Control of Multiple-Herbicide-Resistant Green Pigweed (Amaranthus powellii) with Preemergence and Postemergence Herbicides in Ontario Soybean Production
and Peter H. Sikkema 1Round 1
Reviewer 1 Report
Herbicide resistant weeds are great threatens in soybean production. The management of resistant weeds is one of the hotspot in herbicide resistance fields. This manuscript focused on the chemical management of the multiple herbicide resistant green pigweed in soybean field. The manuscript was properly designed, conducted and written. The results were helpful to the efficacy management of this resistant green pigweed. Background, research design and results were clearly described and presented. My comments are as follows.
Materials and Methods
Please briefly introduce the main weather information of the day spraying herbicides.
Please tell readers the soybean injury was investigated by one or more person. If the injury was evaluated by more than one person, how to avoid deviation?
Table 1 In 2019 experiment, soybean was planted in 8 June and emerged in 6 June and PRE herbicides were sprayed in 11 June. It seemed unreasonable. Please check it.
L134 It seemed that the spray volume was lower, especially in POST treatment. Please tell readers the spray volume used locally.
Results and Discussion
Several PPO inhibitors were investigated in the section of POST herbicide study. Due to the light dependent characterization of this class herbicide, it would be better to clarify the weather information of several days after treatment.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The overall objective of this work appeared to show what herbicides can be used to control a herbicide-resistant biotype of Amaranthus powellii selectively within soyabean crops. I found some aspects of this paper a little frustrating. Nothing was mentioned about the history of the fields in which this work was conducted, unless I missed seeing it. The assumption appeared to be made that all the A. powellii present in these crops had the herbicide resistance. It would be good to see something mentioned about why it was thought all of the weed present had this resistance.
Nothing appears to have been published about this resistance from what I could see within this paper. It would have been good to see just how resistant this biotype is to the various herbicides to which it is resistant, especially the imazethapyr. If it has only just been confirmed in 2021, how do we know how much of these resistant weeds were present in these fields used for the work? Presumably the poor control in all experiments by the imazethapyr is the proof required that the resistant biotype was present, though it seems strange if this is a recent phenomenon that such a high percentage of the weed present was resistant. I am not familiar with how effective imazethapyr is normally against A. powellii, but it appears from the discussion it should have been controlled both from pre-emergence and post-emergence applications.
If information was wanted on just which herbicides were no longer effective against the biotype, surely a series of pot experiments under controlled conditions in which the dose response curves of the resistant biotype were compared with a susceptible biotype for each of the herbicides of interest would have been more constructive. Instead this series of field experiments gave some results but led to statements at the end that more research is needed for the closely related herbicides for which variable results were obtained. Doing the dose response curves initially would have given answers more rapidly.
But given that imazethapyr gave poor results in all experiments, one has to assume the resistant biotype was present and so the control of this biotype selectively within the crops grown is useful information for determining how to manage the problem. The work was otherwise conducted in a sound manner.
Note that tables should “stand alone” from the main text, so there needed to be some reference either in the table caption or footnote as to when the density and biomass figures were obtained relative to the WAA data on weed control and the soybean yield data. No mention was made of whether other weeds were present that could affect the results. Grasses were removed using the quizalofop, but were other weeds present apart from the A. powellii that could have affected results? My other query is about the statement in the 2nd line of the abstract. It states that A. powellii can reduce soybean yield by as much as 49%, yet references in the introduction never mentioned reductions of more than 16% for this crop. So was the 49% derived from the current trial results? And if it was, is it certain this was caused only by A. powellii?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
