Relating initial paraquat injury to final efficacy in selected weed species influenced by environmental conditions

Abstract

Weed control of paraquat can be erratic and may be attributable to differing species sensitivity and/or environmental factors, of which, minor guidance is available in commercial labels. Therefore, the objectives of this research were to quantify selectivity of paraquat across select weed species and the influence of environmental factors. Experiments were performed under controlled conditions in the greenhouse and growth chamber. Compared to purple deadnettle (GR50 = 39 g ai ha-1), waterhemp, Palmer amaranth, giant ragweed, and horseweed were 4.9, 3.3, 1.9, and 1.3 times more sensitive to paraquat, respectively. The injury progression rate over 3 d after treatment (DAT) was a more accurate predictor of final efficacy at 14 DAT than the lag phase until symptoms first appeared. For example, at the 17.5 g ha−1 dose, the injury rate of waterhemp and Palmer amaranth was, on average, 3.6 times greater than horseweed and purple deadnettle. The influence of various environmental factors on paraquat efficacy was weed-specific. Applications made at sunrise improved control of purple deadnettle over applications at solar noon or sunset. Lower light intensities (200 or 600 μmol m-2 s-1) surrounding the time of application improved control of waterhemp and horseweed over 1,000 μmol m-2 s-1. Day/night temperatures of 27/16 C improved horseweed and purple deadnettle control over 18/13 C. Though control was positively associated with injury rates in the application time of day and temperature experiments, a negative relationship was observed for waterhemp in the light intensity experiment. Thus, while there are conditions that enhance paraquat efficacy, the specific target species must also be considered. These results advocate paraquat dose recommendations, currently based on weed height, be expanded to address sensitivity differences among weeds. Moreover, these findings contrast with paraquat labels stating temperatures of 13 C or less do not reduce paraquat efficacy.

Looking beyond Glyphosate for Site-Specific Fallow Weed Control in Australian Grain Production

“…This lack of efficacy was more evident when environmental conditions were less conducive for herbicide uptake and activity. For both paraquat and PPO inhibitors, the lower light conditions experienced in S. oleraceus-R1 (Table 2) potentially decreased the immediate destruction of leaf and vascular tissue, improving herbicide mobility [34,35] and, ultimately, efficacy (Table 4). Conversely, the higher light conditions in S. oleraceus-R2 likely limited the efficacy of both paraquat and PPO inhibitor herbicides.…”

“…Overall, effective control (<10% survival) was only achieved in eight instances, seven of which were in S. oleraceus-R1 (glufosinate-ammonium, paraquat, saflufenical, saflufenical + trifludimoxazin and paraquat + PPO inhibitor mixtures), and also glyphosate in C. virgata-R2 (Table 4). Differences in herbicide efficacy between experiments for C. virgata and S. oleraceus were likely due in part to contrasting humidity and cloud cover conditions between respective herbicide application timings that likely lead to variations in herbicide uptake and translocation [33][34][35]. Clearly, though, the evaluated herbicides and application rates would be not suited for routine use as spot-spraying treatments on older/larger weeds.…”