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© 2003 Plant Management Network.
Accepted for publication 30 September 2003. Published 11 November 2003.

Atrazine May Overcome the Time-of-Day Effect on Liberty Efficacy

Brent A. Sellers, Post Doctoral Fellow, and Reid J. Smeda, Assistant 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 A. Sellers. SellersB@missouri.edu

Sellers, B. A., Smeda, R. J., Johnson, W. G. 2003. Atrazine may overcome the time-of-day effect on Liberty efficacy. Online. Crop Management doi:10.1094/CM-2003-1111-01-RS.

Abstract

Liberty is a common postemergence herbicide used for weed control in Liberty Link corn. However, Liberty applied within two hours of sundown fails to control many weeds like velvetleaf. The impact of this time-of-day (TOD) effect on corn grain yield is unknown. Therefore, field experiments were conducted near Columbia and Novelty, MO in 2000, 2001, and 2002 to determine if the TOD effect impacts corn grain yield and whether atrazine added to Liberty could overcome the TOD effect. All plots were treated preemergence with Dual II Magnum followed by a postemergence application of Liberty (0.31 or 0.44 lb a.i./acre) or Liberty plus atrazine (0.25 + 0.83, 0.31 + 0.83, 0.31 + 1.03, or 0.44 + 1.03 lb/acre) at 6, 2, and 0 hours before sundown (HBS). Liberty or Liberty plus atrazine applied to velvetleaf at sundown resulted in reduced weed control when compared to 6 HBS in four of the six site-years. Other weed species exhibited a similar visual response to postemergence applications of Liberty or Liberty plus atrazine. In addition, when a visual TOD effect was observed, total weed biomass following postemergence applications at sundown versus those at 6 and 2 HBS was up to 11-fold greater 3 weeks after treatment. The addition of atrazine increased velvetleaf control when a TOD effect was observed in only one instance (Columbia 2002). When a TOD effect was observed, grain yield was up to 56% lower when Liberty at 0.31 lb/acre was applied at sundown compared to 6 and 2 HBS. In the three site-years that a TOD effect was observed, the addition of atrazine to at least 0.31 lb/acre of Liberty resulted in little yield loss compared to the weed-free control. This indicates that those injured weeds that remained after treatment did not influence crop yield. However, it remains inadvisable for producers to apply Liberty with or without atrazine within two hours of sundown as described by the Liberty label.

Introduction

Liberty is a foliar-applied herbicide used in the non-selective removal of vegetation in horticultural crops, in no-tillage prior to crop establishment, and postemergence in Liberty Link crops. Liberty was applied postemergence to 6% of U.S. corn acreage in 2001 (22). Liberty inhibits glutamine synthetase (E.C. 6.3.1.2), resulting in a buildup of phytotoxic ammonium and a reduction in amino acid pools (14,23). Inhibition of glutamine synthetase ultimately results in reduced photorespiration and photosynthesis (18,23), followed by cell membrane disruption and necrosis (2).

The introduction of Liberty Link crops provided an alternative use for Liberty in agriculture (4,8,10,12). Liberty results in good to excellent control of many agronomically important grass and broadleaf weed species (4,5,7,8,10,11,12,17,20,21). However, Liberty lacks residual soil activity and additional germination of weed species throughout the growing season can result. Therefore, sequential postemergence Liberty applications or the addition of a residual herbicide are needed for optimum weed control and crop yield (5,8,10,12). Atrazine added to Liberty appears to have the greatest impact upon weed control, when compared to Liberty alone, in Liberty Link corn (5,10,12).

Liberty efficacy is influenced by a variety of factors including plant species, plant size, and environment (1,6,16,20). Moreover, Liberty activity was reduced when applied near or after sundown, a response generally referred to as a time-of-day (TOD) effect (3,15). Light is essential for optimum Liberty activity (13,19). Plants subjected to a 24-hour dark period following treatment accumulated less ammonium than plants subjected to light immediately after application (13). In addition, plant biomass was also higher for plants subjected to a 24-hour dark period following Liberty applications compared to plants subjected to light conditions following application of Liberty. Furthermore, Beyers (3) reported that field applications of Liberty at 9:00 pm to velvetleaf (Abutilon theophrasti Medicus), giant foxtail (Setaria faberi Herrm.), common waterhemp (Amaranthus rudis Sauer), yellow foxtail (Setaria glauca (L.) Beauv.), and morningglory (Ipomoea spp.) spp. resulted in higher plant biomass compared to Liberty applied during daytime hours. Martinson et al. (15) also found a TOD effect for Liberty on several weed species including common lambsquarters (Chenopodium album L.), giant foxtail, common ragweed (Ambrosia artemisiifolia L.), and velvetleaf.

Despite identification of a TOD effect, the influence of the remaining weed population on competition with crops and reduction in crop yield has not been measured. Therefore, experiments were conducted to evaluate the effect of TOD of Liberty application on weed control and corn yield. A second objective was to determine if the addition of atrazine could overcome the TOD effect.

Field Experiments

Field experiments were conducted at University of Missouri research centers near Columbia and Novelty in 2000, 2001, and 2002. The soil type at the Columbia location was a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs) soil with a pH of 6.0 and 3% organic matter. At Novelty, the soil type was a Putnam silt loam (fine, smectitic, mesic Vertic Albaqualfs) soil with a pH of 6.6 and 3.5% organic matter. Fall tillage was performed using a chisel plow and the seedbed was prepared using a harrow cultivator in the spring. Ammonium nitrate fertilizer was applied at a rate to deliver 180 lb N/acre on each experimental site at planting. Velvetleaf seed was broadcasted over the entire experimental area prior to seedbed preparation. Pioneer 34T14 Liberty Link corn (Zea mays L.) was planted (27,800 seeds per acre) on 21 April and 1 May 2000 at Columbia and Novelty, respectively. Pioneer 34G28 Liberty Link corn was planted (27,800 seeds per acre) on 25 April 2001 and 18 April 2002 at Columbia and on 19 April 2001 and 22 May 2002 at Novelty. In 2001 at Novelty, corn was replanted on 21 June due to poor weed and crop densities throughout the experimental area. At all locations, corn was planted in 30-inch rows at approximately 1.5-inches deep.

All plots (10 × 35 ft), including the non-treated check, were treated with 0.77 lb a.i./acre Dual II Magnum preemergence to suppress small-seeded grass and broadleaf weeds. Herbicide applications were made with a CO²-pressurized backpack sprayer at a ground speed of 3.0 mph. The spray boom was equipped with flat-fan nozzles calibrated to deliver a spray volume of 20 gal/acre at 30 pounds per square inch.

Postemergence herbicide applications included Liberty alone at 0.31 and 0.44 lb/acre, and Liberty + atrazine (0.25 + 0.83 lb/acre and 0.31 + 1.03 lb/acre). These rates were chosen based upon recommendations from the Liberty and the Liberty-atrazine (LibertyATZ) labels. All treatments included ammonium sulfate at 3.4 lb/acre, and crop oil concentrate at 1 qt/acre was added when atrazine (Aatrex Nine-O) was included. These herbicide treatments were applied at 6, 2, and 0 hours before sundown (HBS). In 2001 and 2002, two additional treatments (Liberty + atrazine applied at 0.31 + 0.83 lb/acre and 0.44 + 1.03 lb/acre) were added to determine if increasing either the atrazine or the Liberty rate would overcome the TOD effect. Weed-free and untreated check plots were also included to compare weed control and crop yields. Weed-free plots were maintained by applying Liberty + atrazine at 0.31 + 1.0 lb/acre followed by sequential Liberty applications at 0.31 lb/acre as needed.

Control of velvetleaf and other weeds present at each location was estimated visually 2 and 4 weeks after treatment. Visual estimations ranged from 0 to 100, where 0 indicates no weed control, and 100 indicates complete weed control. Plant biomass was also measured 3 weeks after treatment to evaluate weed control. Late-season weed control was evaluated by measuring plant biomass prior to grain harvest in 2001 and 2002. Plants were harvested from three randomly placed 3.6-ft² quadrats in each plot. Plants were severed at the soil surface, separated by species, counted, and fresh weight determined. Dry weights were recorded after placing samples in a dryer at 100°F for at least 4 days. Corn yield was determined by harvesting the center two rows of each plot using a plot combine. Yields were corrected to 15.5% moisture.

The experimental design was a randomized complete block with a factorial arrangement of herbicide treatment and time of application. Each treatment was replicated four times. For total weed biomass, data for all weed species were summed and are presented as total dry biomass (oz/ft²). For statistical analysis, each site and year was designated as a single environment (site-year). All data were subjected to ANOVA using a general linear model for each site-year. Data were not pooled over site-years because of a significant site-year by treatment interaction. The factorial components were orthogonally partitioned from overall treatments. Check and untreated control treatments were not included in the analysis. Data were pooled over individual factors unless significant at P = 0.05 or individual factors were both significant. Means were separated using Fisher’s protected LSD at P = 0.05.

The TOD Effect and Yield

Velvetleaf was not the predominant weed species in three of the six site-years. Additional weeds were visually evaluated, but data are presented only for those weeds that were present in at least three site-years. In addition, visual weed control was similar at 2 and 4 weeks after treatment. Therefore, only visual control data at 4 weeks after treatment are presented. Weed control ratings were also subjected to arcsine square root transformations, but results were not different from the untransformed data. Therefore, untransformed data are presented.

Velvetleaf. TOD effects were present for velvetleaf in four of six site-years (Table 1). However, at Columbia in 2000 control was > 96%, causing a discussion of a TOD effect to be agronomically irrelevant for this site-year. In the other three site-years, velvetleaf control was up to 85% lower when Liberty at 0.31 or 0.44 lb/acre was applied at sundown compared to 6 HBS. Although atrazine increased overall velvetleaf control in all three site-years, the addition of atrazine removed the TOD effect in only one of these site-years (Columbia in 2002).

Table 1. Mean visual control of velvetleaf 4 weeks after treatment at Columbia and Novelty, MO in 2000, 2001, and 2002.* A rating of 0% indicated no weed control and a rating of 100% indicated complete weed control.

* Abbreviations: atra = atrazine; Lib = Liberty; HBS = hours before sundown.

Leaf angle of velvetleaf may be the greatest cause of the TOD effect (19). Spray interception by velvetleaf plants with -90° leaf angles was approximately 55% less than that intercepted by plants with -10° leaf angles (19). Less spray interception ultimately resulted in a reduction of herbicide activity (19). Although dew is another possibility when examining TOD effects (9), dew was not present in any site-year when herbicides were applied. However, it is possible that dew formed after application, and it may be the reason for reduced weed control at Columbia in 2002 and Novelty in 2000 and 2001. Fausey and Renner (9) determined that when dew was allowed to form on redroot pigweed (Amaranthus retroflexus L.) leaves, flumiclorac efficacy was reduced by as much as 21% compared to plants without dew formation. They attributed the decrease in herbicide efficacy to herbicide loss through runoff, but further stated that dilution of the herbicide on the leaves may have also contributed.

Common cocklebur. Common cocklebur (Xanthium strumarium L.) was present in all three years at Columbia and in 2001 at Novelty. A TOD effect was observed in only two of these site-years (data not shown). Common cocklebur control was at least 66% lower when either Liberty rate was applied at sundown compared to 6 and 2 HBS. The addition of atrazine removed the TOD effects in these site-years when 1.03 lb/acre atrazine was added to Liberty at 0.44 lb/acre.

Total weed biomass. All weed biomass from each treatment were summed over species and expressed as total dry biomass (oz per square ft) (Table 2). Time-of-day effects were present in two of six site-years at 3 weeks after treatment and time of application was the only significant factor in 2000 at Novelty. In 2002 at Columbia, total biomass was at least 11 times greater when Liberty at 0.31 and 0.44 lb/acre was applied at sundown compared to 6 and 2 HBS. The addition of atrazine overcame the TOD effect in this site-year. At Novelty in 2000, total weed biomass 3 weeks after treatment was at least 9-fold greater when herbicides were applied at sundown compared to 6 and 2 HBS. In 2001 at Novelty, total plant biomass was 5 to 20 times greater when herbicide treatments were applied at sundown compared to 6 and 2 HBS. A TOD effect was no longer present in any site-year at harvest.

Table 2. Total weed biomass 3 weeks after treatment and at the end of the growing season prior to harvest at Columbia and Novelty, MO in 2000, 2001 and 2002.

Grain yield. Corn grain yield was variable among site-years (Table 3). However, the observed TOD effects on weed control caused yield reductions in four site-years; time of herbicide application was significant for grain yield at Novelty in 2000 and 2001. At Columbia in 2000, grain yield was, at most, 9% less than that of the weed-free control. Weed control was high in this site-year and adequate rainfall had been received. Therefore, discussion of a TOD effect influencing grain yield for this site-year is irrelevant. Weed competition resulting from the observed TOD effect in Liberty-only treatments caused a reduction in grain yield at Columbia in 2002. Grain yield was reduced by 56 and 18% in plots treated with Liberty at 0.31 and 0.44 lb/acre, respectively, at sundown compared to those treated at least 2 hours before sundown. Yield ranged from 93 to 130% of the weed-free control when atrazine was added to Liberty. This suggests that the addition of atrazine provided sufficient weed control to prevent a yield response. At Novelty in 2000 and 2001, grain yield was up to 6% lower when Liberty plus atrazine was applied at sundown compared to 6 and 2 HBS. This suggests that atrazine was not beneficial in these site-years.

Table 3. Effect of time of day applications on corn yield expressed as a percentage of the weed-free control at Columbia and Novelty, MO in 2000, 2001 and 2002.*

* Abbreviations: atra = atrazine; Lib = Liberty; HBS = hours before sundown.

In general, the addition of 1.03 lb/acre atrazine to Liberty reduced weed competition and grain yield loss even when a TOD effect was observed. In some environments, corn yield has been shown to be more strongly dependent upon grass than broadleaf weed control (5). Since Dual II Magnum effectively controlled grasses in all years and locations (data not shown), it is possible that herbicide activity in each plot was great enough to prevent a yield response.

The results of this study suggest that the TOD effect is quite variable across environments. A TOD effect was observed for velvetleaf and common cocklebur in four of six and two of four site-years, respectively, with regards to visual control. Total biomass, however, revealed that a TOD effect was present 3 weeks after treatment in only three of six site-years. Of these three site-years, applying both Liberty and atrazine at sundown decreased grain yield in two site-years (Novelty 2000 and 2001). In the other site-year (Columbia 2002), the addition of atrazine to at least 0.31 lb/acre Liberty resulted in little yield loss compared to the weed-free control when applied at sundown. This indicates that the addition of atrazine provided sufficient weed control to prevent a yield loss when a TOD effect was observed. In addition, atrazine provided residual weed control, which prevented late-season weed competition with the crop. In contrast, when Liberty was applied alone, weed control was inconsistent and weeds that emerged after application likely impacted grain yields.

This study is consistent with previous research that demonstrated that the addition of atrazine to Liberty increases weed control in Liberty Link corn (5,8,18). Research has shown that environmental conditions affect Liberty efficacy (6,19). It is probable that environmental conditions also influence the TOD effect. However, environmental data in the six site-years investigated (data not shown) did not reveal any clear correlations between the presence of a TOD effect and relative humidity or temperature. Additionally, dew cannot be ruled out as a causative factor for the TOD effect, even though it was not present at the time of application at any experimental site. If dew formation is heavy, it could contribute to herbicide run-off from plant leaves, especially plants that have diurnal leaf movements such as velvetleaf, common cocklebur, and morningglory species.

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