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© 2004 Plant Management Network.
Accepted for publication 21 October 2004. Published 8 December 2004.

Waterhemp Control in Transgenic and Conventional Corn Varieties

Brent A. Sellers, Post Doctoral Fellow, Joseph C. Cordes, former Graduate Research Assistant, Reid J. Smeda, and Associate Professor of Agronomy, Department of Agronomy, University of Missouri, Columbia 65211; and William G. Johnson, Assistant Professor of Weed Science, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907

Corresponding author: Brent Alan Sellers. SellersB@missouri.edu

Sellers, B. A., Cordes, J. C., Smeda, R. J., and Johnson, W. G. 2004. Waterhemp control in transgenic and conventional corn varieties. Online. Crop Management doi:10.1094/CM-2004-1208-01-RS.

Abstract

Common waterhemp has become a major problem in the western corn belt. Recent surveys in Missouri indicate that waterhemp is the number one problem weed in corn production. To determine the most effective herbicide combinations and application timings in transgenic and conventional corn varieties for waterhemp control, experiments were conducted at three locations over two years. In general, all herbicide strategies provided excellent waterhemp control within five weeks after the last herbicide application. In Roundup Ready, Liberty Link, and Clearfield herbicide programs, waterhemp density in the middle of August was at least 1.8-fold lower using a preemergence application of atrazine plus acetochlor followed by the appropriate postemergence  herbicide strategy for each program in all site-years, except in one instance. With conventional corn varieties, waterhemp densities were similar among herbicide programs. In only one site-year, corn yields in the late-postemergence herbicide strategy were at least 10% lower than the other three herbicide strategies in transgenic corn. In conventional corn, the 1-X preemergence followed by mid-postemergence applications of herbicides resulted in 14% greater grain yields, but only under irrigated environments. These data suggest that waterhemp can be controlled using various approaches in both transgenic and conventional corn varieties.

Introduction

Common waterhemp (Amaranthus rudis Sauer), hereafter referred to as waterhemp, is a dioecious annual, and field grown female plants are capable of producing as many as 290,000 seeds per plant (16). It was also considered to be the most problematic weed species in Missouri corn fields during the 1998 and 2000 growing seasons (10) (Fig. 1). Several factors have contributed to waterhemp becoming problematic including the potential to produce large numbers of seed, genetic diversity, extended emergence period, changes in agronomic practices, and herbicide resistance.

Fig. 1. Common waterhemp quickly becomes established in corn fields where no preemergence herbicides have been applied. Photo by Brent Sellers.

Herbicide usage has changed since the introduction of postemergence (POST) herbicides. In the early 1970s and 1980s, full rates of soil-applied herbicides were applied to provide season-long weed control (18), but the introduction of POST  herbicides made weed control more flexible. Currently, soil-applied herbicides are applied at lower rates due to environmental concerns, which reduces season-long weed control in many instances. Since waterhemp emergence occurs over a large period of the growing season (7), POST herbicide applications may be necessary. Additionally, seed production of those weeds that are not controlled can increase the waterhemp seedbank in the soil even under shading from a crop canopy (17). Therefore, it is likely that producers need to adopt a total herbicide program including both preemergence (PRE) and POST herbicides.

Conservation tillage practices have increased over the last decade creating an environment conducive to small-seeded species (1). Small-seeded weeds such as green foxtail (Setaria viridis) and redroot pigweed (Amaranthus retroflexus) are more difficult to control in chisel- and no-tillage than in conventional tillage production systems (3). Since waterhemp seeds are similar in size to redroot pigweed, waterhemp seeds are likely to remain near the soil surface in conservation tillage systems. In addition, conventional tillage practices evenly disperse seeds throughout the upper 12 inches of the soil profile (4,19). Studies have shown that waterhemp density is higher in plots under conservation tillage than in those under conventional tillage (6,11). Since cultivation is reduced in conservation tillage practices, reliance upon herbicides for weed control has increased (2,5).

Waterhemp populations respond differently to herbicides (12). This variable response could be due to resistance to a particular herbicide or to differing levels of susceptibility due to extreme genetic variability among biotypes (12). Currently, several waterhemp biotypes are resistant to herbicides that inhibit acetolactate synthase (sulfonylureas, imidazolinones, and triazolopyrimidines), protoporphyrinogen oxidase (diphenylethers), and photosynthesis (triazines), with some suggesting resistance to enolpyruvyl-shikimate-phosphate synthase (glycines) (8,14,20). Since waterhemp can emerge over a long period of the growing season, a single herbicide may not provide adequate control of waterhemp. Therefore, other herbicides or herbicide combinations must be considered.

The introduction of transgenic crops provided producers with additional options for controlling waterhemp. Roundup Ready, Liberty Link, and Clearfield corn varieties are resistant to glyphosate (Roundup, etc.), glufosinate (Liberty), and imazethapyr + imazapyr (Lightning) herbicides, respectively. Such herbicide programs are options for weed control in addition to those herbicides currently used for weed control in conventional (non-transgenic) corn varieties. However, it is necessary to identify the most effective herbicide combinations and application timings to control waterhemp in these herbicide programs. The objective of these field studies was to determine the most effective herbicide combination and application timing to control common waterhemp using Roundup Ready, Liberty Link, and Clearfield corn varieties. In a separate experiment, the same objective was evaluated using a conventional corn variety.

Site Descriptions and General Methods

All field experiments were conducted in 2001 and 2002 on University of Missouri research farms located in Columbia (central), Novelty (northeast), and Albany (northwest), Missouri. The Columbia location was on a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs) with 2.8% organic matter, 10% sand, 58% silt, 32% clay, and pH 6.5. The Novelty location was on a Putnam silt loam (fine, smectitic, mesic Vertic Albaqualfs) with 2.7% organic matter, 14% sand, 54% silt, 32% clay, and pH 6.6. The Albany location was on a Grundy silt loam (fine, montmorillonitic, mesic Aquic Argiudolls) with 3.2% organic matter, 18% sand, 60% silt, 22% clay, and pH 5.5. Corn was planted in 30-inch rows and seeded at a population of 27,700 seeds per acre at a depth of 1.5 inches in late April or early May. Nitrogen was applied at a rate of 160 lb of N per acre two weeks prior to planting. Experimental plot size was 10 feet wide by 35 feet long and all experiments were replicated 4 times. Each experimental site was in a corn/soybean rotation, chisel plowed in the fall, and field cultivated at planting. All herbicides were applied with a CO₂-pressurized backpack sprayer equipped with XR8002 flat fan nozzles calibrated to deliver 15 gal/acre at 20 psi. Crop injury and weed control ratings were collected at 2 and 5 weeks after the last herbicide application. Visual weed control was rated on a scale from 0 to 100% where 0 represents no crop injury or weed control and 100 represents complete death of the crop or weed. Late-season waterhemp control was determined by recording density for the entire length of the plot between the center two rows in the middle of August. The center two rows were mechanically threshed with a combine and yield was adjusted to 15.5% moisture prior to statistical analysis.

Common Waterhemp Management in Herbicide-Resistant Corn

The experimental design of this study was a split-plot with herbicide program (transgenic corn hybrid) as the main plot and herbicide strategy as the sub-plot. Three corn hybrids, ‘Asgrow RX740 RR’ (Roundup Ready), ‘Pioneer 33G28 LL’ (Liberty Link), and ‘Pioneer 34B28 CL’ (Clearfield), were used for Roundup Ready, Liberty Link, and Clearfield herbicide programs, respectively. Four herbicide strategies, a hand-weeded check and a weedy check were evaluated for each corn variety as shown in Table 1. Applications applied mid-postemergence (MPOST) and late postemergence (LPOST) were applied to corn that was at the V3 to V4 and V5 to V6 growth stages, respectively.

Common Waterhemp Management in Conventional Corn

Corn ‘Pioneer 33G26’ was planted at each location as described previously. The experimental design was a randomized complete block. Herbicide treatments are shown in Table 2. MPOST treatments were applied to 5-inch weeds at the V3 to V4 corn growth stage.

Table 2. Herbicide programs for conventional corn. A weedy check
was also included for comparison of all treatments.

 ^x Herbicides trades names include: atrazine, Aatrex Nine-O;
isoxaflutole, Balance Pro; simazine, Princep Caliber 90; mesotrione,
Callisto; flumetsulam & clopyralid, Hornet; dicamba & atrazine,
Marksman; dicamba & diflufenzopyr, Distinct; carfentrazone, Aim;
atrazine & nicosulfuron & rimsulfuron; Basis Gold; dicamba &
diflufenzopyr & nicosulfuron, Celebrity Plus; dicamba & primisulfuron, NorthStar.

 ^y 1-X PRE + refers to a full rate of atrazine & metolachlor (1.85 &
1.44 lb ai/acre; Bicep II Magnum) in addition to the herbicides in
the column; 1-X PRE fb MPOST refers to a full rate of atrazine &
metolachlor followed by a mid-postemergence application of the
herbicides listed in the column; 1/3-X PRE fb MPOST refers to a
1/3-X rate of atrazine & metolachlor (0.62 & 0.48 lb ai/acre) followed
by a mid-postemergence application of the herbicides listed in the
column.

Statistical Analysis

Each location and year was designated as a site-year for statistical analysis due to a significant location by year interaction. Data for the herbicide-resistant trials were analyzed using ANOVA based on a split-plot design with herbicide program as the main plot and herbicide strategy as the subplot. The conventional trials were analyzed as randomized complete blocks. Weed control, weed density, and grain yield data were tested for homogeneity of variances (13) and data were combined when no significant interactions were present.

Results from the conventional corn variety experiments were inconclusive as waterhemp control and grain yield were not statistically different among herbicide treatments. Therefore, treatments were separated into three herbicide programs: (i) PRE only; (ii) 1-X PRE followed by MPOST; and (iii) 1/3-X PRE followed by MPOST, plus an untreated check to determine if differences in waterhemp control and grain yield could be detected. Contrast statements were performed to determine if differences existed among herbicide programs and between the programs and the untreated check.

Waterhemp Management in Herbicide-Resistant Corn

Visual waterhemp control was similar at two and five weeks after treatment (data not shown). Therefore, only visual ratings recorded at 5 weeks after treatment will be discussed. Herbicide treatments applied to Roundup Ready and Liberty Link corn provided greater than 85% control of waterhemp at all locations except Novelty in 2002 where waterhemp control was lower than 65% in plots treated with LPOST applications and the PRE application of atrazine (PRE at/) strategy (data not shown). Lightning applied to Clearfield corn provided greater than 80% waterhemp control, except at Novelty in 2002 where LPOST, PRE at/, and PRE application of isoxaflutole (PRE if/) strategies provided 69, 0, and 25% waterhemp control, respectively, and at Albany in 2001 where the PRE at/ strategy provided 0% waterhemp control. The PRE application of atrazine plus acetochlor (PRE at + ac/) strategy provided greater than 95% control with all herbicide programs at all locations investigated. Previous research has shown that waterhemp control ranged from 35 to 96% when atrazine plus acetochlor was applied PRE in no-tillage corn using the same herbicide programs (9). Similarly, Schuster and Smeda (15) found that acetochlor plus atrazine PRE followed by glyphosate POST provided greater than 95% waterhemp control five weeks after treatment (15).

Overall, waterhemp densities in August were generally lower in the PRE at + ac/ strategy for each herbicide program in all site-years (Table 3). Waterhemp densities were similar among herbicide programs in 2001 at Columbia, but the PRE at/ herbicide strategy had at least 1.7-fold more waterhemp than the other herbicide strategies as only herbicide strategy was significant for this site-year. In 2002 at Columbia, there were no differences in waterhemp density among herbicide programs or herbicide strategies. At Novelty in 2001, waterhemp densities were similar among strategies for the Liberty Link and Clearfield herbicide programs, but were at least 2.6-fold lower than strategies in the Roundup Ready program. In 2002 at Novelty, waterhemp densities were similar among strategies in Roundup Ready corn. However, the LPOST strategy in Liberty Link corn and the PRE at/ and PRE if/ strategies in Clearfield corn had at least 3-fold higher waterhemp densities than the other strategies for each herbicide program. Waterhemp densities were highest in the untreated check at the Albany location in 2001 with as many as 683 plants per 88 square feet. Herbicide strategies reduced waterhemp densities in the Roundup Ready and Liberty Link programs, but not with the PRE at/ strategy in the Clearfield program in this site-year. In 2002 at Albany, all herbicide strategies reduced waterhemp densities compared to the untreated check.

Grain yields were different among site-years. However, there were no differences among strategies within herbicide programs or among herbicide programs in any site-year (Table 4). However, the main plot effect of variety was significant in three of the six site-years. In 2001 at Columbia, grain yield of the Clearfield variety was 13% lower than yields from the Roundup Ready and Liberty Link corn varieties. In contrast, Liberty Link corn yield was 8% higher than that of the Roundup Ready and Clearfield corn varieties at Novelty in 2002. At Albany in 2002, corn yields of Liberty Link and Clearfield corn varieties were at least 16% lower than that of the Roundup Ready variety. Herbicide strategy was the only significant factor at Novelty in 2002 where corn yields in the LPOST strategy were at least 10% lower than the other herbicide strategies employed in this experiment. At Albany in 2001 and at Columbia in 2002, there were no statistical differences among corn varieties or herbicide strategies. These data indicate that all herbicide strategies for each herbicide program provided sufficient waterhemp control to protect corn yields in most cases, but LPOST herbicide strategies may result in reduced grain yields under certain environments.

Waterhemp Management in Conventional Corn

Waterhemp control five weeks after the MPOST application was greater than 85% for all herbicide programs in all site-years (data not shown). The only site-year in which waterhemp control was less than 90% was at Albany in 2001 in plots treated with the 1/3-X PRE followed by some MPOST herbicide program. In all other site-years there were no significant differences between the herbicide programs investigated, indicating that all herbicide treatments provided good control of waterhemp. This is in contrast to previous research in no-tillage corn which suggested that two-pass weed control programs provide the most consistent waterhemp control (9,15). Therefore, our data suggest that tillage can be another tool for waterhemp control in addition to herbicides.

Waterhemp densities in treated plots in August were less than four plants per 88 square feet at Columbia in 2001 and 2002, Novelty in 2002, and Albany in 2002 (Table 5). At Columbia in 2002, waterhemp densities were at least 1.5-fold higher in the 1-X PRE followed by a MPOST program than in the PRE only program or the untreated check. Although this result was significant, 3 plants per plot likely did not impact crop yields. At Novelty in 2001, waterhemp densities were similar among herbicide programs, but densities in the untreated check were significantly greater than either of the two programs where MPOST herbicides were applied. At Albany in 2001 and 2002, waterhemp densities were similar among herbicide programs, with all programs lower than the untreated check.

Table 5. Average waterhemp density (per 88 ft²) recorded in August in herbicide programs for conventional corn.

Abbreviations: PRE = preemergence; MPOST = mid-postemergence; fb = followed by. Means followed by the same letter are not significantly different at P = 0.05 using contrast statements.

Weather likely impacted the differences in densities observed between site-years. The Columbia location was irrigated as needed in both years, whereas other sites relied solely on rainfall (data not shown). Drought was prevalent throughout Missouri in 2002, which may explain the relatively low waterhemp densities at Novelty and Albany in 2002. At Columbia, low waterhemp density in August was likely the result of rapid and full corn canopy closure as shading has been shown to reduce waterhemp germination, growth, and reproduction (15).

Corn yield varied among site-years (Table 6) and was likely due to differing environmental conditions. Under the irrigated environments at Columbia in 2001 and 2002, the 1-X PRE fb MPOST herbicide program had the highest yield of any herbicide program investigated. It is likely that this program prevented weeds other than waterhemp from infesting individual plots. In all other site-years there were no differences in crop yield among herbicide programs, suggesting that a herbicide program consisting of a full rate of a PRE herbicide followed by some MPOST herbicide(s) should be implemented to reduce weed competition and increase crop yield under irrigated environments. In contrast, in sites relying solely on rainfall, any of the three herbicide programs could be implemented to achieve adequate waterhemp control as well as grain yield.

Table 6. Average grain yield (bu/acre) among herbicide programs in conventional corn.

Abbreviations: PRE = preemergence; MPOST = mid-postemergence; fb = followed by. Means followed by the same letter are not significantly different at P = 0.05 using contrast statements.

Summary

Although waterhemp is and will likely continue to be a problematic weed in the western corn belt, many options for waterhemp control are currently available. From this research, we conclude the following:

• Herbicide programs for transgenic and conventional corn varieties can provide excellent waterhemp control.

• Herbicide programs should contain a PRE herbicide to minimize the potential for yield loss as LPOST applications without a PRE herbicide can result in early-season weed competition with the crop and reduce grain yield.

• Herbicide strategies in transgenic corn with atrazine plus acetochlor applied PRE followed by glufosinate, glyphosate or imazethapyr & imazapyr MPOST should provided excellent waterhemp control under conventional tillage practices.

• Waterhemp control and late-season waterhemp densities are much less variable when a herbicide program includes a PRE herbicide containing atrazine plus acetochlor or metolachlor.

Acknowledgments

This project was funded, in part, by the University of Missouri Mission Enhancement Program Water Quality Initiative.

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