I wrote last week on managing heavy glyphosate-resistant (GR) Palmer amaranth infested fields in a corn rotation. Several have commented that this leads to the next question of how to manage these fields going back into conventional, Roundup Ready or Liberty Link soybeans. Of course, the best option is to rotate to corn but that may not be feasible.
In looking at growing conventional soybeans in a heavily infested Palmer pigweed field, the basic idea is to overlap residual herbicides starting 30 to 15 days before planting. Good choices here would be dicamba plus glyphosate plus 3 ozs/A of Valor or 5 ozs of Verdict (Sharpen + flufenacet). This tankmix should control any GR horseweed that is up plus provide enough residual for about 20 days. Then within a couple days plus/minus of planting, apply Gramoxone + metribuzin or Boundary (S-metolachlor + metribuzin). It is absolutely critical that the soybeans emerge without GR Palmer and that is why Gramoxone is needed with near planting applications. Then about 10 to 14 days after planting come back with a fomesafen based product. I prefer Prefix (S-metolachlor + fomesafen) at this timing. Provided you get some timely rains that activate these herbicide applications and a good soybean stand that can shade the soil early, weed control should be in pretty good shape. However, if no precipitation occurs to activate the near planting and early post applications, you may end up with a lot of Palmer emerging with the soybeans and no recourse other than some kind of mechanical control measure or, in some cases, replant.
The Roundup Ready soybean system is very similar to the conventional system. It has to be as the conventional herbicides have to control the Palmer. Therefore start out 30 to 15 days before planting with the glyphosate+ dicamba+ Valor or glyphosate + Verdict burndown then, near planting Gramoxone + a metribuzin based herbicide and finally follow up with Prefix + glyphosate 10 to 14 days after planting. Of course, glyphosate will not help us on the GR Palmer but in fields with other weeds like morningglory complex, prickly sida, etc., glyphosate can help. Like the conventional program, timing is everything on how well the Palmer will be controlled. The timing of precipitation around the herbicide application as well as not spraying Palmer that is over 3” tall with fomesafen will dictate the success or failure of the Palmer amaranth control in these fields.
Liberty Link soybeans are the best choice to be planted in fields with intense Palmer amaranth seed banks. I say this with some concern, as utilizing Liberty right out of the gate in fields with a lot of Palmer increases the odds of finding that rare Palmer that is resistant to Liberty. As a result we must utilize a lot of the herbicides outlined above in the conventional and Roundup Ready system to keep Liberty active on Palmer for as long as possible. Therefore, start out 30 to 15 days before planting with the glyphosate + dicamba + Valor or glyphosate + Verdict burndown then, near planting Gramoxone + a metribuzin based herbicide and finally follow up with Dual Magnum + Liberty 10 to 14 days after planting. If you are able to apply Liberty on Palmer prior to it getting 4” tall, it will control it. However, if it is more than 4” tall, plan to come back 7 to 10 days after the first application with a follow-up application of Liberty to control the regrowth. This is the biggest difference between the Liberty Link system and the conventional or Roundup Ready system. In those systems, if Palmer grows back from a fomesafen Post application, no follow-up herbicide application will control it; where a 29 to 36 oz application of Liberty will control regrowth provided the regrowth is not coming from 10 + inches Palmer.
In all these systems timing is everything. The Liberty system will be the most forgiving, but even it has its limits particularly if you are delayed on a sequential application. That is why corn is still the best option to plant into fields that had heavy Palmer pressure last year.
Larry, I highly beg to differ with putting any glyphosate in any of the programs, at least in our case, it’s pretty well useless. You could dump the stuff straight out of the jug, and it’ll do nothing for morningglory, it’ll do no more than burn the top out of sicklepod, and is also useless on Johnson grass. Other products need to be picking up these weeds along with the Palmer. Select is going to be product of choice for anything in the grass area, both pre and post.
Monsanto might not like it that their product is getting kicked out of the program, but there is no use in paying for the product, or the tech fees related, when the product has no value. We are going to have to realize the “easy days” that developed this mess are over, and that one had better start using what’s between the ears figuring where the products we currently have in the arsenal are going to help solve the problem.
Chris
You make a good point. There is no doubt the value of glyphosate in a program is greatly deminished. This is particularly true for those fields that have almost become a mono culture of glyphosate-resistant Palmer.
However, over the last couple years as we rate 100s of plots we consistently find that we only get about 60% control of weeds like morningglories with herbicides like Valor or even Prefix. The glyphosate added into one of these systems does improve the control into the low 90s typically. Moreover, it is good to use glyphosate from time to time to at least delay resistance developing in weeds to herbicides like Select.
The bottom line is I hate throwing out any potential weed control tools. We need to try to keep them all in the mix where possible. Diversity is the key to long term weed control. No one plants all 1 soybean variety across all their fields to spread their risk. We need to use the same concept with weed managment. By going whole hog to any one herbicide or technology is not spreading your risk of resistance developing. Continually use different technologies where you can along with cultural practices is the only chance we have to manage weeds particularly Palmer long-term.
Larry I think the metribuzin or Boundry are a good choice where Palmer Pigweed infestations are moderate, but in Tennessee Counties that touch the Miss. River it may not be enough. We have had good results with Authority XL at 4 oz. + 1/4 to 1/3 lb. Metribuzin DF based on soil type & OM or Authority MTZ at 10 oz. and spike the Metribuzin up to get desired rate. At 14 days after planting we go with 32 oz. Prefix if we miss our timing and Pigweeds get to 4″ we go 71 oz. Flexstar GT + MSO. Authority XL has also done well on Morning Glories and also Gaint Ragweed. Must watch plant back restrictions but have not had any problems at these low rates on Cotton which is the olny concern for us in these counties. We also go with the early burn down of RR/Dicamba/Valor. It’s almost impossible for one shoe to fit all so I always go on a field by field reccomendations, but we don’t have many field that don’t concern me when it comes to Pigweed. I have also seen a lot of help from RR on MG, Sicklepod if done early before they are too large. Timeing is everything when it comes to Pigweed control, Spraying will have to come first on most farms if we are to succeed in the battle to manage Pigweeds. There will have to be days when the planter is stopped to spray.
Steve
You made some good suggestions and comments. You are correct for fields with intense Palmer pressure Authority spiked with metribuzin would be a good choice. Like you I have not seen Authority carryover be an issue in cotton. However, we do need to be mindful of that plant back restriction because with late soybean planting coupled with a dry summer it could be a problem. You also made a great point about stopping the planter to spray. This caught a number of folks last year who were harvesting wheat or planting soybeans while the Palmer in their early planted soybeans grew from 1 to 6″. Some of these fields had to be disked up and replanted because the Flexstar application was a few days late.
Thanks for the comments,
Larry