Water quality can affect herbicide performance

The efficacy of weak-acid herbicides is improved with acidic spray water pH. Hard-water antagonism is more prevalent with weak-acid herbicides. Spray solution temperature between 18 C and 44 C is optimum for some weak-acid herbicides. The effect of water turbidity is severe on herbicides with low soil mobility such as glyphosate. Although adjuvants are recommended to overcome the negative effect of spray water hardness or pH, the response has been inconsistent with the herbicide chemistry and weed species.

A literature review was conducted to provide an overview of the impacts of spray water quality on herbicide performance and what strategies can be implemented their impacts.

pH impacts on performance

Sulfonylurea (Group 2) herbicides broke down (hydrolyzed) more rapidly in acidic spray water (pH<7) than in alkaline (pH.7) and neutral pH. This effected performance. For example, nicosulfuron (Accent) was more effective on common and large crabgrass with alkaline pH spray water compared to acidic water — control of large crabgrass was reduced by 40% with acidic spray water. Generally, the solubility of sulfonylurea herbicides is increased, and weed control is improved at neutral or alkaline pH.

Saflufenacil (Group 14; i.e. Heat) efficacy was 56% greater on giant ragweed, common lamb’s-quarters and field corn when applied in a spray solution of pH 7.7 compared to pH 4.0.

Research found that Group 27 (HPPD inhibitors) herbicides were variably affected by water pH. Mesotrione (Callisto; Group 27) had better performance on horseweed with acidic compared to alkaline spray water. However, performance was better with alkaline spray water for barnyard grass. The contrast was likely related to differences in leaf characteristics such as cuticle and trichomes.

For weak acid chemistries such as clethodim (Group 1; i.e. Select), sethoxydim (Group 1; Poast Ultra), bentazon (Group 6; i.e. Basagran), glyphosate (Group 9), glufosinate (Group 10; Liberty), and 2,4-D (Group 4), solubility is generally lower with acidic water pH compared to alkaline water pH. However, the uptake through the leaf cuticle is greater in acidic compared to alkaline water pH for these weak-acid herbicides. For example, glufosinate efficacy on Palmer amaranth and giant ragweed was reduced by 10% to 12% at a spray water level of pH 9 compared to spray water pH 4.

Water hardness and performance

Hard water cations such as calcium (Ca2+), magnesium (Mg2+), iron (Fe2+ or Fe3+), aluminum (Al3+), zinc (Zn2+), and manganese (Mn2+) can negatively affect herbicide performance.

These cations bind to negatively charged herbicide molecules, which are not readily absorbed and translocated in the plant. Researchers have reported hard water antagonism on weak-acid herbicides such as sethoxydim, clethodim, tralkoxydim (Group 1; i.e. Achieve), aminopyralid (Group 4; i.e. Reclaim), diflufenzopyr (Group 14; Distinct), dicamba, 2,4-D, MCPA amine, glufosinate, imazethapyr (Group 2; Pursuit), diquat (Group 22), and glyphosate.

The literature review found that glyphosate is the most widely studied herbicide with hard water antagonism.  Most of the research found a reduction in glyphosate weed control in the presence of hard water cations. For example, glyphosate activity was reduced on velvetleaf, common lambs- quarters, giant foxtail, smooth pigweed, and large crabgrass because of Mn2+ cations in the spray water.

Water temperature and performance

Herbicide performance can be affected by water temperature by affecting the rate of hydrolysis, degradation, physiochemical properties of spray mixture, and droplet size distribution.

For some weak-acid herbicides, a spray solution temperature of 18C to 44C is optimum, but performance can be reduced at low (5 C) or high (56 C) water temperatures. For example, glufosinate, mesotrione, 2,4-D choline, and dicamba + glyphosate performance on giant ragweed, pitted morning glory, Palmer amaranth, and horseweed was reduced at C or 56 C temperature, but not affected at spray water temperature between 18C and 44C.

Spray water turbidity

Sediments, organic matter, and sand, silt, and clay can be suspended in dirty spray water and bind to herbicide molecules to reduce performance.

Spray water turbidity has been associated with reduced efficacy of paraquat, diquat, glyphosate, nicosulfuron (Group 2; i.e. Accent), and clethodim.

Spray mixture storage

Research has found that the time between spray tank mixing and application can influence herbicide performance. Over time, the herbicide molecule can break down, resulting in reduced weed control. Plant growth regulator herbicides such as dicamba and 2,4-D are especially susceptible.

The impact of storage time on herbicide effectiveness is variable, depending on the chemistry and target weed. Research has found that isoxaflutole and dicamba control of velvetleaf and common lamb’s-quarters was reduced 3 to 7 days after mixing. But glyphosate, glufosinate, mesotrione + atrazine, and premix dicamba + diflufenzopyr were not affected over the same time delay.

Other research found that storage of a spray mixture up to 9 days did not affect S-metolachlor (Group 15), flumioxazin (Group 14; i.e. Fierce), and imazethapyr control of common lamb’s-quarters, Palmer amaranth, and broadleaf signal grass control with herbicides.

Adjuvants for improved performance

Spray adjuvants such as oils, wetting agents, and surfactants can help to improve emulsification, dispersion, absorption, and penetration of herbicides on targeted plants. They can also adjust solution pH, contain water conditioners to amend hard water, and use suspension aids to enhance the mixing of the herbicide formulation.

Ammonium sulfate (AMS) and dipotassium phosphate are commonly used to overcome hard-water antagonism and improve herbicide efficacy. However, the literature review found the response of AMS varies with the herbicide and targeted weed species.

Research found that glyphosate performance is improved with AMS, because of increase absorption by the weed.

The addition of AMS also improved mesotrione, glufosinate, dicamba, and 2,4-D choline efficacy on common lamb’s-quarters, redroot pigweed, giant ragweed, horseweed, and Palmer amaranth by reducing the antagonistic effect of Ca, Mg, and Mn cations.

Some herbicide labels provide recommendations on the addition of adjuvants. For example, Merge adjuvant is required for Poast Ultra when applied alone or with any registered broadleaf herbicide. Many other herbicides are already formulated with an adjuvant.

Overall, the researchers concluded that “information on the effect of spray water quality on various herbicide chemistries, weed species, and adjuvants is limited; therefore, it is difficult to develop guidelines for improving weed control efficacy. Further research is needed to determine the effect of spray water factors and develop specific recommendations for improving herbicide efficacy on problematic weed species.”

Daramola, O., Johnson, W., Jordan, D., Chahal, G., & Devkota, P. (2022). Spray water quality and herbicide performance: A review. Weed Technology, 36(6), 758-767.

OPEN ACCESS: https://doi.org/10.1017/wet.2022.97

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