Research Shows Better Control

Using different adjuvants with Roundup

Published online: Apr 01, 2011 Feature Don W. Morishita
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Growers often ask if there are differences among the many different nonionic surfactants (NIS) and proprietary blends of NIS and ammonium sulfate (AMS) that can be used with glyphosate for weed control in Roundup Ready sugarbeets. We began comparing a few of the adjuvants two years ago and did not see any difference in the products we tested. In 2009, we compared Roundup PowerMax applications without any additional adjuvant to Roundup that included different AMS rates, AMS + Coverage G-20, AMS + In-Place, and Wet Sol. The AMS rates we used were 6-, 11-, and 17-lb AMS/100 gallons of spray. All of these comparisons were made with Roundup applied at 22 fluid ounces per acre. Infiltration We did not see any difference in weed control or sugarbeet yield with the different AMS rates or with the different spray adjuvants. Coverage G-20 and In-Place are drift-reduction agents. Wet Sol is a nonionic surfactant that is used as a spreader/sticker and soil conditioner. This product was added to a tank mixture of Roundup, Outlook and different AMS rates. One of the reasons Wet Sol was included with the Roundup and Outlook was to see if the Wet Sol influenced crop injury potential as well as weed control with the Outlook, since it is a soil-active herbicide. Our question was if Wet Sol increases water infiltration, could it increase soil-active herbicide infiltration too? Wet Sol does not appear to affect soil-active herbicide movement into the soil. In other studies we have done with soil-active herbicides and Wet Sol, we have not seen any increase in injury potential. Spray Adjuvants In 2010, we again looked the effect of different AMS rates and spray adjuvants used in combination with Roundup (Table 1). Each of these adjuvants has similar ingredients, but also differ slightly depending on the manufacturer. It was decided that we look at these different adjuvants using a lower Roundup rate because, we learned, this can help show differences in weed control between the adjuvants. Roundup was applied with each of these products at rates of 11 and 22 fluid ounces per acre. Field studies conducted at Kansas State University recently showed marked differences in weed control with a below-label Roundup rate using different spray adjuvants. Their studies showed how weed control varied from one weed species to another with the different spray adjuvants. Weed Species In our study, none of the treatments injured the crop (Table 2). As you might expect, Roundup applied at 22 fluid ounces per acres with or without any of the adjuvants tested had better overall weed control than Roundup applied at 11 fluid ounces per acre. However, some weed species such as redroot pigweed, hairy nightshade, and green foxtail were effectively controlled with 11 ounces of Roundup with or without an adjuvant (data not shown). This indicates how sensitive these three weed species are to glyphosate. The response of other weeds, including kochia, common lambsquarters and Russian thistle to 11 ounces of Roundup were quite variable, depending on the adjuvant used. Kochia control eight days after the last application (DALA) ranged from 89- to 91-percent with 11 ounces of Roundup without any adjuvant, or with Class Act NG, Alliance, or the combination of Bronc Max + R-11 + Coverage G-20. However, kochia control was reduced with the addition of Bronc at 3 or 9 pounds per 100 gallons of spray to Roundup. By 86 DALA, which was just before harvest, kochia control with the addition of Bronc at 3 and 9 lb had dropped to only 53- and 75-percent, respectively. Using Alliance, Class Act NG or no adjuvant controlled kochia the best with the reduced Roundup rate. DALA For common lambsquarters control eight DALA, the best adjuvants were Alliance and Class Act NG. Not using any adjuvant was as good or better than all of the other adjuvants tested. Common lambsquarters control just before harvest was best with Alliance followed by Bronc Plus Dry EDT. Russian thistle control 8 DALA without any adjuvant was among the lowest at 68-percent. Bronc at 3 or 9 pounds, Bronc Plus Dry EDT, Class Act NG, and Alliance had the best Russian thistle control. At the evaluation just before harvest (86 DALA); Russian thistle control was best with Alliance, Class Act and Bronc at 9 pounds per 100 gallons of spray. When weed control is compared among these same adjuvants using the full Roundup rate of 22 fluid ounces per acre, the differences with these adjuvants disappears. Kochia control with all of these treatments ranged from 89- to 95-percent eight DALA and 98- to 100-percent 86 DALA. Common lambsquarters and Russian thistle control responded similarly to the full Roundup rate. At 8 DALA, common lambsquarters and Russian thistle control ranged from 91- to 99-percent and 88- to 94-percent with all treatments, respectively. This was consistent with the weed control evaluation 86 DALA, which was just before harvest. Bottom Line Sugarbeet root yield and recoverable sugar yield with all of the adjuvants applied with Roundup at 22 ounces were statistically equal and ranged from 29 to 34 tons per acre. These results are similar to what we have seen in previous studies where we made all our comparisons at the labeled Roundup rate of 22 fluid ounces per acre. It is interesting that some species are easily controlled without an adjuvant while some species are controlled better with an adjuvant. The bottom line is that addition of AMS and a surfactant can increase the consistency of controlling a broad spectrum of weeds. Editor's note: Morishita is University of Idaho Professor of Weed Science and Extension Specialist, he can be contacted at 208-736-3616 or email: don@uidaho.edu. Sidebar: Table 1. Description of adjuvants tested with Roundup in 2010. Table 2. Crop injury, weed control and sugar beet yield with Roundup and various adjuvants, near Kimberly, ID1.