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© 2003 Plant Management Network.
Accepted for publication 9 August 2003. Published 19 September 2003.

Control of Eastern Redcedar with Broadcast and Individual Tree Treatments of Selected Herbicides in Pasture

Stevan Z. Knezevic, Assistant Professor, Haskell Agricultural Laboratory, University of Nebraska, Concord 68728; Adam J. Kantrovich, Assistant Professor, Morehead State University, Morehead, KY 40351, Former University of Nebraska Extension Educator; and Robert A. Masters, Rangeland Scientist, Dow Agro Sciences, Lincoln, NE, 68506

Corresponding author: Stevan Z. Knezevic. sknezevic2@unl.edu

Knezevic, S. Z., Kantrovich, A., and Masters, R. A. 2003. Control of eastern redcedar with broadcast and individual tree treatments of selected herbicides in pasture. Online. Crop Management doi:10.1094/CM-2003-0919-01-RS.

Abstract

Field studies were conducted in 2001 and 2002 in northeast Nebraska to determine the response of eastern redcedar to selected chemical control treatments. Herbicides were applied either broadcast or as high volume foliar sprays to individual trees. Tree height was an important factor influencing level of chemical control in broadcast treatments. Treatment efficacy declined with increased tree height. Eastern redcedar control was greatest when picloram was a component of herbicide treatments either broadcast-applied to trees or when individual trees were sprayed. Eastern redcedar control did not exceed 26% when triclopyr-containing treatments were applied. Broadcast applications of picloram + fluroxypyr at 5 pt/acre, picloram + 2,4-D at 6 and 8 pt/acre, and picloram used alone at 2 pt/acre provided excellent control (> 85%) of trees that were a foot or less in height. In contrast, these treatments provided poor control (< 60%) of trees that were more than 2 ft in height. Eastern redcedar control was excellent (> 85%) when individual trees were treated with premixes of picloram + fluroxypyr, or picloram + 2,4-D, applied at 1.5% and 2.0% (volume per volume) of product solution, and with picloram applied at 1.0 % solutions.

Introduction

Eastern redcedar (Juniperus virginiana L.) is one of 13 juniper species native to the United States. It is the most widespread tree-sized conifer in the US and is native to every state east of the 100th meridian. Throughout this vast range, eastern redcedar grows on many soils and under varying climatic conditions (9). Eastern redcedar reduces pasture forage production (7) and alters the microclimate which encourages a shift from desirable warm-season to less desirable cool-season forage grasses (5). Furthermore, high-density tree infestations impede livestock handling (8) and reduce pasture and rangeland value (9). For these reasons, various methods of control have been studied and reported in the literature including prescribed fire (1), individual tree ignition (4), cutting (2,10), and herbicides (2,3). Reports of efficacy and the economics of control suggested that some methods were more economical than others, but largely depended on the site circumstances (6). For example, prescribed fire followed by treatment of surviving eastern redcedar trees with picloram was the most economical strategy (6). Herbicides are an important component of eastern redcedar integrated management. The objective of these studies was to determine efficacy of selected herbicides for eastern redcedar control.

Field Studies

Broadcast and individual-tree studies were conducted in 2001 near Center and St. James, NE. In 2002, the broadcast-applied herbicide study was conducted near Center, and individual-tree study was conducted near Newcastle, NE. Sites were from 2 to 45 miles from each other. The sites were located on pastures ranging in elevations from 1200 to 1800 ft and slopes varied from 10 to 40%. Average yearly precipitation in the general area is about 23 inches; 75% of the rain usually falls from April to August. Pasture conditions were generally good with grassy species dominated by smooth bromegrass (Bromus inermis L.), Kentucky bluegrass (Poa pratensis L.), and Scribner panicum (Dichanthelum oligosanthles (Shalt.)(Gowld) var scribnerianum (Nash).

Herbicides were applied as both broadcast treatments and directed to the foliage of individual trees. Herbicides were applied on May 31, 2001 at the Center and St. James sites, and on June 6 and June 11 at Newcastle and Center in 2002. Visual evaluation of percent tree injury was conducted at approximately 100 days after treatment. All ratings were based on a scale from 0 to 100 (where 0 = no injury and 100 = tree death).

Both broadcast and individual studies were established as a randomized complete block design with four and ten replications, respectively. Due to differences in elevation and slopes within a site, individual replications (blocks) were positioned perpendicular to the hillside in order to test potential slope-elevation effects. Broadcast studies had ten treatments at each location. The list of treatments included: picloram at 0.66 lbs a.e. per gal + fluroxypyr at 0.66 lbs a.e. per gal applied at the rates of 3, 4, and 5 pt/acre; triclopyr at 1.5 lbs a.e. per gal + fluroxypyr at 0.5 lbs a.e. per gal at the rate of 4 pt/acre; triclopyr at 1.0 lbs a.e. per gal + fluroxypyr at 0.33 lbs a.e. per gal at the rate of 4 and 6 pt/acre; picloram at 0.54 lbs a.e. per gal + 2,4-D at 2.0 lbs a.e. per gal at the rate of 6 and 8 pt/acre; and picloram at 2.0 lbs a.e. per gal at the rate of 2 pt/acre. Picloram + fluroxypyr will be marketed as “Surmount,” while tryclopyr at 1.5 lbs/acre + fluroxypyr at 0.5 lbs/gal will be marketed as “Pasturegard” pending approval for range and pasture use by the Environmental Protection Agency. Tryclopyr at 1.0 lbs a.e. per gal + fluroxypyr at 0.33 lbs a.e. per gal is not commercially available in the United States at the present time. The picloram + 2,4-D treatment is commercially available as “Grazon P&D,” while picloram is available as “Tordon 22K.” All herbicide treatments were applied with a CO₂-operated backpack sprayer that delivered 20 gal/acre at 30 lbs per square inch in through a 10-ft boom with six XR-11002VS nozzles. Plots were 10 ft wide and 30 ft long. Number and height of trees were determined in each plot before herbicides were applied. There were an average of 15 trees per each plot that ranged in height from 0.5 to 6.0 ft.

The individual-tree studies had seven treatments at each location. The list of treatments included: picloram + fluroxypyr each at 0.66 lbs a.e. per gal at the rates of 0.5%, 1.0% and 1.5% volume per volume (v/v) solution; triclopyr at 1.0 lbs a.e. per gal + fluroxypyr at 0.33 lbs a.e. per gal applied as 1.0% v/v solution; picloram at 0.54 lbs a.e. per gal + 2,4-D at 2.0 lbs a.e. per gal at 2.0% v/v solution; and picloram at 2.0 lbs a.e. per gal at the 1.0% v/v solution. All herbicide treatments were applied with a CO₂-operated backpack sprayer that delivered approximately 1.5 oz of spray solution per foot of tree height at 20 lbs per square inch by a single nozzle boom with a XR-8002VS nozzle. Herbicide rates were expressed on a volume per volume basis (e.g., volume of herbicide per volume of water). This is a common practice by land managers used for selective spot spraying utilizing backpack or ATV- mounted sprayers. Average tree height was 6 ft. There were ten trees per treatment and with each tree representing a replication.

Analysis of variance (ANOVA) of visual injury ratings was performed using PROC GLM to test data normality and significance (P < 0.05) of the year, location, replication, treatments, and their interactions. Treatment differences were based on an LSD test (95% confidence level). In order to reduce the LSD values, the non-sprayed control treatment that resulted in zero injury was excluded from data analysis. There were no significant replication- or replication-by-treatment interactions, indicating no effects of elevation or slope, thus data from all replications were combined and mean calculated for each site. Furthermore, there were no significant location- or year-by-treatment interactions, thus data from all sites were pooled over years and locations into a single data set. There was a significant treatment-by-height interaction so control (tree injury) data were presented by tree height.

Broadcast study. Since tree height was the most important plant factor influencing the level of chemical control (tree injury) with broadcast treatments, the efficacy data were categorized by tree height (Table 1). In general, herbicide efficacy decreased as height of treated trees increased. The greatest control was achieved when picloram was a component of the herbicide treatments. For example, excellent control (> 85%) of trees of up to one foot tall was achieved with a premixes of: picloram + fluroxypyr at 5 pt/acre (Treatment 3), picloram + 2,4-D at 6 and 8 pt/acre (Treatments 7 and 8), or picloram used alone at 2 pt/acre (Treatment 9) (Table 1). However, the same treatments, with the exception of picloram + 2,4-D, provided poor control (< 50%) of trees taller than 2 ft. Eastern redcedar injury was no better than 26% when triclopyr-containing treatments were applied, regardless of the tree height (Table 1). Short-term grass injury in the form of leaf yellowing and top growth burning was evident among all treatments, especially Treatments 2 and 3 (Table 1). The average cost of Treatments 7, 8, and 9 was determined to be $21, 26, and $22 per acre, respectively.

Table 1. Percent eastern redcedar and grass injury levels at about 100 days after treatment as influenced by the tree height (feet) where herbicide treatments were broadcast applied.

^aTreatments 1, 2, and 3 were mixtures of picloram + fluroxypyr each at 0.66 lbs a.e. per gal.

Treatment 4 was triclopyr at 1.5 lbs a.e. per gal + fluroxypyr at 0.5 lbs a.e. per gal.

Treatments 5 and 6 were triclopyr at 1.0 lbs a.e. per gal + fluroxypyr at 0.33 lbs a.e. per gal.

Treatments 7 and 8 were picloram at 0.54 lbs a.e. per gal + 2,4-D at 2.0 lbs a.e. per gal.

Treatment 9 was picloram at 2.0 lbs a.e. per gal.

Individual-tree study. Similar to the broadcast study, the greatest tree control was achieved when picloram was a component of the herbicide treatments. For example, excellent eastern redcedar control (> 85%) resulted when individual trees were treated with premixes containing picloram + fluroxypyr applied in 1.5 or 2.0% (v/v) solutions, picloram + 2,4-D, or picloram used alone (Table 2). Similar to the broadcast study, there was temporary grass injury following application of all treatments, especially for Treatments 3, 5, and 6 (Table 2).

Table 2. Percent of eastern redcedar and grass injury at 100 days after treatment in individual-tree study.

^aTreatments 1, 2, and 3 were mixtures of picloram + fluroxypyr each at 0.66 lbs a.e. per gal.

Treatment 4 was triclopyr at 1.0 lbs a.e. per gal + fluroxypyr at 0.33 lbs a.e. per gal.

Treatment 5 was picloram at 0.54 lbs a.e. per gal + 2,4-D at 2.0 lbs a.e. per gal.

Treatment 6 was picloram at 2.0 lbs a.e. per gal.

^bDose was a herbicide/water solution on a volume/volume basis

Management Implications

From a practical standpoint, the data from this study suggests that herbicides can be used as an effective tool for redcedar control. However, the use of selective herbicide treatments should be based on tree height. Broadcast treatments can be effective only on short trees (< 2 ft tall), while the medium height trees (2 to 10 ft) can be controlled with individual-tree treatments. Taller trees (> 10 ft) can be controlled by cutting, which is the most effective control method (6).

Acknowledgements

Published as University of Nebraska Agricultural Research Division Journal Series No. 14170.

Literature Cited

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