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© 2005 Plant Management Network.
Accepted for publication 22 June 2005. Published 19 July 2005.

Comparison of Glyphosate Herbicides in Nebraska

Brady F. Kappler, Extension Educator, Stevan Z. Knezevic, Associate Professor, Robert F. Klein, Professor, Drew J. Lyon, Professor, Alex R. Martin, Professor, Fred W. Roeth, Professor, and Gail A. Wicks, Department of Agronomy University of Nebraska, Lincoln 68583-0915

Corresponding author: Stevan Knezevic. sknezevic2@unl.edu

Kappler, B. F., Knezevic, S. Z., Klein, R. F., Lyon, D. J., Martin, A. R., Roeth, F. W., and Wicks, G. A. 2005. Comparison of glyphosate herbicides in nebraska. Online. Crop Management doi:10.1094/CM-2005-0719-01-RS.

Abstract

The proliferation of glyphosate products into the glyphosate-resistant crop market is unprecedented. Due to widespread public interest in these products, field experiments were conducted to compare the efficacy of various glyphosate herbicides on weed control at six locations across Nebraska in various cropping systems. In eastern Nebraska, all herbicide treatments provided excellent weed control (> 90%) regardless of the rate or brand name. In Western Nebraska, herbicide rate and the year were significant, but there was no significant difference among glyphosate brands. In general, generic glyphosate products provided equal level of weed control compared to the usually more expensive brand name products.

Introduction

Glyphosate, a non-selective systemic herbicide, is the most widely used herbicide in the world, and literature about its use and characteristics is extensive (2,3,8). Glyphosate use in the United States increased rapidly with the introduction of glyphosate-resistant (Roundup-Ready) crops, which growers readily integrated into their crop production practices. For example, currently more than 80% of 65 million acres of soybeans grown in the USA annually are cultivars genetically engineered to be tolerant to glyphosate. In Nebraska, over 90% of the soybeans and 20% of the corn in 2004 were glyphosate-resistant hybrids (1).

The widespread adoption of glyphosate-resistant crops greatly expanded the market demand for glyphosate, which in turn, resulted in many generic and brand name products after the patents held by Monsanto expired. There are more than 40 glyphosate herbicides registered for use in Nebraska (1). The influx of the generic glyphosate herbicides has also resulted in price reductions as distributors attempt to remain competitive. Producers are interested in getting the best weed control for the lowest cost, creating an interest in the efficacy of these products. Therefore, the objective of this study was to compare efficacy of a variety of glyphosate products on weed control in several cropping systems in Nebraska.

Field Studies at Six Nebraska Locations

Field studies were conducted for three years (2001, 2002, and 2003) at six sites across Nebraska. Weather data for each site and year combination was reported in Table 1. Glyphosate-resistant soybean was used at Clay Center, Concord, and Lincoln in 2001 and 2002. Glyphosate-resistant corn was used at Clay Center in 2002. The study was also conducted in wheat stubble fallow at two sites near North Platte (3 miles apart) and one location near Sidney (Table 2). A total of 6, 6, and 4 glyphosate products were tested at each site in 2001, 2002, and 2003, respectively (Table 3). The experiments were established as a randomized complete block design (RCBD) with 12, 12, and 8 treatments for 2001, 2002, and 2003, respectively, including a non-treated control, with three replications. Each plot was 30 ft long and 10 ft wide. All products were applied with the addition of ammonium sulfate at a 2% weight per volume (w/v). Herbicides were applied at V5-V6 leaf stage of corn and soybean, and when most weeds were 8 to 12 inches tall. Each glyphosate product was applied at 0.375 and 0.75 lb ae/acre at a spray volume of 10 gal/acre using a backpack or tractor-mounted sprayer pressured with CO₂ (Table 4). Visual ratings of percent control of overall weed control were conducted at approximately 15 and 30 days after treatment (DAT). There was no difference in the level of weed control between the 15 and 30 DAT therefore the data from 30 DAT was presented. Control ratings were based on a scale from 0 to 100, where 0 = no injury and 100 = plant death.

Table 1. Monthly rainfall (inches) for each location during 2001, 2002,
and 2003 spraying seasons.

Table 2. List of locations by years and crops used in the study and
herbicide application dates.

Table 3. List of glyphosate herbicide formulations used at each of the
five sites during the 2001, 2002, and 2003 year.

Table 4. List of glyphosate based product, formulation types, and rates (lb ae/acre) tested in the study.

 ^x Glyphosate can be formulated in a form of several salts: IPA = Isopropylamine; MA = monoammonium; DA = diammonium; K = potasium salt.

 ^y The concentration of glyphosate are usually expressed as "salt or active ingredient" (ai) or "acid-equivalent" (ae).

 ^z The two rates in acid equivalent (ae) per acre used in this study, and respective equivalent rates of product per acre.

Weed species composition differed among sites and years. At Clay Center, Concord, and Lincoln, common weeds included velvetleaf (Abutilon theophrasti), common waterhemp (Amaranthus rudis), sunflower (Helianthus annuus), and a mix of foxtail species (Setaria spp). At Sidney and both North Platte locations, common weeds were kochia (Kochia scoparia), Russian thistle (Salsola iberica), narrow leaf lambsquarters (Chenopodium desiccatum), buffalobur (Solanum rostratum), and a mix of foxtail species (Setaria glauca and S. viridis).

Analysis of variance (ANOVA) of visual ratings was performed using PROC MIXED procedure to test data normality and significance (P < 0.05) of the year, location, replication, glyphosate rates, glyphosate brands, and their interactions. There was no significant effect of the location when the data sets were compared within the eastern and western parts of the state. Thus data were combined over the three eastern locations (Lincoln, Clay Center, and Concord) and for the three western locations (North Platte site 1, North Platte site 2, and Sidney) and reanalyzed. Hereafter, these combined data sets will be referred to as Eastern Nebraska and Western Nebraska. There were no significant effects of the year, rate, year-by-treatment, or rate-by-treatment interaction in the Eastern Nebraska data, thus data was combined over years and rates. Due to significant effect of the glyphosate rate, year, and year-by-treatment interaction in the Western Nebraska data, results are presented separately for each year and rate. Treatment differences were based on Fisher’s protected LSD test (P = 0.05). In order to reduce the LSD values, the non-sprayed control treatment, which provided no weed control, was excluded from data analysis.

Herbicide Efficacy

Most herbicide treatments provided acceptable weed control. Due to a lack of interactions in Eastern Nebraska data, herbicide efficacy was combined across years, locations, and rates. In Eastern Nebraska, all herbicide treatments provided excellent weed control (> 90%) regardless of the brand name (Table 5). For example, there was no significant difference in the level of weed control for a brand name Roundup Ultra Dry when compared to the generic product such as Clearout41 Plus, or any other herbicide (Table 5).

Table 5. Percent weed control at 30 days after treatment in
 Eastern Nebraska (data were combined from 3 years, 3
locations and 2 rates). Weeds in the plots included velvetleaf,
common waterhemp, sunflower, and green and yellow foxtail.

At Western Nebraska sites, herbicide rate and the year were significant, while there was no significant effects of the site or the glyphosate product, thus data were combined among sites and presented by year and rate. In each year, the 0.75 lb ae/acre rate provided significantly higher level of weed control than the 0.375 lb ae/acre rate (Table 6). In 2001, weed control ratings for 0.75 lb ae rate ranged from 94% to 99% compared to 68% to 74% at 0.375 lb ae. Similar trends were observed in 2002 and 2003 (Table 6). There was no significant difference in weed control among glyphosate products within each of the two rates tested. For example, in 2003, the level of weed control was not significantly different for a brand name product such was Roundup WeatherMax when compared to a generic product such as Clearout41 Plus for either the 0.375 and 0.75 lb ae/acre rate (Table 6).

Table 6. Overall weed control ratings (%) at 30 days after treatment
at Western Nebraska in 2001, 2002, and 2003. Common weeds were
kochia, Russian thistle, narrow leaf lambsquarters, Buffalobur, green
and yellow foxtail.

Observed differences in weed control among years and rates in Western Nebraska was likely due to dryer weather conditions (e.g., low rainfall) around the time of herbicide application, especially in 2003. The timing of application to wheat stubble was often in late July to early August, which is normally the driest period of the year in this part of the state. Herbicide efficacy is often dercreased during dry periods due to slow weed growth and reduced herbicide uptake and translocation (S. Z. Knezevic, personal communications).

Practical Implications

From a practical standpoint, there was no difference in the level of weed control among glyphosate products tested. These findings were similar to the results reported by others (4,6,7). Therefore, generic glyphosate products can provide similar weed control at a lower cost compared to name brand glyphosate products. Perhaps the more important decision by producers should be to select the appropriate herbicide rate based upon weed species present and environmental conditions, rather than choosing a specific glyphosate product.

With the growing popularity of the relatively inexpensive generic glyphosate products, there is an even greater need for herbicide stewardship. Their value can be preserved only by proper management. This becomes even more important if other Roundup-Ready crops become more readily available (e.g., Roundup-Ready corn and Roundup-Ready alfalfa), as it is easy to fall into a trap of overusing glyphosate when one glyphosate-resistant crop is grown after another. Therefore, proper use of glyphosate technology, as a component of integrated weed management program, is the key to preserving the long-term benefits of this technology while avoiding many of the concerns about its use, or misuse (5).

Acknowledgments

Published as University of Nebraska Agricultural Research Division Journal Series No. 14579.

Literature Cited

1. Anonymous. 2004. Guide for Weed Management. Univ. Nebr. EC-130.

2. Franz, J. E., Mao, M. K., and Sikorski, J. A. 1997. Glyphosate: A Unique Global Hebricide. Monogr. 189. Washington, DC. Am. Chem. Soc.

3. Grossbeard, E. and Atkinson, D. 1985. The Herbicide Glyphosate. Boston, MA, Butterworths.

4. Hillger, D. E., Bauman, T. T., and White, M. D. 2002. Performance and economic comparisons of different glyphosate products in glyphosate resistant corn (Zea mays) and soybean (Glycine max) Weed management systems. CD-ROM computer file. N. Central Weed Sci. Soc. Abstr. 57. Champaign, IL.

5. Knezevic, S. Z., and Cassman, K. G. 2003. Use of herbicide-tolerant crops as a component of an integrated weed management program. Online. Crop Management doi:10.1094/CM-2003-0317-01-MG.

6. Johnson, W. G., Wait, J. D., Schmidt, A. A., Guttikonda, S. K., Cordes, J. C. 2001. Glyphosate formulations. CD-ROM computer file. N. Central Weed Sci. Soc. Res. Rep. 58:292. Champaign, IL.

7. Maxwell, D. J., Sprague, C. L., Hasty, R. F. 2002. Different glyphosate salts for weed control in soybean. CD-ROM computer file. N. Central Weed Sci. Soc. Res. Rep. 59:233. Champaign, IL.

8. Woodburn, A. T. 2000. Glyphosate: Production, pricing and use worldwide. Pest Manag. Sci. 56:309-319.