The influence of water pH, water hardness, and co-applied herbicides and fertilizers on the efficacy of selected herbicides

Jared M Roskamp, Purdue University

Abstract

Water comprises 99% of most spray solutions and properties of the water used as carrier can influence the efficacy of herbicides. Water pH is one property of water which can limit the solubility of some herbicides or cause degradation of herbicides through hydrolysis. Water hardness, or the amount of dissolved cations in water, can reduce herbicide efficacy due to binding of herbicide molecules to cations. The presence of these cations can cause reduced absorption or translocation of the herbicide into plants. Glyphosate is the most studied herbicide with respect to the influence of water pH and hardness. Some herbicides have also exhibited reduced efficacy when co-applied with other herbicides or foliar fertilizers. Antagonism of herbicides has been documented with protoporphyrinogen IX inhibiting herbicides and glyphosate. Glyphosate has been documented to exhibit reduced phytotoxicity when applied with manganese fertilizer. The objective of this research was to evaluate the response of selected herbicides applied in water with varying pH and hardness, or co-applied with various herbicides and foliar fertilizers. Saflufenacil, a protoporphyrinogen IX inhibitor, was tested to determine a response to five pH levels (4.0, 5.2, 6.5, 7.7, and 9.0) or three water hardness levels (0, 310, and 620 mg L-1). A field study on common lambsquarters and giant ragweed was conducted as well as a greenhouse bioassay on horseweed and corn to determine the influence of these water properties on saflufenacil efficacy. In both the field and greenhouse experiments, no influence of water hardness was seen when proper adjuvants were included in the spray solution. However, water pH at 4.0 reduced the efficacy of saflufenacil on common lambsquarters in the field experiments and corn in the greenhouse experiments. High pressure liquid chromatography testing on the solubility of saflufenacil found that the solubility of saflufenacil was reduced from 3,687 mg L-1 at pH 7.7 to 10 mg L-1 at pH 4.0. This reduction in solubility may have led to the reduced efficacy on corn in the greenhouse where a pH of 4.0 showed only 79% reduction in growth as compared to 87% reduction in growth with a pH of 7.7. A second study on saflufenacil was conducted to test if adjusting the pH of the saflufenacil solution would influence efficacy. Saflufenacil was mixed in five different carrier waters with initial pH values of 4.0, 5.2, 6.5, 7.7, or 9.0 and adjusted to one of four final pH levels (4.0, 6.5, 9.0, or no change). When initial pH levels were acidic (4.0 or 5.2) and raised to a more neutral pH, saflufenacil efficacy on corn was increased. It was also observed that when initial pH was 6.5 or 7.7 and reduced to 4.0, saflufenacil efficacy on corn was reduced. Saflufenacil was also tested for interaction with fourteen herbicides. A field study was conducted on fallow ground containing common lambsquarters, giant ragweed, and giant foxtail. Saflufenacil co-applied with glyphosate or glufosinate provided the most consistent and best control of giant foxtail and common lambsquarters at 14 days after treatment. At 21 days after treatment, control of common lambsquarters was best with 2,4-D and dicamba mixed with saflufenacil. All tank mixtures provided 85% or more control of common lambsquarters except imazaquin, imazethapyr, or chlorimuron-methyl co-applied with saflufenacil. All tank mixtures provided better control of weeds as compared to saflufenacil alone. 2,4-D and dicamba, two growth regulator herbicides, were tested for a reduction in efficacy when applied in water with calcium ions, magnesium ions, manganese fertilizer, or zinc fertilizer. 2,4-D control of horseweed or redroot pigweed was decreased when ammonium sulfate was not included in the spray solution regardless of the presence of cations or fertilizers. Overall, 2,4-D efficacy was reduced when applied in water containing manganese fertilizer as compared to 2,4-D applied in deionized water. The presence of calcium ions decreased 2,4-D control of common lambsquarters when ammonium sulfate was not used. Dicamba control of common lambsquarters and redroot pigweed was decreased when applied in water with magnesium or manganese in the absence of ammonium sulfate. No reduction in dicamba control of horseweed was noted across cation solutions, as was seen with 2,4-D.

Degree

M.S.

Advisors

Johnson, Purdue University.

Subject Area

Agriculture|Plant sciences

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