Response of Weeds to Zinc-Glyphosate Mixtures

Linda Benedict  |  8/23/2008 1:05:47 AM

Figure 1. Visual weed control 14 days after treatment in the 2006 field study averaged across glyphosate products.

Figure 2. Visual weed control 14 days after treatment in the 2007 pot study averaged across glyphosate products.

Weeds were grown in trade-gallon nursery containers planted three per pot and thinned to one per pot before treatment.(Photo by Derek M. Scroggs)

Roundup WeatherMax at 22 ounces per acre applied alone at 14 days after treatment.(Photo by Derek M. Scroggs)

Roundup WeatherMax at 22 ounces per acre plus TraFix Zn at 2 quarts per acre at 14 days after treatment.(Photo by Derek M. Scroggs)

Derek M. Scroggs, Alexander "Sandy" M. Stewart, Donnie K. Miller,B. Rogers Leonard, James L. Griffin and David C. Blouin

Production input expenses for crop producers continue to rise. To reduce these costs, particularly fuel and labor, growers can co-apply multiple pesticides and eliminate trips across fields. However, co-application strategies must recognize the concurrent need to control multiple pests.

For optimum performance of all products, the pest problems must occur simultaneously, and the treatment timings should coincide with the need to control multiple pests. With many of Louisiana’s crops tolerant to post-emergence applications of the herbicide glyphosate (Roundup Ready), the potential exists for glyphosate to become the primary herbicide in a co-application strategy for pest management. Products such as insecticides and plant growth regulators have been evaluated as co-application partners with glyphosate. In those trials, most products did not negatively affect weed control, and most proved to be beneficial when applied with glyphosate. Limited research has evaluated the potential for plant nutrients to be co-applied with glyphosate.

Past research has suggested no negative effects on weed control when boron, nitrogen or sulfur were co-applied with glyphosate. However, when manganese was co-applied with glyphosate, weed control was negatively affected. The plant micronutrient zinc had not been evaluated as a tank-mix partner with glyphosate.

Value of zinc
Zinc is important to plants as an enzyme activator and is involved in such plant processes as transporting carbon dioxide in photosynthesis, protein synthesis, starch formation and nitrogen uptake. Zinc deficiencies can occur in high pH soils, fields that recently have been leveled or areas with low organic matter. Symptoms of zinc deficiency can include yellowing of Blouin the leaves between the veins, reduced shoot growth (or stunting) and reduced leaf size. To overcome or even prevent zinc deficiencies from occurring, zinc sulfate or chelated zinc can be applied to the soil or to the plant foliage.

Because both glyphosate for weed control and foliar zinc could be used at the same time during the growing season, foliar zinc and glyphosate could be co-applied. The objective of this research was to observe and quantify glyphosate efficacy on selected weed species when co-applied with two commonly used zinc sulfate products.

Field trials
Field trials were conducted on cotton in 2006 and 2007 at the LSU AgCenter's Dean Lee Research and Extension Center in Alexandria. Three glyphosate products – Cornerstone Plus, Roundup WeatherMax and Touchdown Total – were each applied alone, co-applied with a 7 percent zinc sulfate product (RSA) or co-applied with a 10 percent zinc sulfate product (TraFix Zn). The three glyphosate formulations were applied at the rates of 32, 22 and 24 ounces per acre, respectively, and the zinc products were applied at the rate of 2 quarts per acre. Weeds evaluated were barnyardgrass, browntop millet and Palmer amaranth (pigweed). Weed control was evaluated at 7, 14 and 28 days after treatment (DAT) using a scale of 0-100, with 0 = no control and 100 = total weed death.

In 2006, when glyphosate was applied alone, control of barnyard grass was 97 percent, browntop millet was 97 percent and Palmer amaranth was 96 percent at 14 DAT (Figure 1). When RSA was included with glyphosate, weed control decreased to 63 percent for barnyardgrass, 64 percent for browntop millet and 64 percent for Palmer amaranth. Similar results were observed when TraFix was applied with glyphosate. Weed control decreased to 49 percent for barnyardgrass, 52 percent for browntop millet and 65 percent for Palmer amaranth.

The 2007 tests produced similar results. At 14 DAT glyphosate alone averaged 96 percent control of all weeds evaluated, but weed control was reduced by approximately 74 percent across all target weeds when the zinc products were included with glyphosate.

Pot study
A pot study was conducted twice in 2007 at the Dean Lee Center. All treatments were identical to those used in the field study. The target weeds consisted of barnyardgrass, browntop millet, johnsongrass, ivyleaf morningglory and redroot pigweed. Weeds were grown in trade-gallon nursery containers planted three per pot and thinned to one plant per pot before treatment. Applications were made to weeds when the plants were 3 to 6 inches tall and had 3 to 5 leaves. Visual weed control was estimated at 7 and 14 DAT using a scale of 0-100, with 0 = no control and 100 = total weed death. Additionally, all plants were harvested and weighed. Biomass from each plant was compared with the non-treated control plants. Therefore, plants with the least biomass were controlled the most by the treatment they received.

When glyphosate was applied alone, weed control for all weeds evaluated ranged from 82 to 98 percent (Figure 2), but weed control was reduced between 43 and 59 percent when zinc was added to glyphosate. Additionally, the greatest reduction in biomass (88 to 96 percent) occurred when glyphosate was applied alone. In co-applications of zinc with glyphosate, biomass was reduced by an average of only 41 percent.

Results of these studies suggest that glyphosate efficacy is negatively affected in co-applications with the foliar formulations and rates of the zinc products evaluated. These antagonistic effects were consistent across all glyphosate formulations and suggest weed control problems could exist with other plant nutrient products as well. None of the glyphosate labels used in this study suggest detrimental weed control effects from co-applications with micronutrients such as zinc. Therefore, growers should be aware of this potential for antagonism, and these co-applications should be avoided.

Derek M. Scroggs, Instructor, and Alexander "Sandy" M. Stewart, Associate Professor, Dean Lee Research and Extension Center, Alexandria, La.; Donnie K. Miller, Associate Professor, Northeast Research Station, St. Joseph, La.; B. Rogers Leonard, Professor, Macon Ridge Research Station, Winnsboro, La.; James L. Griffin, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; David C. Blouin, Professor, Department of Experimental Statistics, LSU AgCenter, Baton Rouge, La.

(This article was published in the summer 2008 issue of Louisiana Agriculture.)

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