Spray Water pH and Herbicide Performance

Spray water pH is one of the most critical water quality factors influencing herbicide performance (Roskamp et al. Reference Roskamp, Turco, Bischoff and Johnson2013a). Acidic or alkaline spray water pH can adversely affect herbicide efficacy by affecting the solubility, hydrolysis, dissociation, or chemical breakdown of the herbicide molecule (Green and Hale Reference Green and Hale2005; Roskamp and Johnson Reference Roskamp and Johnson2013; Sarmah and Sabadie Reference Sarmah and Sabadie2002). A lower- or higher-than-optimal water pH may result in reduced solubility or rapid dissociation of active herbicide ingredient into an inactive degradative product, which subsequently affects the herbicide absorption and translocation (Green and Cahill Reference Green and Cahill2003; Grzanka et al. Reference Grzanka, Sobiech, Skrzypczak and Piechota2021; Roskamp et al. Reference Roskamp, Turco, Bischoff and Johnson2013a).

The influence of spray water pH on hydrolysis and resulting efficacy depends on herbicide chemistry and targeted weed species. In acidic spray water (pH <7), the sulfonylurea herbicides such as prosulfuron, primisulfuron, rimsulfuron, nicosulfuron, chlorimuron, chlorsulfuron, trifloxysulfuron, thifensulfuron-methyl, and metsulfuron-methyl hydrolyzed more rapidly to non-herbicidal molecules than in neutral water pH (Berger and Wolfe Reference Berger and Wolfe1996; Green and Cahill Reference Green and Cahill2003; Green and Hale Reference Green and Hale2005; Matocha and Sensemen Reference Matocha and Senseman2007; Sarmah and Sabadie Reference Sarmah and Sabadie2002). There was no difference in the rate of hydrolysis between alkaline and neutral pH (Green and Cahill Reference Green and Cahill2003; Green and Hale Reference Green and Hale2005; Matocha and Sensemen Reference Matocha and Senseman2007; Sarmah and Sabadie Reference Sarmah and Sabadie2002). The active molecule of trifloxysulfuron was degraded by 10% in about 48 h after mixing in acidic water, while it required more than 120 h for an equivalent degradation in neutral or alkaline water (Matocha and Senseman Reference Matocha and Senseman2007). This indicates that trifloxysulfuron is more stable and degrades less in spray water with alkaline pH, leading to greater absorption and translocation (Sarmah and Sabadie Reference Sarmah and Sabadie2002). Matocha et al. (Reference Matocha, Krutz, Senseman, Koger, Reddy and Palmer2006) demonstrated that the absorption of ¹⁴C-trifloxysulfuron on Palmer amaranth (Amaranthus palmeri S. Watson) and Texasweed (Caperonia palustris L.) was 15% greater when applied with carrier water pH 9 compared with pH 5. Additionally, the study by Matocha et al. (Reference Matocha, Krutz, Senseman, Koger, Reddy and Palmer2006) showed that the higher absorption of ¹⁴C-trifloxysulfuron at pH 9 translated into greater ¹⁴C-trifloxysulfuron translocation, indicating a potential for increased efficacy at alkaline spray water pH. Nicosulfuron also showed greater activity on common cocklebur (Xanthium strumarium L.) and large crabgrass (Digitaria sanguinalis L.) at alkaline compared to acidic water pH (Green and Cahill Reference Green and Cahill2003). Similarly, the efficacy of saflufenacil on giant ragweed, common lambsquarters (Chenopodium album L.), and field corn (Zea mays L) was greater by 56% when applied with spray solution at pH 7.7 compared to pH 4.0 (Roskamp et al. Reference Roskamp, Turco, Bischoff and Johnson2013a).

The reduction in the efficacy of trifloxysulfuron, nicosulfuron, and saflufenacil at low or acidic spray water pH may be due to reduced solubility of the herbicides. Water solubility of the sulfonylurea herbicides is directly related to the acid dissociation (pKa) constant (Table 1), which ranges from 3.3 to 5.2 (Sarmah and Sabadie Reference Sarmah and Sabadie2002; Senseman Reference Senseman2007; Shaner Reference Shaner2014). Previous studies have shown that herbicide solubility is low when the spray solution pH is below the pKa constant, and the herbicides are not ionized but form dispersions and precipitates (Sarmah and Sabadie Reference Sarmah and Sabadie2002; Senseman Reference Senseman2007). This could negatively affect the absorption and subsequent translocation of the herbicide molecules on targeted plants (Green and Cahill Reference Green and Cahill2003; Matocha et al. Reference Matocha, Krutz, Senseman, Koger, Reddy and Palmer2006; Roskamp et al. Reference Roskamp, Turco, Bischoff and Johnson2013a). For example, when nicosulfuron solubility was reduced at acidic spray water pH, control of large crabgrass was reduced by 40% (Green and Cahill Reference Green and Cahill2003). The reduction in absorption has been reported for herbicides that form precipitates or crystals on the leaf surface (Nalewaja and Matysiak Reference Nalewaja and Matysiak2000). Generally, the solubility of sulfonylurea herbicides is increased, and weed control is improved at neutral or alkaline pH (Sarmah and Sabadie Reference Sarmah and Sabadie2002; Senseman Reference Senseman2007).

Table 1. Mode of action, acid dissociation constant, and water solubility of the herbicides mentioned in the review ^a

^a Abbreviatons: ACCase, acetyl co-enzyme A carboxylase; ALS, acetolactate synthase; EPSPS, 5-enolpyruvylshikimate 3-phosphate synthase; pKa, acid dissociation constant; PPO, protoporphyrinogen oxidase.

For herbicides such as mesotrione, tembotrione, sulcotrione, and bipyrazone that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD), improved efficacy has been reported with acidic or alkaline spray water pH; however, the response was variable with the weed species (Grzanka et al. Reference Grzanka, Sobiech, Skrzypczak and Piechota2021; Sobiech et al. Reference Sobiech, Idziak and Skrzypczak2018, Reference Sobiech, Skrzypczak and Grzanka2019, Reference Sobiech, Grzanka, Skrzypczak, Idziak, Włodarczak and Ochowiak2020). The efficacy of mesotrione on horseweed increased with acidic compared to alkaline spray water (Devkota et al. Reference Devkota, Spaunhorst and Johnson2016a). In contrast, mesotrione efficacy was greater with alkaline compared to acidic spray water for barnyardgrass [Echinochloa crus-galli (L.) Beauv.; Sobiech et al. Reference Sobiech, Idziak and Skrzypczak2018]. Variation in results may be attributed to the differences in the leaf morphological characteristics (cuticle, trichomes, etc.) among the weed species, and potential for crystalline salt formation (Hall et al. Reference Hall, Hart and Jones2000; Harr et al. Reference Harr, Guggenheim and Schulke-Falk1991; Liu et al. Reference Liu, Gaskin and Zabkiewicz2004). For instance, the epicuticle wax content for barnyardgrass was reported at 19 µg cm⁻² (Sanyal et al. Reference Sanyal, Bhowmik and Reddy2006) but varied between 1 to 80 µg cm⁻² for horseweed (Koger and Reddy Reference Koger and Reddy2005). Such differences can result in variations in herbicide penetration and lead to inconsistent weed control activity.

Although the water solubility of weak-acid herbicides is generally low at acidic compared to alkaline water pH (Table 1), the uptake through the leaf cuticle is greater in acidic compared to alkaline water pH for weak-acid herbicides such as clethodim, sethoxydim, bentazon, glyphosate, glufosinate, and 2,4-D (Matocha et al. Reference Matocha, Krutz, Senseman, Koger, Reddy and Palmer2006; Muir and Hansch Reference Muir and Hansch1955). This is because the pKa’s of the functional weak-acid groups range between 1 and 7, and at pH below the pKa, the functional group will mostly be protonated, non-ionic, and diffuse readily into the plant cuticle (Green and Hale Reference Green and Hale2005). When the spray water pH is below the herbicide pKa, and solubility is a limiting factor for herbicide uptake, increasing pH can increase solubility. However, when solubility is not a limiting factor, and the pH is raised above the pKa, weak-acid herbicides become ionic, and thus penetration through the lipophilic cuticle, negatively charged cell wall, and membrane is reduced (Green and Hale Reference Green and Hale2005; Nalewaja et al. Reference Nalewaja, Woznica and Matysiak1991; Stirling Reference Stirling1994). Ruiz and Ortiz (Reference Ruiz and Ortiz2005) reported reduced glyphosate efficacy on broadleaf signalgrass (Brachiaria extensa L.) with an alkaline compared to acidic spray water pH. Similar results were observed for glyphosate efficacy on palisade grass (Brachiaria brizantha L.; Dan et al. Reference Dan, Dan, Barroso and Souza2009). Glufosinate efficacy on Palmer amaranth and giant ragweed was reduced by 10% to 12% at spray water pH 9 compared to spray water pH 4 (Devkota and Johnson Reference Devkota and Johnson2016a). The reduced efficacy of these herbicides at alkaline spray water pH was attributed to the dissociation of the herbicide molecules when dissolved in alkaline water, which could have resulted in lower accumulation in the plant and reduced effectiveness (Dan et al. Reference Dan, Dan, Barroso and Souza2009; Matocha et al. Reference Matocha, Krutz, Senseman, Koger, Reddy and Palmer2006). Conversely, increased efficacy at acidic spray water pH was attributed to a higher proportion of the herbicide molecules in an undissociated form that can easily diffuse through the leaf cuticle (Liu Reference Liu2002; Devkota and Johnson Reference Devkota and Johnson2016a).

Spray Water Hardness and Herbicide Performance

Herbicide spray mixtures are prepared with water from various sources, which may consist of high levels of cations such as calcium (Ca²⁺), magnesium (Mg²⁺), iron (Fe²⁺ or Fe³⁺), aluminum (Al³⁺), zinc (Zn²⁺), and manganese (Mn²⁺). The divalent and polyvalent charges of these cations can bind efficiently to the negatively charged herbicide molecules and form a less soluble or inactive herbicide-salt complex, which is not readily absorbed and translocated in the plant (Bailey et al. Reference Bailey, Poston, Wilson and Hines2002; Roskamp et al. Reference Roskamp, Chahal and Johnson2013b). Furthermore, interaction of herbicide molecules and cations can increase the spray droplet size, resulting in less retention and reduced uptake of the active ingredient (Hoffmann et al. Reference Hoffmann, Fritz and Martin2011).

The presence of hard water cations in the spray mixture can have a negative effect on herbicide efficacy. Several researchers have reported hard water antagonism on weak-acid herbicides such as sethoxydim (Matysiak and Nalewaja Reference Matysiak and Nalewaja1999); clethodim and tralkoxydim (De Villiers et al. Reference De Villiers, Kudsk, Smit and Mathiassen2001); aminopyralid, diflufenzopyr, dicamba, and tembotrione (Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2011); 2,4-D (Roskamp et al. Reference Roskamp, Chahal and Johnson2013b; Schortgen and Patton Reference Schortgen and Patton2021; Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2010); MCPA amine and glufosinate (Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2010; Devkota and Johnson Reference Devkota and Johnson2016a); imazethapyr (Aliverdi et al. Reference Aliverdi, Ganbari, Mohassel, Nassiri-Mahallati and Eskandar2014; Gronwald et al. Reference Gronwald, Jourdan, Wyse, Somers and Magnusson1993); paraquat and diquat (Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2010); and glyphosate (Bailey et al. Reference Bailey, Poston, Wilson and Hines2002; Bernards et al. Reference Bernards, Thelen and Penner2005; Nalewaja and Matysiak Reference Nalewaja and Matysiak1991; Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2010). These studies indicate that the impact of hard water cations on herbicide performance is often related to the cation, herbicide, and weed species. Studies have shown that trivalent cations such as Al³⁺ and Fe³⁺ result in a greater negative effect on glyphosate activity than monovalent and divalent cations (Bernards et al. Reference Bernards, Thelen and Penner2005; Nalewaja and Matysiak Reference Nalewaja and Matysiak1991). A few studies have reported that Fe³⁺ cation resulted in greater antagonism on glyphosate than divalent cations (Stahlman and Phillips Reference Stahlman and Phillips1979; Sundaram and Sundaram Reference Sundaram and Sundaram1997). Furthermore, a higher chelation constant is reported for methyl-glyphosate with Fe³⁺ than with Mn²⁺, which results in a very stable glyphosate-Fe³⁺ complex in the solution (Motekaitis and Martell Reference Motekaitis and Martell1985). Bernards et al. (Reference Bernards, Thelen and Penner2005) reported that there was a rapid absorption of glyphosate-Fe³⁺ complex into the treated leaf; however, it bound tightly in the apoplast, reducing further translocation.

Glyphosate is the most widely studied herbicide with hard water antagonism, and most of the studies have reported a reduction in efficacy with the presence of hard water cations (Abouziena et al. Reference Abouziena, Elmergawi, Sharma, Omar and Singh2009; Chahal et al. Reference Chahal, Jordan, Burton, Danehower, York, Eure and Clewis2012; Devkota et al. Reference Devkota, Spaunhorst and Johnson2016a; Hall et al. Reference Hall, Hart and Jones2000; Mueller et al. Reference Mueller, Main, Thompson and Steckel2006; Pratt et al. Reference Pratt, Kells and Penner2003). The presence of Ca²⁺, Mg²⁺, and Zn²⁺ in the spray water reduced glyphosate efficacy on velvetleaf (Abutilon theophrasti L.), barnyard grass, and yellow nutsedge (Cyperus esculentus L.; Abouziena et al. Reference Abouziena, Elmergawi, Sharma, Omar and Singh2009; Hall et al. Reference Hall, Hart and Jones2000). Mueller et al. (Reference Mueller, Main, Thompson and Steckel2006) reported that presence of Mg²⁺ in spray water reduced glyphosate efficacy on broadleaf signalgrass, pitted morningglory, Palmer amaranth, and yellow nutsedge. This result was attributed to the formation of glyphosate-magnesium complex and inactivation of isopropylamine in glyphosate molecules, thereby reducing plant uptake. A nuclear magnetic resonance analysis of the effect of Ca²⁺ and Mg²⁺ on isopropylamine formulation of ¹⁴C-glyphosate demonstrated inactivation of isopropylamine through the formation of a less readily absorbed calcium-glyphosate and magnesium-glyphosate conjugate salt with the phosphate and carboxylic groups of glyphosate molecule, thereby reducing herbicide efficacy (Thelen et al. Reference Thelen, Jackson and Penner1995).

Glyphosate activity was reduced on velvetleaf, common lambsquarters, giant foxtail (Setaria faberi Herrm.), smooth pigweed (Amaranthus hybridus L.), and large crabgrass (Digitaria sanguinalis L.) due to the presence of Mn²⁺ in the spray water (Bernards et al. Reference Bernards, Thelen and Penner2005). In a similar study, Bailey et al. (Reference Bailey, Poston, Wilson and Hines2002) reported that the antagonistic effect might be due to the chelation of glyphosate molecules with cations, which led to the precipitation of the herbicide from the solution and limited the penetration of the herbicide through the leaf cuticle. However, Bernards et al. (Reference Bernards, Thelen and Penner2005) suggested that the effect of Mn²⁺ antagonism on glyphosate efficacy is not only limited to reactions in the spray tank or leaf surface but also in the cytoplasm, where glyphosate may form a complex with Mn²⁺ and result in lower efficacy. According to Caetano et al. (Reference Caetano, Ramalho, Botrel, da Cunha and de Mello2012), the formation of herbicide-cation complexes with Ca²⁺, Mg²⁺, Mn²⁺, Zn²⁺, and Fe³⁺ could prevent glyphosate from binding to the enzyme 5-enolpyruvylshikimate 3-phosphate synthase and result in reduced performance.

Increasing carrier water hardness from 0 to 1,000 mg L⁻¹ reduced mesotrione efficacy by 28%, 18%, and 18% on giant ragweed, horseweed, and Palmer amaranth, respectively (Devkota et al. Reference Devkota, Spaunhorst and Johnson2016a). Similarly, Devkota and Johnson (Reference Devkota and Johnson2016b) reported a linear trend in reducing 2,4-D choline and 2,4-D choline plus glyphosate efficacy on giant ragweed, horseweed, and Palmer amaranth control when carrier water hardness (resulting from CaCl₂ and MgSO₄) increased from 0 to 1,000 mg L⁻¹. According to Devkota and Johnson (Reference Devkota and Johnson2016b), increases in water hardness from 0 to 1,000 mg L⁻¹ reduced weed control by 20% or greater with 2,4-D choline and premixed 2,4-D choline plus glyphosate. Schortgen and Patton (Reference Schortgen and Patton2021) also reported that water-soluble amine and choline formulations of 2,4-D were antagonized by hard water at 600 mg CaCO₃ L⁻¹. In a similar study, common lambsquarters, horseweed, redroot pigweed (Amaranthus retroflexus L.), and common ragweed control with 2,4-D was reduced with the presence of Ca²⁺ and Mg²⁺ in the spray water (Izadi Darbandi et al. Reference Izadi Darbandi, Nessari and Azarian2011; Roskamp et al. Reference Roskamp, Chahal and Johnson2013b). The result was attributed to the cations binding to the negatively charged herbicide ions, reducing the absorption into plants and rendering them less effective (Roskamp et al. Reference Roskamp, Chahal and Johnson2013b).

Hard-water antagonism of weak-acid herbicides has been associated with variable response on weed species. Foxtail millet (Setaria italic L.) and yellow foxtail (Setaria pumila L) control by terbuthylazine plus mesotrione was reduced by hard-water antagonism; however, there was no response for common lambsquarters, common ragweed, and velvetleaf control (David and Mate Reference David and Mate2010). Common lambsquarters control with 2,4-D was reduced by hard-water antagonism, whereas a similar response was not observed for horseweed (Erigeron canadensis L.) control (Roskamp et al. Reference Roskamp, Turco, Bischoff and Johnson2013a). The efficacy of glufosinate on velvetleaf, foxtail millet (Setaria italic L.), and red amaranth (Amaranthus cruentus L.) was reduced by hard-water antagonism (Zollinger et al. Reference Zollinger, Nalewaja, Peterson and Young2010, Reference Zollinger, Nalewaja, Peterson and Young2011). In contrast, the effect of water hardness was not observed on glufosinate for green foxtail, common lambsquarters, redroot pigweed, barnyard grass, and velvetleaf (Soltani et al. Reference Soltani, Nurse, Robinson and Sikkema2011). Roskamp et al. (Reference Roskamp, Chahal and Johnson2013b) observed hard-water antagonism on 2,4-D efficacy on control of common lambsquarters, but a similar response did not occur for saflufenacil efficacy. The variable response of weed species may be due to the differences in morphology and rate of herbicide absorption and translocation as affected by hard-water antagonism. For instance, Zollinger et al. (Reference Zollinger, Nalewaja, Peterson and Young2010) observed greater absorption of ¹⁴C-glufosinate by velvetleaf and giant foxtail than by common lambsquarters in the presence of Ca²⁺ and Mg²⁺ in the spray solution.

Spray Water Temperature and Herbicide Performance

Spray water temperature is influenced by ambient air if stored in an outside holding tank prior to herbicide application. The temperature of groundwater in the United States can vary from 3 C in northern states such as Indiana, Kansas, and Minnesota to 22 C in southern states such as Alabama, Florida, and Georgia depending on the application timing of the year (USEPA 2016). Moreover, when herbicide solution is stored in a spray tank, the herbicide spray water tends to be at equilibrium with the prevailing ambient air temperature (Ellis and Griffin Reference Ellis and Griffin2002). The few available studies on this have shown that spray water temperature can adversely affect herbicide performance (Beltran et al. Reference Beltran, Fenet, Cooper and Coste2000; Devkota Reference Devkota2016; Singh et al. Reference Singh, Punia and Malik2010). Spray water temperature can influence herbicide performance by affecting the rate of hydrolysis, degradation, physiochemical properties of spray mixture, and droplet size distribution (Beltran et al. Reference Beltran, Fenet, Cooper and Coste2000; Miller et al. Reference Miller, Tuck, Salyani and Lindner2005). Higher spray water temperature may lead to rapid conversion of herbicide active ingredient to an inactive compound, thereby reducing its efficacy (Beltran et al. Reference Beltran, Fenet, Cooper and Coste2000). For example, the degradation rate of isoxaflutole was faster at 50 C than at 22 C, although the inactive degradation product was not detected (Beltran et al. Reference Beltran, Fenet, Cooper and Coste2000). However, Rouchaud et al. (Reference Rouchaud, Neus, Callens and Bulcke1998) reported that the degradation of isoxaflutole in spray water led to a rapid conversion into 2-methanesulfonyl-4-trifluoromethylbenzoic acid, an inactive benzoic acid derivative, resulting in reduced herbicidal activity on the target plants. Similarly, the half-life of chlorsulfuron was reduced from 5.59 d at a solution temperature of 20 C to 0.08 d at 55 C, indicating rapid dissipation and a possible efficacy reduction at a higher solution temperature (Grey and McCoullough Reference Grey and McCullough2012). Higher spray water temperature is also found to reduce surface tension, viscosity, and spray droplet size, resulting in increased vapor drift, reduction in spray interception by targeted plants, and sub-optimum droplet coverage (Miller et al. Reference Miller, Tuck, Salyani and Lindner2005; Miller and Tuck Reference Miller and Tuck2005). Miller et al. (Reference Miller, Tuck, Salyani and Lindner2005) demonstrated that conventional nozzles resulted in smaller droplets size as spray water temperature increased up to 25 C. Similarly, Hoffmann et al. (Reference Hoffmann, Fritz and Martin2011) reported a reduction in surface tension, viscosity, and spray droplet size with increasing spray water temperature from 10 to 40 C.

Spray water temperature can also influence the solubility of the herbicide in water. Herbicides can become less soluble and form a precipitate at the bottom of the spray tank at a lower water temperature, which can result in poor target delivery of the active ingredient (Singh et al. Reference Singh, Punia and Malik2010). For example, the water solubility of simazine decreased from 84 mg L⁻¹ at 85 C to 6.2 mg L⁻¹ at 22 C and 2 mg L⁻¹ at 0 C (Wauchope et al. Reference Wauchope, Buttler, Hornsby, Augustijn-Beckers and Burt1992). Similarly, acetyl co-enzyme A carboxylase (ACCase) inhibitors such as cyhalofop, fenoxaprop-p, metamifop, and quizalofop-p were reported to be less water soluble (<1 mg L⁻¹) at temperatures below 25 C (Table 1), indicating a potential for reduced efficacy at lower water temperature (Wauchope et al. Reference Wauchope, Buttler, Hornsby, Augustijn-Beckers and Burt1992). The efficacy of clodinafop formulations on littleseed canarygrass (Phalaris minor L.) was decreased at 8 C compared with 25 C or 40 C, owing to the reduced solubility and subsequent target delivery of the herbicide at lower spray water temperature (Singh et al. Reference Singh, Punia and Malik2010). The efficacy of diquat and endothall on curlyleaf pondweed (Potamogeton crispus L.) was inhibited as the water temperature in the tank decreased from 25 C to 10 C (Netherland et al. Reference Netherland, Skogerboe, Owens and Madsen2000). Similarly, glufosinate, mesotrione, 2,4-D choline, and premixed dicamba plus glyphosate efficacy on giant ragweed (Ambrosia artemisiifolia L.), pitted morningglory (Ipomoea lacunosa L.), Palmer amaranth, and horseweed (Conyza canadensis L.) was reduced at relatively low (5 C) or high (56 C) temperature, but the efficacy was not affected at spray water temperature between 18 C and 44 C (Devkota Reference Devkota2016; Devkota et al. Reference Devkota, Whitford and Johnson2016b). Those authors have also suggested that the herbicide efficacy could be affected at lower or higher spray water temperature due to the formulation instability, which subsequently inhibits uptake and weed control.

Spray Water Turbidity and Herbicide Performance

Inorganic (sand, silt, and clay) and organic matter, and sediments suspended in spray water can bind to herbicide molecules and reduce their performance (Frater et al. Reference Frater, Mikulyuk, Barton, Nault, Wagner, Hauxwell and Kujawa2017). Spray water turbidity has been associated with reduced efficacy of paraquat (Simarmata et al. Reference Simarmata, Taufik and Peranginangin2017); diquat (Fox and Murphy Reference Fox and Murphy1990; Hofstra et al. Reference Hofstra, Clayton and Getsinger2001; Poovey and Skogerboe Reference Poovey and Skogerboe2003, Reference Poovey and Skogerboe2004; Rytwo and Tavasi, Reference Rytwo and Tavasi2003); endothall (Poovey and Skogerboe Reference Poovey and Skogerboe2004); glyphosate (Simarmata et al. Reference Simarmata, Taufik and Peranginangin2017); foramsulfuron (Nosratti et al. Reference Nosratti, Saeidi, Barbastegan, Jalali Honarmand and Ghobadi2016); nicosulfuron (Hajmohammadnia-Ghalibaf et al. Reference Hajmohammadnia-Ghalibaf, Rashed-Mohassel, Nassiri-Mahllati and Zand2015, Reference Hajmohammadnia-Ghalibaf, Mohassel, Mahallati and Zand2016; Nosratti et al. Reference Nosratti, Saeidi, Barbastegan, Jalali Honarmand and Ghobadi2016); clethodim and sethoxydim (Gauvrit and Lamrani Reference Gauvrit and Lamrani2008; Singh et al. Reference Singh, Punia and Malik2010); and imazosulfuron, diflufenican, and iodosulfuron (Shahbazi et al. Reference Shahbazi, Saeedi, Nosrati and Jalai Honarmand2015). Hajmohammadnia-Ghalibaf et al. (Reference Hajmohammadnia-Ghalibaf, Rashed-Mohassel, Nassiri-Mahllati and Zand2015) reported reduced glyphosate and nicosulfuron efficacy on barnyardgrass and velvetleaf with the presence of soil particles in the spray water. Fox and Murphy (Reference Fox and Murphy1990) observed reduced diquat efficacy on submerged weeds with water turbidity. Diquat activity on common waterweed (Egeria densa Planeh) and coontail (Ceratophyllum demersum L.) was reduced with the presence of bentonite clay sediment in the spray water (Hofstra et al. Reference Hofstra, Clayton and Getsinger2001; Poovey and Getsinger Reference Poovey and Getsinger2002). Herbicides such as paraquat and glyphosate with low soil mobility or with high soil adsorption coefficient (K_oc) can bind tightly to the suspended particles in the solution. The negative effect of spray water turbidity on herbicide efficacy has been attributed to the binding of sediment or negatively charged clay particles to the highly polar and positive-charged herbicide molecule, which resulted in a reduction in plant uptake (Poovey and Getsinger Reference Poovey and Getsinger2002). Cationic herbicides such as paraquat and diquat can adsorb strongly to negatively charged suspended particles (Bowmer Reference Bowmer1982; Hofstra et al. Reference Hofstra, Clayton and Getsinger2001; Weber et al. Reference Weber, Perry and Upchurch1965). Diquat efficacy was also reduced because of adsorption to montmorillonite and kaolinite clay particles in the spray solution (Bowmer Reference Bowmer1982; Weber et al. Reference Weber, Perry and Upchurch1965). Additionally, suspended particles in the spray solution can block sprayer nozzles/screens, thereby reducing the delivery of the herbicide to the target species, and also damage spray equipment (e.g., pumps; Aliverdi and Ahmadvand Reference Aliverdi and Ahmadvand2020; Poovey and Getsinger Reference Poovey and Getsinger2002).

Summary

Spray water quality has profound implications on herbicide spray solution and weed control efficacy. The available literature suggests that the weed control potential of herbicides can be antagonized by spray water quality. Among the spray water quality factors, pH is the most important, followed by hardness, temperature, turbidity, and storage duration. These factors can alter the properties of a herbicide active ingredient during mixing, while the spray mixture remains in the tank, during the application processes, and after deposition on the target. These effects can be observed as tank-mixing incompatibility, reduced solubility, rapid hydrolysis and degradation, formation of herbicide-mineral complex, binding of herbicide molecule with suspended particles, change in droplet size and evaporation rate, and crystalline salt deposits on the leaf surface. Furthermore, effects can be pronounced on total absorption and translocation, which reduces overall performance depending upon herbicide chemistry and weed species. The leaf morphology can also play an important role in influencing herbicide efficacy regarding the spray water quality. Leaf structures such as trichomes, nature of epicuticular wax, and cuticle thickness are some of the characteristics that can confound with spray water quality factors and influence herbicide efficacy.

The key findings suggest that the efficacy of sulfonylureas is negatively affected by acidic water pH, whereas the efficacy of weak-acid herbicides such as glyphosate, glufosinate, clethodim, sethoxydim, bentazon, and 2,4-D is improved under this condition. The efficacy of HPPD-inhibitor herbicides can be improved with acidic or alkaline spray water pH depending on the weed species. Control of weed species such as barnyardgrass, broadleaf signalgrass, common cocklebur, giant ragweed, horseweed, large crabgrass, palisade grass, Palmer amaranth, and Texasweed can be reduced by non-optimal spray water pH. Hard-water antagonism is more prevalent with weak-acid herbicides, with trivalent cations such as Fe³⁺⁺ being the most problematic. Control of weed species such as common lambsquarters, giant foxtail, giant ragweed, horseweed, Palmer amaranth, velvetleaf, and yellow nutsedge are primarily affected by hard-water antagonism to various degrees depending on the herbicides and cations. AMS and other ammonium-containing fertilizers can help to overcome the hard-water antagonism of weak-acid herbicides; however, the response varies with the herbicide and targeted weed species. Spray solution temperature between 18 C and 44 C is the optimum for most weak-acid herbicides, whereas efficacy is reduced at relatively low (5 C) or high (56 C) temperatures. Cationic herbicides such as paraquat and diquat, and herbicides with low soil mobility such as glyphosate, are most susceptible to the antagonistic effect of spray water turbidity; therefore, clean water should be used to mitigate water turbidity issue. The reduction in efficacy with respect to spray mixture storage duration is primarily due to herbicide adherence to the spray tank (dicamba), and herbicide degradation to a nonactive compound (isoxaflutole).

Future Research

A significant number of published research findings on spray water quality suggest that herbicide properties are influenced by spray water quality factors before, during, and after application. However, most of these studies have focused on single water-quality factors with greenhouse experiments conducted on individual weed species, which may not truly predict herbicide and weed responses under field conditions. Moreover, information on the effect of spray water quality factors is not available for many of the herbicides currently used to manage glyphosate-resistant weeds. This warrants extensive research on the potential effects of spray water-quality factors on different herbicide chemistries and formulations across diverse weed categories. It is particularly necessary to conduct field research to evaluate multiple water-quality factors and their interactions on herbicide efficacy with a focus on problematic weed species. Most of the published research on the effect of adjuvants in adjusting water quality are primarily focused on using AMS for addressing spray water hardness. There is no published research evaluating spray buffers for amending solution pH and optimizing herbicide efficacy against spray water pH. Additionally, research needs to be conducted focusing on adjuvants for addressing spray water hardness and pH for improving herbicide efficacy. Research findings generated from such studies will be important in developing guidelines for optimizing herbicide efficacy for effective weed control.