopy cover of silver sagebrush occurred with all tebuthiuron application rates and both product formulations. The second year reductions in percent live canopy cover for the tebuthiuron application rates of 0.27, 0.54, 0.71, and 0.98 lb ai/ac (0.3, 0.6, 0.8, and 1.1 kg ai/ha, respectively) were 70.6%, 76.0%, 90.6%, and 77.8% for the 10% product formulation, respectively, and 77.4%, 83.3%, 96.6%, and 94.7% for the 20% product formulation, respectively. None of the rates and product formulations of tebuthiuron eradicated silver sagebrush after two years of treatment. Percent live canopy cover of western wheatgrass decreased in areas treated with 0.98 lb ai/ac (1.1 kg ai/ha) tebuthiuron (Whitson and Alley 1984).

Tebuthiuron Herbicide   

Tebuthiuron is an amide-urea derivative herbicide that is soil activated and absorbed by plants through the roots. Tebuthiuron interferes with or inhibits the photosynthetic process, causing premature aging and shedding of the leaves. Several leaf defoliation cycles deplete stored nonstructural carbohydrates and result in death of the plant (Bjerregaard et al. 1978). Tebuthiuron is moderately toxic, with an LD50 of >2000 mg/kg, for a single oral dose and slightly toxic, with an LD50 of >2000 mg/kg, for a single dermal dose (product material safety data sheet). Tebuthiuron may persist in soils for long periods. It is adsorbed to the organic matter and clay particles in the soil. Tebuthiuron resists photodecomposition and volatilization, and its breakdown by microbial activity is slow (Hamilton et al. 2004).

Spike 20P is the trade name for tebuthiuron. It is a surface applied soil-active pelleted product manufactured by Dow AgroSciences for control of woody plants in rangeland, pastureland, and noncropland. This product is also available as a wettable powder. This herbicide is toxic to fish and cannot be applied directly to water or to areas with a shallow water table (5 feet or less). The label rates for control of woody brush on grazingland are a maximum of 1.0 lb ai tebuthiuron (5 lb product) per acre in regions that receive less than 20 inches annual precipitation and a maximum of 2.0 lb ai tebuthiuron (10 lb product) per acre in regions that receive greater than 20 inches annual precipitation. Rates greater than 0.8 lb ai tebuthiuron (4.0 lb product) per acre may cause injury to perennial grasses. The product cannot be applied to an area more than once per year. Tebuthiuron has no grazing restrictions for beef animals at the labeled rates for pastures. The restriction period required between slaughter of animals and their removal from tebuthiuron treated pastures is zero days. Hay for livestock feed cannot be cut for one year after treatment. Intact treated woody plants should not be disturbed by mowing or burning for two years after treatment because the plants go through several defoliation cycles before stored nonstructural carbohydrates are depleted and death occurs (product label 2003). At low rates of 0.25 lb ai tebuthiuron per acre, additional time for control may be required.

Study of Tebuthiuron Management of Silver Sagebrush

A study was conducted to evaluate the effects of tebuthiuron on silver sagebrush, assess the effectiveness of application rates lower than the suggested 0.75 lb ai/ac rate, and evaluate longevity of treatment effectiveness by resampling treatment plots during the twenty-second growing season after herbicide applications.

Procedure

The trial of chemical management of silver sagebrush was conducted on a 0.50 acre (0.20 hectare) study area located in western North Dakota on the SW¼, SW¼, NW¼, sec. 12, T. 143 N., R. 96 W., at the Pyramid Park Experimental Area of the Dickinson Research Extension Center, from 1983 to 1986 and resampled in 2004. The study area was on a loamy overflow range site, and the soils were Glendive fine sandy loam and Haverlon silty loam, with both soils developed from deep alluvial deposits on a stream floodplain terrace. The plant community was Sagebrush type in the classification system of Hanson and Whitman (1938). Western wheatgrass, green needlegrass, and Kentucky bluegrass were the dominant grasses. Yarrow, white sage, white prairie aster, long-headed coneflower, and American vetch were the dominant forbs. Silver sagebrush infestation on the study site was heavy, with about one plant per square meter.

At the time this report was written, tebuthiuron was available as a 20% product formulation. At the time of study initiation, the chemical came in two product formulations, at concentrations of 20% and 40% active ingredient (ai) incorporated into dense clay pellets. Each product formulation was broadcast applied 24 May 1983 with a whirlybird hand spreader at three rates, 0.25, 0.50, and 0.75 lb ai/ac (0.28, 0.56, and 0.84 kg ai/ha, respectively). A control treatment with no herbicide applied was used as the reference. The 22 X 30 foot (6.71 X 9.14 m) plots were arranged in a randomized block design with two replications for the high and low rate treatments and four replications for the middle rate treatment and the control treatment.

Each silver sagebrush plant of each plot was permanently identified with a uniquely numbered tag affixed to a main branch. Measurements were collected 24-25 August 1983, 9-11 July 1984, 8-10 July 1985, 16-18 July 1986, and 8-9 and 12-14 July 2004. Each plant was measured for height from ground level and for crown diameter in two directions, north-south (N-S) and east-west (E-W). Diameter measurements were taken in two directions because the aerial crown of the shrub is more elliptical than round. Two sets of height and crown diameter measurements were collected for each plant. One set of data included only live branches, and the other set included live and dead branches. Measurements for the dead branches were determined from the difference between the live plus dead measurements and the live only measurements. The elliptical crown area per square meter was determined from the N-S and E-W diameter measurements, and the crown volume per cubic meter was determined from the crown area and height measurements for both the live and dead branches. Canopy cover was determined from the percentage of mean crown area covering a square meter of ground. The data collected in 2004 were from silver sagebrush plants with three distinctly different growth types: regrowth that developed from surviving aerial branches of old shrubs, mid aged shrubs produced by vegetative sucker sprouts that developed from surviving belowground stem bases and horizontal rhizomes, and young plants that developed from seedlings. Data for each growth type were treated separately. Total plant measurement values were the sum of the respective mean plant measurements for the mean number of old, mid aged, and young plants growing per square meter. Shrub density--number of plants per square meter--was determined from the total plant population on each plot. Differences between means were analyzed by a standard paired plot t-test (Mosteller and Rourke 1973). Cattle grazed the area each year from mid July to early November.

Results

The short-term and long-term effects that tebuthiuron applied at three rates and two product formulations had on the topgrowth of silver sagebrush and on plant survival/mortality were evaluated from plant measurements of crown diameter, crown height, shrub density, crown area, crown volume, and canopy cover. The collected information was used to describe the annual changes in live and dead branch measurements during the first four growing seasons after tebuthiuron application and to describe the changes in live and dead branch measurements between the fourth and twenty-second growing seasons for each treatment. The differences in live and dead branch measurements for old, mid aged, and young shrubs after twenty-two growing seasons were compared among the tebuthiuron and control treatments. The long-term differences in live branch measurements collected during the twenty-second growing season on the tebuthiuron and control treatments were compared to the respective plant measurements collected during the first growing season on the control treatment.

From 1983 to 1986, the amount of live branch material decreased and the amount of dead branch material increased on silver sagebrush plants treated with tebuthiuron at the rates of 0.25, 0.50, and 0.75 lb ai/ac at both 20% and 40% product formulations. The plant measurements of crown volume per m3 for silver sagebrush on the tebuthiuron treatments changed precipitously between 1983 and 1985, from greater than 90% live branches at the time the herbicide treatments were applied to greater than 90% dead branches after three growing seasons. In 1986 (the fourth growing season), the percent dead branch crown volume per m3 for the treatment rates of 0.25, 0.50, and 0.75 lb ai/ac was 91.9%, 98.2%, and 99.8% for the 20% concentration, respectively, and 92.4%, 98.3%, and 99.4% for the 40% concentration, respectively (table 1, figures 1, 2, 3, 4, 5, and 6). The percent crown volume per m3 for silver sagebrush on the control treatment changed from 91.7% live branches in 1983 to 68.7% dead branches in 1986 (table 1, figure 7).

The plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 for silver sagebrush on the treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates at both the 20% and 40% product formulations of tebuthiuron decreased significantly for the live branches from 1983 to 1986 and increased significantly for the dead branches from 1983 to 1985 (tables 2, 3, 4, 5, 6, and 7). Some of the dead branches were sloughed or broken off the shrubs; this loss caused a decrease in dead branch measurements between 1985 and 1986.

The plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 for silver sagebrush on the control treatment plots decreased annually, but not significantly, for the live branches from 1983 to 1986 and increased annually, but not significantly, for the dead branches from 1983 to 1985 (table 8). Some of the dead branches were sloughed or broken off the shrubs; this loss caused a decrease in dead branch measurements between 1985 and 1986.

The field notes made in 1986 indicated that the shrubs on the control treatment appeared to be healthy and similar to untreated plants in adjacent areas. However, the data from the control treatments (table 8) appear to show decreases in live branch measurements and increases in dead branch measurements greater than natural branch senescence. The annual decrease in the amount of live branch material and the annual increase in the amount of dead branch material were not significant for silver sagebrush plants on the control treatments from 1983 to 1986 (table 8). The cumulative changes during the first four growing seasons were significant except the changes in shrub density. The percent decrease in live branch plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 from 1983 to 1986 was 44.5%, 42.6%, 33.0%, 68.5%, and 31.1%, respectively. The percent increase in dead branch plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 from 1983 to 1986 was 635.6%, 715.3%, 2694.2%, and 446.9%, and 431.8%, respectively. There is a possibility that leaves containing small amounts of tebuthiuron were shed from treated plants and landed on the control treatment plots, where absorption of the herbicide by control plants resulted in death of portions of some branches.

The shrub density tended to decrease slightly on the tebuthiuron treatment plots and control treatment plots between 1983 and 1986, but no significant changes occurred (tables 2, 3, 4, 5, 6, 7, and 8). The shrub density on the tebuthiuron treatment plots did not decrease during the first four years of the study because one or more of the large stem branches remained attached to the stem base of the dying plants.

The field notes made in 1986 indicated that the topgrowth of most of the shrubs on all tebuthiuron treatment plots appeared to be dead. A few plants on the periphery of the plots had miniature leaves growing from the surface of some large stems. The slower tebuthiuron effect on the periphery plants most likely resulted as a remnant of the herbicide application method that attempted to spread tebuthiuron pellets to the edge of the plots but not past the edge into an adjacent plot, causing lower-than-treatment rates along the periphery of the plots. In 1986, the percentage of silver sagebrush plants with topgrowth not totally dead on the tebuthiuron treatment plots with rates of 0.25, 0.50, and 0.75 lb ai/ac was 56.4%, 40.3%, and 19.8% for the 20% concentration, respectively, and 59.1%, 29.9%, and 27.7% for the 40% concentration, respectively (table 9).

All of the herbicide treatment rates of 0.25, 0.50, and 0.75 lb ai/ac tebuthiuron for both 20% and 40% product formulations caused greater than 92% of the topgrowth branches to die by the fourth growing season after one application. There was a trend of slightly increasing percent dead branches with increase in herbicide rate. None of the treatment rates caused total topgrowth kill. Some aerial branches and some belowground stem bases and rhizomes were still alive during the fourth growing season.

A considerable number of the silver sagebrush plants measured in 1986 continued to deteriorate from the effects of tebuthiuron, and both the aboveground and belowground plant parts died sometime after 1986. The percent of the silver sagebrush plants that were measured in 1986 and later totally died on the treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates was 49.4%, 79.0%, and 51.6% for the 20% concentration, respectively, and 38.4%, 68.8%, and 63.6% for the 40% concentration, respectively. On the control treatment, 42.2% of the plants measured in 1986 died at a later date.

Tebuthiuron affects the photosynthetic processes in the leaves, reducing the production of carbohydrates and causing a drawdown of the nonstructural carbohydrate reserves; when these stores are depleted, the plant dies. Numerous silver sagebrush plants required more than four growing seasons with disrupted photosynthesis to deplete their nonstructural carbohydrate reserves completely. Some plants had sufficient carbohydrate stores to outlast the effects from a single application of tebuthiuron.

Evaluation of long-term effects from tebuthiuron treatments was conducted by resampling the study plots in 2004--22 growing seasons after herbicide application. Percent old silver sagebrush plants with regrowth development from surviving aerial branches on the tebuthiuron treatment plots with rates of 0.25, 0.50, and 0.75 lb ai/ac was 26.4%, 11.1%, and 20.8% for the 20% concentration, respectively, and 28.3%, 20.4%, and 24.1% for the 40% concentration, respectively, in 2004. Percent surviving old silver sagebrush plants on the control treatment was 33.6%. Most of the main branches with permanently affixed identification tags had died and broken away from the stem base. Only 2.4% of 1235 tags remained attached to a living main branch in 2004. The largest portion, 40.0%, of these functional tags was still affixed to live branches of plants on the control treatment plots.

From 1986 to 2004, the amount of live branch material increased and the amount of dead branch material decreased on surviving old silver sagebrush plants on plots treated with tebuthiuron at the rates of 0.25, 0.50, and 0.75 lb ai/ac at both 20% and 40% product formulations. The plant measurements of crown volume per m3 for old silver sagebrush on the tebuthiuron treatments changed from greater than 91% dead branches in 1986 to greater than 74% live branches in 2004. The percent live branch crown volume per m3 of old shrubs treated with tebuthiuron at the rates of 0.25, 0.50, and 0.75 lb ai/ac was 96.5%, 75.8%, and 91.7% for the 20% concentration, respectively, and 79.0%, 81.2%, and 74.1% for the 40% concentration, respectively, in 2004 (table 1, figures 1, 2, 3, 4, 5, and 6). The percent crown volume per m3 for silver sagebrush plants on the control treatment changed from 31.3% live branches in 1986 to 83.8% live branches in 2004 (table 1, figure 7).

The plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 for old silver sagebrush on the treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates at both 20% and 40% product formulations of tebuthiuron increased significantly for the live branches and decreased significantly for the dead branches from 1986 to 2004 (tables 2, 3, 4, 5, 6, and 7).

The live branch plant measurements for old silver sagebrush on the control treatment plots increased significantly for the N-S and E-W diameters, height, and crown area per m2 measurements and increased, but not significantly, for the crown volume per m3 measurements from 1986 to 2004. The dead branch plant measurements for old silver sagebrush on the control treatment plots decreased significantly for the N-S diameter measurements and decreased, but not significantly, for the E-W diameter, height, crown area per m2, and crown volume per m3 measurements (table 8).

Shrub density decreased significantly for old silver sagebrush on the tebuthiuron treatment plots and control treatment plots between 1986 and 2004 (tables 2, 3, 4, 5, 6, 7, and 8).

Live silver sagebrush plants on the study plots in 2004 had three distinctly different growth types: (1) old shrubs that had been present on the plots in 1983 and had regrowth that developed from surviving aerial branches, (2) mid aged shrubs that had no surviving old (1983) topgrowth and had been produced by vegetative sucker sprouts that developed from surviving belowground stem bases and horizontal rhizomes, and (3) young shrubs that developed from seedlings.

The percent of the silver sagebrush population comprised of shrubs developed from the regrowth of surviving aerial branches on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates was 43.4%, 30.6%, and 19.8% for the 20% concentration, respectively, and 30.4%, 21.6%, and 39.6% for the 40% concentration, respectively. The percent of the silver sagebrush population comprised of shrubs developed from the vegetative sucker sprouts of surviving belowground stem bases or horizontal rhizomes on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates was 30.2%, 25.0%, and 21.6% for the 20% concentration, respectively, and 31.3%, 11.4%, and 13.2% for the 40% concentration, respectively. The percent of the silver sagebrush population comprised of shrubs developed from successful seedlings on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates was 28.3%, 44.4%, and 58.6% for the 20% concentration, respectively, and 38.4%, 68.2%, and 47.2% for the 40% concentration, respectively. On the control treatment, the percent of the silver sagebrush population comprised of shrubs developed from old surviving plants, from the vegetative sucker sprouts of surviving belowground stem bases or horizontal rhizomes, and from the growth of successful seedlings was 47.5%, 31.3%, and 21.2%, respectively.

The plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 collected in 2004 from the old shrubs of silver sagebrush on the tebuthiuron treatments were not significantly different from the respective plant measurements for the old shrubs on the control treatment plots. The shrub densities of the old silver sagebrush on the tebuthiuron treatments were lower than the shrub densities of the old shrubs on the control treatment, but not significantly, in 2004 (tables 10, 11, 12, 13, 14, and 15).

The plant measurements of N-S and E-W diameters, height, shrub density, crown area per m2, and crown volume per m3 collected in 2004 from the mid aged shrubs of silver sagebrush on the tebuthiuron treatments were not significantly different from the respective plant measurements for the mid aged shrubs on the control treatment plots, except the N-S diameter measurements on the 0.75 lb ai/ac 20% concentration treatment were significantly less and the shrub densities on the 0.50 lb ai/ac 20% concentration and 0.75 lb ai/ac 40% concentration treatments were significantly less than those collected on the control treatments in 2004 (tables 10, 11, 12, 13, 14, and 15).

The plant measurements of N-S and E-W diameters, height, and crown volume per m3 collected in 2004 from the young shrubs of silver sagebrush on the tebuthiuron treatments were less than the respective plant measurements for the young shrubs on the control treatment plots, but not significantly. The measurements of crown area per m2 for the young shrubs on the tebuthiuron treatments were not significantly different from those for the young shrubs on the control treatment. The densities of the young shrubs on the tebuthiuron treatments were not significantly different, except the shrub density on the 0.75 lb ai/ac 20% concentration treatment was significantly greater than that on the control treatment (tables 10, 11, 12, 13, 14, and 15).

The total plant measurements of N-S and E-W diameters, height, shrub density, crown area per m2, and crown volume per m3 resulting from the sum of the respective mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments were not significantly different from those on the control treatment for both the live and dead branches in 2004, except the E-W diameter measurements of dead branches on the 0.25 lb ai/ac 20% concentration and 0.75 lb ai/ac 20% concentration treatments were significantly less than those on the control treatment (tables 10, 11, 12, 13, 14, and 15).

Long-term effects from tebuthiuron treatments after 22 growing seasons were evaluated by comparisons of live branch plant measurements of N-S and E-W diameters, height, shrub density, crown area per m2, and crown volume per m3 collected on tebuthiuron treatment plots in 2004 with the respective measurements collected on the control treatment in 1983.

The total N-S diameter measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments in 2004 were lower than the N-S diameter measurements collected on the control treatment in 1983; measurements collected in 2004 on the 0.25 lb ai/ac 20% concentration, 0.50 lb ai/ac 20% concentration, 0.50 lb ai/ac 40% concentration, and 0.75 lb ai/ac 40% concentration treatments were significantly less than N-S diameter measurements collected in 1983 on the control treatment (table 16).

The total E-W diameter measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments in 2004 were lower than the E-W diameter measurements collected on the control treatment in 1983; measurements collected in 2004 on the 0.25 lb ai/ac 20% concentration, 0.50 lb ai/ac 20% concentration, 0.50 lb ai/ac 40% concentration, 0.75 lb ai/ac 20% concentration, and 0.75 lb ai/ac 40% concentration treatments were significantly less than E-W diameter measurements collected in 1983 on the control treatment (table 16).

The total height measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments in 2004 were lower than the height measurements collected on the control treatment in 1983; measurements collected in 2004 on the 0.25 lb ai/ac 20% concentration, 0.50 lb ai/ac 20% concentration, 0.50 lb ai/ac 40% concentration, and 0.75 lb ai/ac 40% concentration treatments were significantly less than height measurements collected in 1983 on the control treatment (table 16).

The total shrub density measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments in 2004 were lower than the density measurements collected on the control treatment in 1983; measurements collected in 2004 on the 0.50 lb ai/ac 20% concentration and 0.75 lb ai/ac 40% concentration treatments were significantly less than density measurements collected in 1983 on the control treatment (table 17).

The total crown area per m2 measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter on the tebuthiuron treatments in 2004 were lower than the crown area per m2 measurements collected on the control treatment in 1983; measurements collected in 2004 on the 0.50 lb ai/ac 20% concentration and 0.50 lb ai/ac 40% concentration treatments were significantly less than crown area per m2 measurements collected in 1983 on the control treatment (table 18).

On all tebuthiuron treatments but one, the total crown volume per m3 measurements resulting from the sum of the mean measurements of old, mid aged, and young shrubs per square meter in 2004 were lower than the crown volume per m3 measurements collected on the control treatment in 1983; the measurements were greater on the 0.25 lb ai/ac 20% concentration treatment. Measurements collected in 2004 on the 0.25 lb ai/ac 40% concentration, 0.50 lb ai/ac 20% concentration, 0.50 lb ai/ac 40% concentration, and 0.75 lb ai/ac 20% concentration treatments were significantly less than crown volume per m3 measurements collected in 1983 on the control treatment (table 18).

The plant measurements of N-S and E-W diameters, height, density, crown area per m2, and crown volume per m3 collected on the control treatment in 2004 were not significantly different from the respective measurements collected on the control treatment in 1983 (tables 16, 17, and 18).

On the 0.25 lb ai/ac 20% concentration treatment, the plant measurements of N-S and E-W diameters and height collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (table 16). On the 0.25 lb ai/ac 40% concentration treatment, the plant measurements of crown volume per m3 collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (table 18).

On the 0.50 lb ai/ac 20% concentration treatment, the plant measurements of N-S and E-W diameters, height, shrub density, crown area per m2, and crown volume per m3 collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (tables 16, 17, and 18). On the 0.50 lb ai/ac 40% concentration treatment, the plant measurements of N-S and E-W diameters, height, crown area per m2, and crown volume per m3 collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (tables 16 and 18).

On the 0.75 lb ai/ac 20% concentration treatment, the plant measurements of E-W diameter and crown volume per m3 collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (tables 16 and 18). On the 0.75 lb ai/ac 40% concentration treatment, the plant measurements of N-S and E-W diameters, height, and shrub density collected in 2004 were significantly below the respective measurements collected on the control treatment in 1983 (tables 16 and 17).

Canopy cover decreased annually on all tebuthiuron and control treatments between 1983 and 1986. The percent decrease in canopy cover in the fourth growing season after application of tebuthiuron at the rates of 0.25, 0.50, and 0.75 lb ai/ac was 89.6%, 95.5%, and 99.5% for the 20% concentration, respectively, and 88.7%, 95.9%, and 97.5% for the 40% concentration, respectively (table 19). The percent decrease in canopy cover on the fourth growing season on the control treatment was 68.5% (table 19).

A portion of the canopy cover had been recovered by the surviving shrubs sometime after 1986 and before 2004. The canopy cover of surviving silver sagebrush on most of the tebuthiuron treatments during the twenty-second growing season remained substantially below the silver sagebrush canopy cover on the control treatment during the first growing season. In 2004, the percent reduction of the canopy cover of the surviving shrubs on the tebuthiuron treatments with rates of 0.25, 0.50, and 0.75 lb ai/ac was 32.4%, 50.7%, and 55.1% for the 20% concentration, respectively, and 9.8%, 54.9%, and 37.7% for the 40% concentration, respectively, below the canopy cover on the control treatment in 1983 (table 19). The percent reduction of the canopy cover of the surviving shrubs on the control treatment in 2004 was 15.9% below the canopy cover on the control treatment in 1983 (table 19).

Between 1986 and 2004, some silver sagebrush plants developed from successful seedlings on all of the tebuthiuron and control treatments. The canopy cover of the shrubs developed from seedlings was low. On the twenty-second growing season, the canopy cover provided by shrubs developed from seedlings was substantially below the canopy cover of silver sagebrush in the first growing season. In 2004, the percent decrease in canopy cover of the shrubs developed from seedlings on the tebuthiuron treatments at the 0.25, 0.50, and 0.75 lb ai/ac rates was 98.9%, 97.1%, and 96.4% for the 20% concentration, respectively, and 95.1%, 98.0%, and 96.6% for the 40% concentration, respectively, below the canopy cover on the control treatment in 1983 (table 19). In 2004, the percent decrease in canopy cover provided by shrubs developed from seedlings on the control treatment was 95.9% below the canopy cover on the control treatment in 1983 (table 19).

The plant measurements collected in 2004 on the tebuthiuron treatments with 0.25, 0.50, 0.75 lb ai/ac rates at both 20% and 40% product formulation were lower than the respective plant measurements collected in 1983 on the control treatment. After 22 growing seasons following herbicide application, the silver sagebrush plants on the tebuthiuron treatments with 0.25, 0.50, and 0.75 lb ai/ac rates had not recovered fully in plant size and distribution to equal the measurements of untreated plants on the control treatment during the first growing season.

The changes in live and dead branch plant measurements for silver sagebrush plants on the control treatment followed a pattern similar to that of the changes for silver sagebrush plants on the tebuthiuron treatment plots; this similarity may indicate that the plants on the control plots were affected somewhat by herbicide transported to the control plots by leaves shed from treated plants. The recovery of silver sagebrush plants from the effects of inadvertent herbicide movement to control treatments was more rapid than the recovery of plants from the effects of tebuthiuron application on the treatment plots.

Tebuthiuron killed both the topgrowth and belowground plant parts of numerous plants at each of the herbicide treatment rates and product formulations by the time the effects of the chemical played out. Of the plants present in 1983 on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates, the percent that had both the aboveground and belowground plant parts killed was 55.2%, 79.8%, and 56.6% for the 20% concentration, respectively, and 42.5%, 68.8%, and 67.8% for the 40% concentration, respectively (table 20). On the control treatment, 44.3% of the plants were totally killed (table 20). Of the plants present in 1983 on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates, the percent that had entire topgrowth killed was 73.6%, 88.9%, and 79.3% for the 20% concentration, respectively, and 71.7%, 79.6%, and 75.9% for the 40% concentration, respectively (table 20). On the control treatment, 66.4% of the plants had the topgrowth killed (table 20). On some of the plants present in 1983, aerial branches or belowground stem bases and horizontal rhizomes survived the herbicide treatments. Recovery of the plants with surviving parts occurred as regrowth from the surviving aerial branches and as vegetative sucker sprouts from the surviving belowground parts. Of the plants present in 1983 on the tebuthiuron treatment plots with 0.25, 0.50, and 0.75 lb ai/ac rates, the percent that had aboveground or belowground plant parts survive was 44.8%, 20.2%, and 43.4% for the 20% concentration, respectively, and 57.5%, 31.2%, and 32.2% for the 40% concentration, respectively (table 20). On the control treatment, 55.8% of the plants survived (table 20).         

Long-term reduction of silver sagebrush requires killing both the aboveground and belowground plant parts. Chemical management with tebuthiuron is a promising practice for long-term reduction of silver sagebrush. Chemical management of silver sagebrush with one application of tebuthiuron at 0.25, 0.50, and 0.75 lb ai/ac killed 92% to 99% of the aerial branches in four growing seasons, totally killed 43% to 80% of the individual plants in an unknown time period greater than four growing seasons, and prevented full recovery of plant size and distribution for at least 22 growing seasons. The 0.50 and 0.75 lb ai/ac rates were slightly more effective at total plant kill than the 0.25 lb ai/ac rate. Nevertheless, the low rate totally killed about 49% of the individual plants. None of the application rates killed all of the plants. An average of about 38% of the silver sagebrush plants had sufficient stores of nonstructural carbohydrates or regulated carbohydrate drawdown rates and were able to survive longer than the effects from one application of tebuthiuron. High mortality of existing plants is required for long-term management of silver sagebrush because regrowth from surviving plants can reoccupy treated areas faster than shrubs developed from seedlings. The relatively large number of surviving plants suggests that chemical management of silver sagebrush requires multiple applications of tebuthiuron in order to extend the period of effectiveness. Either an application of 0.50 lb ai/ac followed by an application of 0.25 lb ai/ac or a series of three applications of tebuthiuron at 0.25 lb ai/ac would be biologically effective for a longer period than a single application of 0.75 lb ai/ac tebuthiuron. Unfortunately, the interval between herbicide applications was not determined by this study. Extrapolation indicates that the interval between herbicide applications would be greater than four years but less than 22 years and probably less than 10 years. Most likely an application interval of 6 to 8 years would work satisfactorily.

Discussion             

Tebuthiuron affects the photosynthetic processes in shrub leaves, resulting in premature leaf shedding. Production and shedding of successive sets of leaves cost the plant more nonstructural carbohydrates than the leaves produce during the brief period they are active. When the carbohydrate stores are depleted, the plant dies. The effects of tebuthiuron on silver sagebrush topgrowth are quantifiably noticeable in three months, and death of aerial branches occurs rapidly during the first three growing seasons following herbicide application. The rate of plant deterioration slows during the fourth growing season, and presumably the herbicide effects continue at progressively slower rates for an unknown number of growing seasons until the effects from tebuthiuron completely cease.

Numerous silver sagebrush plants had small portions of their stored carbohydrates remaining when the tebuthiuron was depleted. The surviving plants produced regrowth from aerial branches or vegetative sucker sprouts from belowground stem bases or horizontal rhizomes. The rate of recovery is assumed to start slowly and accelerate gradually. From the time of tebuthiuron application, recovery of a silver sagebrush colony requires more than twenty- two growing seasons to reach pretreatment infestation levels. A second application of a small amount of tebuthiuron just prior to depletion of the first application of the chemical would extend the period of effectiveness and result in carbohydrate depletion of most, if not all, of the silver sagebrush plants in the colony. After the chemical from the second application is depleted, the area would be prone to reinfestation by silver sagebrush seedlings. Implementation of biologically effective grazing management practices like the twice-over rotation method would improve the health and competitive abilities of the native grass plants and impede silver sagebrush seedling reinfestation.

Summary

Silver sagebrush is a problem plant that has infested considerable acreage of western grazinglands. This biologically successful shrub infests highly productive prairie soils and shades understory grasses and forbs from sunlight, reducing herbage biomass production and causing diminished carrying capacity and greatly reduced net income captured from grasslands. Infested areas require some type of shrub reduction management. Silver sagebrush topgrowth is not too difficult to kill with a heavy duty brush mower, prescribed burning during fall, or 2,4-D applied at 2.0 lb ae/ac when twigs are growing rapidly. Killing the topgrowth, however, does not solve the problem. Silver sagebrush has well developed mechanisms that provide rapid recovery from severe damage to the aerial crown. Recovery of silver sagebrush occurs with the regrowth from surviving aerial branches, with the vegetative sucker sprouts from surviving belowground stem bases or horizontal rhizomes, and with the growth from successful seedlings.

The herbicide tebuthiuron interferes with the photosynthetic processes in shrub leaves; its application results in the plant’s ability to produce only small amounts of carbohydrates annually and causes the plant to draw down the stored nonstructural carbohydrates. The plant dies when the carbohydrate reserves are depleted. Tebuthiuron is a pelleted herbicide applied to the soil at relatively low cost; application of 0.50 lb ai/ac would cost about $6.00 per acre for the chemical. Chemical management of silver sagebrush could be accomplished by two or three applications of low rates of tebuthiuron at intervals of about 6 to 8 years. Management practices that reduce silver sagebrush long term would improve the health and condition of grassland ecosystems, increase grass herbage production, and increase carrying capacity of infested grazinglands.

Acknowledgment

I am grateful to Sheri Schneider for assistance in production of this manuscript and for development of the tables and figures. I am grateful to Amy M. Kraus for assistance in preparation of this manuscript.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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