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© 2007 Plant Management Network.
Accepted for publication 5 February 2007. Published 24 July 2007.

Foliar Potassium Fertilizer Sources Affect Weed Control in Soybean with Glyphosate

Kelly A. Nelson, Research Assistant Professor, Division of Plant Sciences, University of Missouri, Novelty 63460; and Peter P. Motavalli, Associate Professor, Department of Soil, Environmental and Atmospheric Sciences, University of Missouri, Columbia 65211

Corresponding author: Kelly A. Nelson. nelsonke@missouri.edu

Nelson, K. A., and Motavalli, P. P. 2007. Foliar potassium fertilizer sources affect weed control in soybean with glyphosate. Online. Crop Management doi:10.1094/CM-2007-0724-01-RS.

Abstract

Field research in 2003 and 2004 evaluated soybean injury, weed control, yield response, and cost-effectiveness of foliar-applied, commercially-available K fertilizers with glyphosate. Fertilizer sources were applied at the highest soluble rate for dry formulations or were used as the carrier with glyphosate. All K-fertilizer sources had less than 10% injury 7 and 14 days after treatment (DAT) except 0-0-30-0 (%N-%P₂O₅-%K₂O-%S). Tank mixtures of 0-0-62-0, 14-0-46-0, 0-0-50-17, and 3-18-18-0 with glyphosate controlled common ragweed, common lambsquarters, common waterhemp, and giant foxtail greater than 85% and had grain yields similar to glyphosate plus diammonium sulfate (DAS). Gross margins when glyphosate was tank-mixed with 0-0-62-0, 14-0-46-0, and 0-0-50-17 ranged from $290 to 300/acre which were similar to glyphosate plus DAS. This research demonstrated that several K fertilizer sources may be combined with glyphosate for cost-effective foliar fertilizer applications while minimizing crop injury and maintaining weed control.

Introduction

The incidence of K deficiency in the Midwestern US has increased in recent years due to reduced K availability in areas with compaction and periodic drought, reduced fertilizer applications for soybean [Glycine max (L.) Merr.] because of low commodity prices, increased corn (Zea mays L.) grain yields in corn-soybean rotations (4), and increased acreage of continuous soybean production in states such as Missouri (12). Soil test K data for Missouri indicated that over 50% of the soil samples tested in the low to medium range for K (5), suggesting that large agricultural areas may be responsive to K fertilization.

Several studies have evaluated response of soybean to foliar fertilizer mixtures applied during early and late stages of soybean development (6,9,17). Soybean response to a foliar application of K₂SO₄ increased grain yield over 10 bu/acre when compared with non-treated or MgSO₄ controls on a low to medium soil test K (16). A large carrier volume was required for optimum foliar K sulfate application rate due to the low solubility of K sulfate which would limit the use of this particular fertilizer source as part of an integrated crop management system.

Over 80% of the soybeans grown in the United States are glyphosate- resistant (7). A single-postemergence application of glyphosate (N-(phosphonomethyl)glycine) has been used as a non-injurious, cost-effective weed management treatment for glyphosate-resistant soybean production in the northcentral US (21). Salts in the spray solution have increased, decreased, or had no effect on control of weeds with glyphosate (13,14,15). Diammonium sulfate (DAS) has been used with glyphosate to reduce hard water antagonism of weed control with glyphosate (14,15). In addition, recent formulations of glyphosate as a K-salt have been commercially-available (10,19). Nalewaja and Matysiak (14) reported that K was one of the least antagonistic cations of glyphosate, but that anions were important for overcoming spray solution salt antagonism of glyphosate. Potassium nitrate, K chloride, and K sulfate were predicted to be the least antagonistic cation-anion complexes in the spray solution with glyphosate (14). Compatible, commercially-available fertilizer tank mixtures with glyphosate could help offset application costs associated with separate foliar fertilizer applications and provide essential nutrients to the crop.

Some research has evaluated crop response to commercial fertilizer tank-mixture treatments with glyphosate; however, limited research has evaluated weed control with these production systems (1,11). Most of this research has focused on N fertilizer and not K fertilizer sources. Similarly, little information is available regarding the compatibility of foliar K sources with glyphosate and the effects of these K fertilizer mixtures with glyphosate on crop injury and weed control. In addition, recent formulations of glyphosate as a K-salt have been commercially-available (10,19). We hypothesized that the addition of K fertilizer to the spray mixture of formulated K-glyphosate would not reduce weed control effectiveness compared to application of the glyphosate alone or with DAS. The objectives of this research were to: (i) determine soybean yield response and salt injury from different foliar-applied K fertilizer sources; (ii) assess the impact of foliar K fertilizer sources on weed control when tank-mixed with a glyphosate-based herbicide; and (iii) evaluate the cost-effectiveness of applying K fertilization with glyphosate-based herbicides for no-till glyphosate-resistant soybean production.

Evaluating Fertilizer Tank Mixtures with Glyphosate

Field experiments were conducted in 2003 and 2004 at the University of Missouri Greenley Research Center near Novelty, MO (40°01’N, 92°11’W) on a Putnam silt loam (fine, smectitic, mesic Vertic Albaqualfs) soil. The soil had a high initial soil test K (370 lb/acre in 2003 and 270 lb/acre in 2004) using a 1 M NH₄OAc extracting solution at pH 7. All plots were 10 by 35 ft with four replications. All treatments were applied with a CO₂-propelled hand-boom calibrated to deliver 15 gal/acre at 18 lb/inch², traveling 2.8 mi/h, and equipped with 8002 flat-fan nozzles (Spray Systems CO., Wheaton, IL) spaced 15 inches apart and 16 inches above the canopy. Commercially-available K fertilizer sources included K carbonate (NA-CHURS/ALPINE Solutions, Marion, OH) (0-0-30-0 as %N-%P₂O₅-%K₂O-%S), K chloride (PCS, Potash Corp. of Saskatchewan, Northbrook, IL) (0-0-62-0), K nitrate (SQM North American Corp., Atlanta, GA) (14-0-46-0), K phosphate + urea (NA-CHURS/ALPINE Solutions, Marion, OH) (3-18-18-0), K sulfate (Great Salt Lake Minerals Corp., Overland Park, KS) (0-0-50-17), K thiosulfate (Tessenderlo Kerley Inc., Phoenix, AZ) (0-0-25-17), and K thiosulfate + urea-triazone (Tessenderlo Kerley Inc., Phoenix, AZ) (5-0-20-13). A DAS treatment was included since it is commonly used as an additive with glyphosate at 0.75 lb ae/acre to reduce the antagonistic effects of hard water on weed control (15). Grain was harvested with a small-plot combine and grain yields were reported with grain moisture content adjusted to 13%.

All data were subjected to analysis of variance using PROC ANOVA (Version 9.1, SAS Institute Inc., Cary, NC). Percent data for injury and weed control were transformed to the arcsine prior to the analysis. The transformation did not affect the conclusions so the original data are presented. All data were combined over years and main effects were presented when there was an absence of interactions. Weed control data were presented separately each year due to slight differences in the primary weed species and population density present each year. Means were separated using Fisher’s Protected LSD at P ≤ 0.05.

Glyphosate-resistant soybean, ‘Asgrow 3701’ (Monsanto, St Louis, MO) were no-till planted on 19 May 2003 and 20 May 2004 in 15-inch rows at 180,000 seeds/acre. A factorial arrangement of treatments was used in a randomized complete block design. Fertilizer source was one factor and was either applied alone to weed-free plots or tank-mixed with glyphosate (Roundup WeatherMAX, Monsanto, St. Louis, MO) and applied to weedy plots.

Since K fertilizer sources vary in solubility and possible compatibility with glyphosate, a compatibility test was conducted prior to applying the treatments to the field experiment to determine the solubility of the K source and pH (Oakton Instruments, Vernon Hills, IL) of the spray solution. Solution pH measurements were replicated twice and combined over years. The liquid fertilizers were utilized as the carrier and dry formulations were added until the solution was almost saturated. Based on this preliminary test, the highest soluble rate of each commercially-available foliar K fertilizer was applied with glyphosate with K rates from 1.6 to 46 lb/acre. A slight precipitate formed when 3-18-18-0 or 0-0-25-17 was tank-mixed with glyphosate, but this precipitate did not affect application of these products. Spray boom height was adjusted to maintain 30% overlap when 0-0-30-0 was applied alone or tank-mixed with glyphosate.

At the time of spray application, air temperature was 88°F with 59% relative humidity on 24 June 2003 and 81°F with 45% relative humidity on 23 June 2004. Common lambsquarters (Chenopodium album L.) (number of plants per area at time of application = 10/m²), common ragweed (Ambrosia artemisiifolia L.) (1/ft²), and common waterhemp (Amaranthus tamariscinus Nutt.) (6/ft²) were the primary weeds in 2003, and giant foxtail (Setaria faberi Herrm.) (2/ft²) and common waterhemp (4/ft²) were the primary weeds in 2004. Soybean was 6 to 8 inches tall at the V4 to V5 stage of development (3), giant foxtail was 6 to 10 inches tall with 3 to 4 leaves, common ragweed was 4 to 8 inches tall with 8 to 14 leaves, common waterhemp was 3 to 10 inches tall with 4 to 14 leaves, and common lambsquarters was 4 to 6 inches tall with 12 to 18 leaves at the time of application.

Soybean injury was determined 7 and 14 days after treatment (DAT) on a scale of 0 (no effect) to 100 (complete crop death). Soybean plants and weeds were harvested from a randomly placed 10 by 30-inch quadrat 28 DAT and weighed to determine soybean recovery and weed control. Soybean fresh weight was determined immediately after harvest. Weeds were dried and percent dry weight reduction was calculated as 100[1-(total weed dry weight/non-treated weed dry weight)]. Visual weed control was evaluated 56 DAT on a scale of 0 (no effect) to 100 (complete plant death) to evaluate regrowth of weed species.

A gross margin was calculated as the [(grain yield * market price) – (cost of foliar fertilizer + fertilizer application cost in absence of herbicide)] to determine the economic benefit of using foliar K sources mixed with and without glyphosate for soybean production. The market price for soybean was estimated at $5.40/bu and fertilizer application cost was $5.00/acre (18). Fertilizer cost was estimated for 3-18-18-0 at $43.00/gal, 0-0-30-0 at $39.40/gal, 0-0-25-17 at $39.40/gal, 5-0-20-13 at $60.90/gal, 0-0-50-17 at $1.23/lb K, 0-0-62-0 at $1.28/lb K, 14-0-46-0 at $3.30/lb K.

Soybean Injury

Commercially-available K fertilizers vary in relative solubility which resulted in application rates that ranged from 1.6 to 46 lb of K per acre for the 15-gal/acre carrier volume utilized in this research (Table 1). Spray solution pH was 7.4 to 12.5 prior to the addition of glyphosate. Formulated glyphosate lowered solution pH with all K sources except 3-18-18-0 and 0-0-30-0. Spray solution pH of glyphosate plus DAS, 0-0-50-17, 14-0-46-0, or 0-0-62-0 was similar to glyphosate plus water; however, solution pH ranged from 6.9 to 12.3 when glyphosate was tank-mixed with 3-18-18-0, 5-0-20-13, 0-0-25-17, or 0-0-30-0. Solution pH of the fertilizer and glyphosate mixtures may be confounded by the cation-anion complex in the spray solution (14).

Table 1. Spray solution pH of K fertilizer sources alone and with
glyphosate at 0.75 lb ae/acre prior to application.

 ^x Abbreviations: DAS, diammonium sulfate; 0-0-30-0, K carbonate;
0-0-62-0, K chloride; 14-0-46-0, K nitrate; 0-0-50-17, K sulfate;
3-18-18-0, K phosphate + urea; 0-0-25-17, K thiosulfate;
5-0-20-13, K thiosulfate + urea-triazone.

Soybean injury was primarily necrosis of leaves exposed to the foliar applications of fertilizer additives alone or tank-mixed with glyphosate (visual observation). All treatments, averaged over the weed-free and glyphosate tank mixture treatments, injured soybean less than 10% 7 DAT except 0-0-30-0 and 5-0-20-13 (Table 2). Glyphosate tank-mixed with 0-0-30-0 or 5-0-20-30 injured soybean greater than the fertilizer source applied alone 14 DAT. The adjuvants present in the glyphosate formulation possibly increased uptake of the foliar K fertilizers which increased soybean injury. Soybean fresh weight was reduced by 0-0-30-0 when compared to the non-treated control 28 DAT.

Table 2. Soybean injury main effect 7 DAT, injury interaction between K additives applied alone in a weed-free environment and tank-mixed with glyphosate at 0.75 lb ae/acre 14 DAT, and fresh weight main effect 28 DAT.

 ^x Abbreviations: DAS, diammonium sulfate; DAT, days after treatment;
0-0-30-0, K carbonate; 0-0-62-0, K chloride; 14-0-46-0, K nitrate;
0-0-50-17, K sulfate; 3-18-18-0, K phosphate + urea;
0-0-25-17, K thiosulfate; 5-0-20-13, K thiosulfate + urea-triazone.

Weed Control

Glyphosate plus 3-18-18-0, 14-0-46-0, 0-0-62-0, or DAS reduced weed dry weights similar to the weed-free control 28 DAT (Table 3). Control of common ragweed, common lambsquarters, common waterhemp, and giant foxtail was greater than 85% with glyphosate plus 3-18-18-0, 0-0-50-17, 0-0-62-0, 14-0-46-0, or DAS 56 DAT (Table 4). None of these treatments except 3-18-18-0 affected spray solution pH when compared to glyphosate alone or with DAS (Table 1). Glyphosate solubility was probably reduced at the high pH levels which reduced herbicide absorption into the plant (13,20) and resulted in poor weed control when 5-0-20-13, 0-0-25-17, or 0-0-30-0 were combined with glyphosate.

Table 3. Weed dry weight reduction with glyphosate at 0.75 lb ae/acre
tank-mixed with K fertilizer sources 28 days after treatment (DAT) in
2003 and 2004.

 ^x Abbreviations: DAS, diammonium sulfate; 0-0-30-0, K carbonate;
0-0-62-0, K chloride; 14-0-46-0, K nitrate; 0-0-50-17, K sulfate;
3-18-18-0, K phosphate + urea; 5-0-20-13, 0-0-25-17, K thiosulfate;
K thiosulfate + urea-triazone.

 ^y Total dry weight reduction was calculated as: 100[1-(total weed dry weight/non-treated weed dry weight)]. Weeds included common
lambsquarters in 2003, common ragweed in 2003, common water-
hemp in 2003 and 2004, and giant foxtail in 2004. The non-treated
weed dry weight was 0.21 lb/ft².

Table 4. Control of common lambsquarters in 2003, common ragweed in 2003, common waterhemp in 2003 and 2004, and giant foxtail in 2004 56 DAT with glyphosate at 0.75 lb ae/acre plus K fertilizer sources on a high soil test K.

 ^x Abbreviations: DAS, diammonium sulfate; 0-0-30-0, K carbonate;
0-0-62-0, K chloride; 14-0-46-0, K nitrate; 0-0-50-17, K sulfate;
3-18-18-0, K phosphate + urea; 5-0-20-13, 0-0-25-17, K thiosulfate;
K thiosulfate + urea-triazone.

Yield and Gross Margins

The average soybean grain yield in 2003 and 2004 for plots which had weeds and were not treated with glyphosate was 33.7 bu/acre (data not shown). The addition of DAS to glyphosate increased soybean grain yields 3.8 bu/acre compared to when glyphosate was added alone (Fig. 1A). Significant reductions in grain yield occurred when 0-0-25-17 and 0-0-30-0 were mixed with glyphosate due to a combination of crop injury and reduced weed control. The weed-free application of 0-0-30-0 increased soybean grain yield 3 bu/acre and was the most injurious K source when compared with the non-treated, weed-free control. This response may be similar to treatments that cause crop injury yet increase grain yield due to a reduction in the incidence of disease (2,8). This has been observed when herbicides such as lactofen were applied to soybean (2). Tank mixtures of 0-0-50-17, 0-0-62-0, and 14-0-46-0 with glyphosate or applied alone had grain yields and gross margins (Fig. 1B) similar to glyphosate plus DAS. Gross margin return was significantly reduced when 0-0-30-0, 0-0-25-17 and 5-0-20-13 were tank-mixed with glyphosate which may be due to a combination of crop injury, reduced weed control, and/or cost due to the high application rates of these fertilizer additives in this research.

Fig. 1. The effect of fertilizer additive on soybean grain yield (A) and gross margin (B) in a weed-free environment and tank mixtures with glyphosate at 0.75 lb ae/acre with a high soil test K. Fertilizer additives included: DAS, diammonium sulfate; 0-0-25-17, K thiosulfate; 0-0-30-0, K carbonate; 0-0-50-17, K sulfate; 0-0-62-0, K chloride; 3-18-18-0, K phosphate + urea; 5-0-20-13, K thiosulfate + urea-triazone; 14-0-46-0, K nitrate. The non-treated control grain yield was 33.7 bu/acre. Gross margins were calculated as [(grain yield * market price) – (cost of foliar fertilizer + fertilizer application cost in absence of herbicide)].

Conclusion

Tank mixing K fertilizer sources with glyphosate may be a flexible management option for foliar K application to reduce K-deficiency in soybean. Mixtures of K fertilizer with glyphosate could help offset application costs associated with separate foliar K fertilizer applications and allow for in-season response to K deficiency when management or climatic conditions warrant application. However, proper selection of K fertilizer source is important because some K fertilizers may reduce weed control or cause crop injury that may lower grain yield.

The amount of K available to the plant may depend on the source solubility and compatibility with glyphosate. This research has determined that 14-0-46-0, 0-0-50-17, 3-18-18-0, and 0-0-62-0 controlled weeds similar to glyphosate plus DAS, and soybean grain yields were similar to the weed-free control in a soil without K deficiency. The greatest amount of K available to the plant would be with 0-0-62-0 or 3-18-18-0 tank-mixed with glyphosate while maintaining crop safety and weed control. Potassium uptake through the leaf may be affected by the presence of an adjuvant in the spray solution that may influence phytotoxicity. Soil test K at the research site used in this study was relatively high, but the primary objective was to evaluate the impact of K source on weed control with glyphosate.

Additional research may need to compare foliar K uptake with soil uptake to determine the impact of K source on glyphosate solubility and the resulting effects of the K source on herbicide absorption, and assess the economically optimal rates of K sources for weed control while providing a supplemental K source. This research demonstrated that several K fertilizer sources may be combined with glyphosate for cost-effective foliar fertilizer applications while minimizing crop injury and maintaining weed control.

Acknowledgments

The authors would like to thank Heather Collier, Sandra Devlin, Josh Intveld, Adam Jones, Matthew Jones, Sam Loyd, Eduardo Navarro, Sutham Phurahong, and Randall Smoot for their technical assistance with this research. In addition, a special thanks is extended to the Fluid Fertilizer Foundation and Missouri Fertilizer and Ag Lime Board for their financial support of this research.

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