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© 2009 Plant Management Network. Evaluating Postemergence Herbicides for Relative Corn Safety Mark J. VanGessel, Professor and Extension Specialist, Quintin R. Johnson, Extension Associate, and Barbara A. Scott, Research Associate, Department of Plant and Soil Sciences, University of Delaware Research and Education Center, 16483 County Seat Highway, Georgetown, DE 19947 Corresponding author: Mark J. VanGessel. mjv@udel.edu VanGessel, M. J., Johnson, Q. R., and Scott, B. A. 2009. Evaluating postemergence herbicides for relative corn safety. Online. Crop Management doi:10.1094/CM-2009-0806-01-RS. Abstract When determining the need for a postemergence herbicide, growers consider many factors including yield loss due to competition from existing weeds, effectiveness of various herbicide options, timing of application, as well as crop safety. Few trials have been conducted that compare corn injury and yield loss due to various active ingredients. Injury and potential yield loss from various postemergence herbicides were evaluated under weed-free conditions. At labeled use rates, chlorosis and stunting were generally at acceptable levels (> 11%) and did not impact yield. In separate experiments with twice the labeled use rate, stunting was the most pronounced symptom and lasted longer than twisting or chlorosis. Yield was reduced with foramsulfuron, foramsulfuron plus iodosulfuron, primisulfuron plus dicamba, and dicamba plus atrazine compared to the untreated control. Predicting yield loss based on visual symptoms was not reliable. In addition, corn responded differently to various herbicide mixtures containing similar active ingredients. Knowledge of herbicides that have a higher risk of injury will allow farmers to make more informed decisions on postemergence herbicides, particularly when conditions are favorable for corn injury. Introduction Corn (Zea mays L.) needs a weed-free period to eliminate yield loss due to weed interference. This weed-free period is difficult to define since it is impacted by many factors. A study across 35 sites in the North Central region found that this time-period coincides with no more than 23 days after corn planting and when corn was at a maximum of four-collar stage (3). Corn growers often need a postemergence herbicide application to achieve weed control throughout this weed-free period. When selecting a postemergence herbicide corn injury (or response) due to herbicide and/or additives is an issue. While many herbicide modes of action are contact and the crop quickly recovers, others are absorbed by the corn and are translocated throughout the plant. Symptoms from translocated herbicides can be observed for weeks after application. Growers are concerned about the impact of herbicide injury on corn and often inquire about relative crop safety. However, most trials examining crop safety usually include only one active ingredient or a limited range of herbicides (1,2,4). There is a lack of trials examining multiple herbicide modes of action for crop safety and the impact on yield to make comparisons among various active ingredients. Furthermore, most field trials evaluating a specific active ingredient examine crop injury as well as weed control (5,6,7,8). However, when herbicide injury is observed, it is difficult to determine if it has an impact on yield due to a number of issues such as treatment differences in weed control or differences in timing of application. In order to know if herbicide injury does impact yield, the study needs to be done under weed-free conditions to avoid confounding. Thus, the objective of these studies was to determine if crop response with many of the common postemergence herbicides has an impact on yield. In addition, since most of the postemergence herbicides have both foliar and soil activity, response on coarse-textured soils is necessary to make recommendations for coastal plains soils. Herbicides and Rates Examined and Design Study I. Two field trials were conducted in 2002 and one in 2003 at the University of Delaware’s Research and Education Center (UD-REC) located near Georgetown, DE. A fourth trial was established in 2002 near Middletown, DE. All locations were conventionally tilled each year with chisel plowing, discing, and field cultivation in the spring. Plots were 25 ft long and 10 ft wide and rows were 30 inches apart (4 rows per plot). The soils for the studies at UD-REC were loamy sand or sandy loam (ranging from 79 to 81% sand), with pH range of 5.6 to 6.6 and organic matter ranged from 1.0 to 1.5%. The site at Middletown was a silt loam, with 41% sand, soil pH of 6.1, and1.8% organic matter. One field in 2002 at UD-REC was irrigated and it was seeded on May 13 at 28,000 seeds/acre of Doebler’s 82XPRR. None of the other locations were irrigated. The site at Middletown was planted on May 15 with 22,000 seeds/acre of DeKalb 551, and the second site at UD-REC was at planted May 24 with 19,000 seeds/acre of DeKalb 551. At UD-REC in 2003, 19,000 seeds/acre of Doebler’s 797RYG was planted April 30. Starter fertilizer was used and tefluthrin at 5.5 lbs/acre was applied in-furrow. Supplemental nitrogen of 130 to 190 lbs/acre was applied at side-dress, depending on yield expectations. The entire trial was treated with s-metolachlor at 1.0 lb ai/acre plus 1.2 lb ai/acre atrazine (Bicep II Magnum) within 48 h of planting. Glyphosate was applied postemergence at 0.8 lb ae/acre to keep the experimental area weed-free. Treatments were the labeled rates of nicosulfuron (Accent), foramsulfuron (Option), prosulfuron plus primisulfuron (Exceed), nicosulfuron plus rimsulfuron plus atrazine (Basis Gold), nicosulfuron plus rimsulfuron plus atrazine plus dicamba (Basis Gold plus Banvel), diflufenzopyr plus dicamba (Distinct), primisulfuron plus dicamba (NorthStar), flumetsulam plus clopyralid (Hornet), dicamba plus atrazine (Marksman), 2,4-D plus atrazine (Shotgun), mesotrione plus atrazine (Callisto plus atrazine), and an untreated control. Herbicide rates and adjuvants are given in Table 1. Herbicide treatments were applied to corn at 5-collar stage and 18 inches tall for the irrigated site at UD-REC in 2002, 4-collar stage and 10 inches tall at Middletown, and 3-collar stage and 12 inches tall at the non-irrigated site at UD-REC in 2002. The UD-REC in 2003 was treated when the corn was at 6-collar stage and 14 inches tall. Treatments were applied with a tractor-mounted compressed air sprayer in a spray volume of 25 g/acre at 29 lb/inch², traveling 3 mph, equipped with 11003XR (Teejet) nozzles. All treatments were repeated four times and the trial was arranged as a randomized complete block design. Corn response was rated at 3, 7, 14, and 21 days after treatment (DAT). Corn was rated for percentage of tissue with chlorosis or whitening, percent of corn tissue with leaf burn, amount and severity of twisted or leaning corn, and amount of corn biomass reduction. The two center rows were harvested with a combine at physiological maturity at all locations. Yields were corrected to 15.5% moisture content. Treatments were checked for normality and appropriate transformations were used if needed; arcsine square root for stunting, square root for twisting, and natural log for leaf burn, chlorosis, and whitening. Data were combined over years if treatment by year interaction was not significant. Treatment means were separated using LSD test at the 5% level of probability. Untransformed data are presented in the tables. Study II. Herbicide rates were increased after the 2003 season due to lack of yield response (Table 2). Twice the labeled herbicide rate was used. Due to changes from the manufacturers, the formulation of nicosulfuron plus rimsulfuron plus atrazine was switched (Steadfast ATZ replaced Basis Gold), and foramsulfuron plus iodosulfuron (Equip) replaced nicosulfuron plus rimsulfuron plus atrazine plus dicamba (Basis Gold plus Banvel). The study was repeated annually until 2008 at UD-REC and production practices were the same unless noted. From 2004 until 2008, all sites were irrigated, and the soil was sandy loam each year, with a range of soil pH from 5.9 to 6.4 and soil organic matter from 1.0 to 1.7%. Seeding rate was 28,000 seeds/acre, and supplemental nitrogen of 146 to 163 lbs/acre was applied at side-dress, with a total nitrogen rate of 163 to 205 lbs/acre. Glyphosate-resistant hybrids TA 6752, TA 7762, TA 775-02, DKC66-23, and DKC6424 were planted in 2004 through 2008, respectively. Corn was planted the last week of April or first week of May. Herbicide treatments were applied to corn at the four to five collar stages, which coincided with 10 inch tall corn (except 2004 when it was 14 inches tall). The entire trial was kept weed-free with either multiple applications of glyphosate (2004 to 2006) or with s-metolachlor at 0.5 lb/acre plus 0.6 lb ai/acre atrazine (Bicep II Magnum) applied within 24 h of planting, followed by glyphosate applied postemergence at 0.8 lb ae/acre (2007 and 2008). Treatments were arranged as a randomized complete block with three replications. Data collection and analysis remained the same as Study I. Corn Responses and Yield Due to Postemergence Herbicide Treatments Study I. Chlorosis was only observed in 2003 and was highest for foramsulfuron, prosulfuron plus primisulfuron, nicosulfuron plus rimsulfuron plus atrazine, primisulfuron plus dicamba, and flumetsulam plus clopyralid, averaging 10% (data not presented). Stunting (7 DAT) at UD-REC in 2002 (both sites) was highest with foramsulfuron, averaging 17% (Table 1). All other treatments averaged 10% stunting or less. At Middletown and UD-REC in 2003, treatments were not significantly different from one another, < 7% stunting. At 14 DAT, significance was only observed in 2003, but all treatments were less than 10% stunting (data not presented). No yield differences were observed in this study (Table 1). At labeled use rates, none of the herbicides and herbicide combinations evaluated reduced corn yield. Table 1. Corn stunting and yield due to various postemergence herbicides applied at labeled use rates. Two field trials were conducted in 2002 and one in 2003 at the University of Delaware’s Research and Education Center, and a fourth trial was established in 2002 near Middletown, DE; these sites are designated site I, site II, site III, and site IV, respectively.
* Means within a column followed by the same letter are not significantly different at P ≤ 0.05 according to Fisher’s Protected LSD test. Study II. Since corn response and yield with labeled use rates did not allow for observation of differences among the various herbicides, the rates were increased to twice the label rate. Corn response varied depending on the herbicide applied. Leaf burn, or necrosis, was only observed in 2004, and the highest level was < 5% (data not presented). Twisting or bending of the corn was observed only with treatments containing dicamba or 2,4-D (Table 2). The treatment of dicamba plus atrazine resulted in the greatest amount of twisting and bending, and this treatment had a higher rate of dicamba than dicamba plus diflufenzopyr or dicamba plus primisulfuron. The amount of twisting ranged from 10 to 33% at 14 DAT (data not presented). Table 2. Twisting at 7 days after treatment and chlorosis or whitening at 3 days after treatment caused by various postemergence herbicides applied at twice the labeled use rates at University of Delaware’s Research and Education Center from 2004 to 2008.
* Means within a column followed by the same letter are not significantly different at P ≤ 0.05 according to Fisher’s Protected LSD test. ** Chlorosis and whitening was not recorded at 3 DAT in 2004. Chlorosis was observed at 3 and 7 DAT with all treatments containing an ALS-inhibiting herbicide. The 3 DAT data, pooled for 2005, 2006, and 2008, ranged from 11 to 19% chlorosis (mean of 16%) for ALS-inhibiting herbicides (Table 2). In 2007 at 3 DAT, there was 10 to 23% chlorosis with nicosulfuron plus rimsulfuron plus atrazine and prosulfuron plus primisulfuron having the highest percent chlorosis (Table 2). By 7 DAT, nicosulfuron alone and flumetsulam plus clopyralid exhibited only 2% chlorosis (data not presented). All other ALS-inhibiting herbicides had significantly higher levels of chlorosis (13 to 16%). By 14 DAT, chlorosis was not observed at more than 6% for any treatment (data not presented). Mesotrione plus atrazine caused temporary whitening only in 2007. Stunting was the symptom that most pronounced and lasting longer than twisting or chlorosis (Table 3). In addition, it was the most inconsistent symptom from year to year. At 7 DAT, none of the years could be combined for analysis. Nicosulfuron plus rimsulfuron plus atrazine and foramsulfuron plus iodosulfuron, two treatments that often exhibited the highest level of stunting, had only 2 and 6% reduction in biomass, respectively, in 2004 (data not presented). However, in 2006, the same treatments were reduced by at least 32%. Table 3. Corn stunting at 14 days after treatment and yield when treated with various postemergence herbicides applied at twice the labeled use rates at University of Delaware’s Research and Education Center from 2004 to 2008. Yield data is combined over the 5 years due to lack of year by treatments interaction.
* Means within a column followed by the same letter are not significantly different at P ≤ 0.05 according to Fisher’s Protected LSD test. Corn stunting data at 14 DAT could be pooled for three years, and there were some clear trends across the years (Table 3). First, stunting was not as severe as at 7 DAT, but still significant for most treatments. There was less than 10% stunting observed with mesotrione plus atrazine, atrazine plus 2,4-D, and nicosulfuron. Flumetsulam plus clopyralid was not consistent for stunting across the years, ranging from 0 to 20%. In 2006, stunting was still significant for seven treatments at 32 DAT, ranging from 7 to 13% reduction. Yield was reduced for foramsulfuron, foramsulfuron plus iodosulfuron, primisulfuron plus dicamba, and dicamba plus atrazine compared to the untreated control (Table 3). Yield reduction for these four treatments ranged from 8 to 11%. Symptoms were not a reliable predictor of yield loss. For example, nicosulfuron plus rimsulfuron plus atrazine had similar levels of chlorosis and stunting as foramsulfuron plus iodosulfuron, however, yield with nicosulfuron plus rimsulfuron plus atrazine was greater than foramsulfuron plus iodosulfuron; and nicosulfuron plus rimsulfuron plus atrazine was not different than the untreated control. Likewise, rates of specific active ingredients were not reliable either. For example, dicamba at 0.4 lb ai/acre did not reduce yields when it was in prepackaged mixture with diflufenzopyr, but it did reduced yields at this rate when in a pre-packaged mixture with primisulfuron. Furthermore, primisulfuron plus prosulfuron did not reduce yield. Predicting potential yield loss based on visual symptoms is not reliable. In addition, corn responds differently to various herbicide mixtures containing similar active ingredients. While labeled use rates did not cause yield loss in this trial, being aware of those herbicides that have a higher risk of injury will allow farmers to make more informed decisions on postemergence herbicides when conditions are favorable for corn injury. Literature Cited 1. Bunting, J. A., Sprague, C. L., and Riechers, D. E. 2004. Corn tolerance as affected by the timing of foramsulfuron applications. Weed Technol. 18:757-762. 2. Doohan, D. J., Ivany, J. A., White, R. P., and Thomas, W. 1998. Tolerance of early maturing corn (Zea mays) hybrids to DPX-79406. Weed Technol. 12:41-46. 3. Gower, S. A., Loux, M. M., Cardina, J., Harrison, S. K., Sprankle, P. L., Probst, N. J., Bauman, T. T., Bugg, W., Curran, W. S., Currie, R. S., Harvey, R. G., Johnson, W. G., Kells, J. J., Owen, M. D. K., Regehr, D. L., Slack, C. H., Spaur, M., Sprague, C. L., VanGessel, M., and Young, B. G. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-year multistate study. Weed Technol. 17:821-828 4. Green, J. M. 1998. Differential tolerance of corn (Zea mays) inbreds to four sulfonylurea herbicides and bentazon. Weed Technol. 12:474-477. 6. Mitra, S., and Bhowmik, P. C. 1999. Effect of growth stages on quackgrass (Elytrigia repens) control in corn (Zea mays) with rimsulfuron. Weed Technol. 13:37-42. 7. Nurse, R. E., Hamill, A. S., Swanton, C. J., Tardif, F. J., and Sikkema, P. H. 2007. Weed control and yield response to foramsulfuron in corn. Weed Technol. 21:453-458. 8. Sikkema, P. H., Knezevic, S. Z., Hamill, A. S., Tardif, F. J., Chandler, K., and Swanton C. J. 1999. Biologically effective dose and selectivity of SAN 1269H (BAS 662H) for weed control in corn (Zea mays). Weed Technol. 13:283-289. |
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