Evaluation of glyphosate and 2,4-Dichlorophenoxyacetic acid combinations to control glyphosate-resistant hairy fleabane [(Conyza bonariensis (L.) Cronquist]

Abstract

Herbicide resistance has become a global threat to sustainable weed management. It is an avoidable consequence of continuous and repeated use of herbicides with single modes of action, use of unregistered mixtures and use of higher or lower recommended rates. Selection pressure on weeds has been exerted as farmers battle to combine weed management strategies. To date 509 cases in 266 weed species have been reported to have developed herbicide resistance globally and of these 51 to glyphosate (Heap 2021). Glyphosate resistance was first reported in 1996 in an apple orchard. Since the development of glyphosate, more than 71 different countries have reported glyphosate resistance, and the number of glyphosate resistant weed species is expected to increase. Three species in South Africa have developed glyphosate resistance namely: Buckhorn plantain (Plantago lanceolate L.), rigid ryegrass (Lolium rigidum) and hairy fleabane (Conyza bonariensis). Glyphosate has become one of the leading herbicides in the markets since its introduction in 1974. It is used on 180 weed species in over 40 countries. It is the only herbicide that inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), a nuclear- encoded, chloroplast-localized enzyme in the shikimic acid pathway of plants and micro-organisms. In the last decade weed researchers have been advocating for herbicide combinations (with different modes of action) as one of the strategies to mitigate or even delay glyphosate resistance (GR). Glyphosate is often combined with 2,4-D or dicamba, to control broadleaf weeds including glyphosate-resistant Conyza bonariensis populations that are hard to control with glyphosate alone. 2,4-D is a systemic herbicide that has been in use since 1940. It controls broadleaves and causes uncontrolled, abnormal growth and results in plant death. Several studies have investigated the possible interaction of the 2,4-D and glyphosate. Although the combination has proven to be effective, SA farmers use various doses of glyphosate and 2,4 D and the only registered mixture for many years is the 2 L ha-1 Roundup® Turbo (RT) with 2 L ha-1 2,4 D. What is unknown is whether doses at rates lower than the lethal dose of 2,4-D in combination with glyphosate at the recommended rate can overcome GR in C. bonariensis. The objectives of the study were to i) determine whether the addition of lower concentrations of 2,4-D to the recommended dose of glyphosate can overcome glyphosate resistance (GR) as a result of herbicide-induced selection, ii) the influence of growth-stage on GR in C. bonariensis, (iii) to measure shikimic acid levels in a glyphosate-susceptible and resistant populations over time with High Performance liquid chromatographic (HPLC) analysis. All experiments were pot trials and were conducted at the xi phytotron facilities at the University of Pretoria’s Experimental Farm in Hatfield, Pretoria. Two populations of C. bonariensis were used in the study; Piketberg (glyphosate-resistant) and Hatfield (sensitive to glyphosate). The results indicated that acceptable control of the glyphosate resistant biotype could be achieved by adding 2,4-D to the recommended rate of glyphosate. The combination treatments performed significantly higher 1x glyphosate + 0.5x 2,4-D, 2x glyphosate+ 0.5x 2,4-D and 1x 2,4-D. The treatment 1x 2, 4-D (recommended rate) completely killed the GR Conyza bonariensis and these results did differ statistically from the two combination treatments. The treatment 1x glyphosate and 2x glyphosate had the least control, at 36% and 60 %, respectively. The results confirmed resistance on the Piketberg biotype as the recommended glyphosate dose could not kill the biotype. 2,4-D alone did not perform any better than the combination treatments excluding 1x glyphosate+0.75x 2,4-D and 1x 2,4-D (1L ha-1) treatments. The results in both dose response experiments suggest that there is a benefit in adding 2,4-D to glyphosate as it enhances the performance of glyphosate. The results further indicated that increasing the rate of 2,4-D in the glyphosate mixture to 1x glyphosate x 1.25x 2,4-D and 1.5x x 2,4-D did not significantly increase control. The combination of glyphosate and 2,4-D at (0.25x or 0.25 L ha-1) could provide effective control of GR Conyza bonariensis biotypes in South Africa. The three-way interaction of population, herbicide treatment and growth stage had significant (P<0.01) effect on percent control of glyphosate- resistant C. bonariensis. At the 2-8 leaf growth stage, when glyphosate was either applied individually or combined was most effective 0.25x, 0.5x and 1L) on both populations (Hatfield and Piketberg) with a control of 95 to 100%. Growth stage was shown to play an important factor in the control of both GR as well as sensitive C. bonariensis populations, with the level of control decreasing as plants mature. There was a significant difference between the two growth stages 2-4 and 8-15. There were also no significant differences between the Hatfield (GS) and (GR) biotypes with shikimic acid accumulation in response to time when treated with the combination treatments 1x glyphosate + 0.25x and 0.5x. In both treatments, shikimic acid decreased in the Piketberg biotype following treatment, hence indicating resistance. A similar pattern in shikimic acid accumulation was observed in the GS biotype. Moreover this combination will control both the grasses and broadleaves in one run compared to sole 2,4-D treatment. The results show that glyphosate resistance can be overcome by the combination 1 x glyphosate + 0.25 x 2,4-D at correct timing of application xii of 2-4 leaf stage. The study has also confirmed that timing of control is key in weed management