Characterization of Foliar Symptom Progression and Final Efficacy of Paraquat on Problematic Weed Species
Paraquat is a common, non-selective herbicide for management of emerged weeds in numerous sites, including prior to corn or soybean planting for control of problematic winter annual weeds. Use of paraquat for control of herbicide-resistant weed species, such as glyphosate-resistant horseweed [Conyza canadensis (L.) Cronq.] has become imperative over the past few decades. However, applications of paraquat can result in inconsistent efficacy and the underlying factors associated with optimal or sub-optimal paraquat activity have not been well characterized. The primary objective of this research was to determine the most important application and environmental factors that influence paraquat efficacy and quantify the relationship of the speed of plant symptom development from paraquat activity with final herbicide efficacy. Greenhouse experiments were conducted to determine the relative efficacy and speed of symptomology for paraquat on select problematic weed species: Palmer amaranth (Amaranthus palmeri S. Wats.), tall waterhemp [Amaranthus tuberculatus (Moq.) Sauer], giant ragweed ( Ambrosia trifida L.), horseweed, and purple deadnettle ( Lamium purpureum L.). Palmer amaranth and tall waterhemp were the most susceptible species to paraquat with GR50 values of 11 and 8.4 g ai ha-1, respectively. Giant ragweed, horseweed, and purple deadnettle were less sensitive to paraquat with GR50 values of 21, 30, and 39 g ha-1, respectively. Symptomology from the 70 g ha-1 of paraquat evaluated was first observed at 2 HAT on Palmer amaranth, tall waterhemp, and giant ragweed, 4 HAT on purple deadnettle, and 6 HAT on horseweed. The greatest correlation between final efficacy and symptom development occurred when symptoms from paraquat occurred the quickest. Further greenhouse experiments were conducted to determine the influence of application time of day, light intensity, air temperature, and adjuvants on paraquat efficacy. In most cases, speed of initial activity from paraquat did not translate into greater final efficacy. In some cases in purple deadnettle, faster symptom development resulted in greater final efficacy than paraquat symptoms developed slower. In only one instance under low light conditions on tall waterhemp, slower initial symptom development from paraquat resulted in greater final efficacy than symptoms that progressed more rapidly. Thus, the extent of final paraquat efficacy was not predictable across all weed species and experimental factors based on the speed of initial plant response to paraquat. Field trials were conducted in 2014 and 2015 to investigate the influence of application time of day for paraquat or the addition of metribuzin on overall efficacy on horseweed. In 2014 at 1 day after treatment (DAT), paraquat plus metribuzin applied to 15 to 20 cm horseweed plants exhibited less visual injury (17 to 38%) than plants that received paraquat alone (61 to 66%) across both application times of day. By 3 DAT both paraquat and paraquat plus metribuzin treated plants were equal in visual control (77 vs 76% at solar noon and 74 vs 73% at sunset, respectively). However, by 28 DAT horseweed plants sprayed with paraquat plus metribuzin exhibited greater control (98% dry weight growth reduction) versus paraquat applied alone (70 to 87% growth reduction). In 2015, the paraquat plus metribuzin mixture resulted in less visual control at 1 DAT compared to paraquat alone (47 to 48% vs 73 to 86%, respectively). However, by 28 DAT, greater than 93% control was achieved with paraquat or paraquat + metribuzin regardless of application time of day. Based on this research we conclude that paraquat efficacy is species dependent, a factor that has not been considered on the federal label and recommendations for paraquat use. The speed of plant symptom development from paraquat did not consistently relate to the extent of final paraquat efficacy. Rapid progression of plant symptoms from paraquat were generally associated with greater levels of final herbicide efficacy. The addition of metribuzin to paraquat on horseweed slowed the speed of herbicide efficacy, but ultimately resulted in similar or greater final herbicide efficacy than paraquat applied alone. Thus, the additivity (and synergy) observed with this herbicide mixture would be best explained as a joint contribution from each herbicide, rather than metribuzin just slowing paraquat efficacy, which may enhance final efficacy from paraquat. This is contrary to previous research that proposed the delayed activity of paraquat allowed for potentially more translocation in plants. An overall greater understanding of how paraquat efficacy can be optimized for the applicator on individual weed species will help reduce inconsistent efficacy and allow paraquat to serve a greater role in modern weed management systems that are focused on herbicide-resistant weed biotypes.
Young, Purdue University.
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