Dicamba, 2,4-D, and glufosinate tolerance for soybeans and cotton have opened a new world of opportunity for using these herbicides for post-emergence weed control. However, these opportunities also introduce challenges with antagonism when multiple herbicides are tank-mixed.
Herbicide antagonism occurs when a tank mix of multiple herbicides results in weaker weed control vs. the individual herbicides applied alone (Colby, 1967). Herbicide antagonism has been documented for many decades and will likely become more challenging as tank mixes become more complex. Since the release of dicamba, 2,4-D, and glufosinate tolerant crops, these herbicides have been shown to antagonize grass-control herbicides like clethodim (Merrit et al., 2020; Meyer et al., 2020). Adjuvants have proven to help overcome antagonism, as shown in studies by Penner (1989) and Bernards et al., 2017.
Further investigations on how adjuvants can minimize antagonism are critical in today’s agriculture, where weed resistance drives application costs and jeopardizes crop yield.
Figure 1: Volunteer corn control from Select Max and Enlist One tank mixtures in Sharon, WI. The number above the column is percentage control. Lowercase letters indicate significant differences at the alpha=0.05 level.
*Contains patented Penetration Factor technology 21 DAA: LSD = 6.63; P = 0.0001
Figure 2: Volunteer corn control from Select Max and Enlist One tank mixtures in Winamac, IN. The number above the column is percentage control. Lowercase letters indicate significant differences at the alpha=0.05 level.
*Contains patented Penetration Factor technology 21 DAA: LSD = 6.63; P = 0.0001
Exacto conducted research studies in Sharon, WI and Winamac, IN to evaluate clethodim antagonism by 2,4-D. The results showed that adding Enlist One® (2,4-D choline, 32 fl oz/ac) to Select Max® (clethodim, 4 fl oz/ac) significantly reduced volunteer corn control in Sharon, WI (Figure 1) and Winamac, IN (Figure 2). At both sites, increasing the Select Max rate up to 8 fl oz/ac overcame the antagonism. However, adding oil-enhancing adjuvants to the tank mix overcame the antagonism with the lower rate of Select Max at both sites (Figure 3).
Figure 3: Volunteer corn showing signs of control with Select Max (4 oz/ac) applied alone (top left). When Enlist One was added to the tank mix, Select Max no longer provided control (top right). Adding an adjuvant overcame the antagonism with the 4 oz/ac rate of Select Max (bottom left). Increasing the Select Max rate to 8 oz/ac also overcame the antagonism (bottom right), but might be more costly.
Do not skip the adjuvant in your post-emergence weed control applications, especially when antagonism could occur. Adding adjuvants can help the application reach the target, help it affect the target pest more effectively, and help overcome antagonism from complex tank mixes.
While increasing the clethodim rate overcame antagonism in these studies, more herbicide results in costly applications and overuse of active ingredient. While active ingredient rates can be increased within a given label’s range, there is a limit to how much those rates can be increased. Over time, if weed populations become more and more tolerant to a given active ingredient, increasing the rate will be less and less effective.
Successful applications are dependent on many factors: tank mix water properties, compatibility of tank-mix components, spray equipment and settings, environmental conditions at the time of application, application timing, and weed ecology of a given field. Adjuvants provide a valuable tool to get the most out of every application and help applicators get more consistent results in the face of increasingly complex tank mixes and challenges.
References:
Bernards, M., B. Young, G. Obear, and F. Sexton. 2017. Non-AMS adjuvants effect on dicamba + clethodim tank-mixture antagonism in control of Palmer amaranth and Volunteer corn. WSSA Annual Meeting Abstract.
Colby, S.R. 1967. Calculating synergistic and antagonistic responses of herbicide combinations. Weeds. 15:20-22.
Merrit, L.H., J.C. Ferguson, A.E. Brown-Johnson, D.B. Reynolds, T. Tseng, and J.W. Lowe. 2020. Reduced herbicide antagonism of grass weed control through spray application technique. Agron. 10:1131. https://doi.org/10.3390/agronomy10081131
Meyer, C.J., J,K. Norsworthy, and G.R. Kruger. 2020. Antagonism in mixtures of glufosinate + glyphosate and glufosinate + clethodim on grasses. Weed Tech. 35:12-21.
Penner, D. 1989. The impact of adjuvants on herbicide antagonism. Weed Tech. 3:227-231.
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