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Current Challenges for May 2019: Large Weeds, and PPO Damage


This blog is going to be a little bit of a mashup of topics covering some of the more critical issues we’ve been seeing in the field. Extended rain has made field work impossible, leaving not only planters sitting in the shop, but sprayers as well. Consequently, we’ve have some optimum conditions for weed growth, and no way to control them. This blog post will cover some of the options for controlling larger size weeds as well as adjusting herbicide strategies on the go. We’ve also been seeing some soybean seedling diseases and herbicide injury. We will touch on what this issue looks like in the field, and what management strategies you can employ to mitigate them.


Weed Size and Herbicide Management Strategies

With our latest rain delay, some fields that should have gotten sprayed still haven’t, and unfortunately, the weeds haven’t stopped growing to give us more time. This has resulted in some new flushes of weed seedlings, as well as some growth of weeds to sizes larger that we like to manage.


Our window where weeds are the most competitive with corn or soybeans will depend on many factors and is hard to pinpoint generally. (Depends on row spacing, planting date, environmental conditions, etc) Some sources say that if weeds are 3-4 inches tall when the corn is at V3-V4, that is one of the most competitive stages. At this point, if the weeds are not controlled, you start to lose 3 bushels per acre per day. Additionally, the weeds begin to sequester N that you won’t get back until the next growing season. This could result in a shortfall of available N to the crop.


You are at particular risk if you didn’t get a fall or spring burndown with some residual on. This means we are putting our entire weed management program on the shoulders of a post emerge application that has been delayed longer that it likely should. If you find yourself in this situation, consider the herbicide mix you are using and add in another mode of action. This will help ensure we have a potent enough mix of chemical with enough variety of modes of action to ensure we get a good kill.


Maybe you find yourself in this situation: you did get a burndown on with some residual. Great! However, with this extended amount of rainfall, it is possible your residual will not be as effective anymore. Residual herbicides can be leached from the profile based on the solubility of the herbicide, as well as how strongly attracted the herbicide is to soil particles. Herbicides like Princep, Prowl, and Treflan are less likely to leach because of their low solubility and tightly bound nature. Herbicides like Dicamba, Sencor, Tricor, and Stinger are more likely to leach when we have extended rainfall and ponding like we’ve just experienced. If you’ve used one of these more soluble herbicides, check your fields routinely to determine the amount of control they are keeping over the weeds. Be aware that it may not be as long as normal, and have a post emerge plan in place for when control breaks.


Finally, your weeds have gotten a lot larger than you would like. We need to consider several things before we move forward with application.

--Are they still on label? We need to be VERY careful to not apply off label or too low of rates on large plants. This is just setting us up for low level resistance of weeds. We have enough problems with weed control—we don’t need to make more problems for ourselves.

---Our goal is always to have a higher concentration on smaller plants. That is why we rarely have resistance issues with residual herbicides. We are treating the smallest sized plant possible with a heavy dose of herbicide. Although we are already past this point of treatment, the principle still is the same. Get in the field as soon as you can to treat the weeds before they get any bigger.

--Its not likely that we are going to be getting any new herbicides any time soon. So we need to rely on the best management practices that we can. Work hard to make sure all components of your spray program are correct---rates, droplet size, timing, etc.

--Escapes are a big deal, whether you like it or not. One missed weed produces enough seed to cause a big problem in a short amount of time. Keep your eye out for escapes—and get out there and pull them if you have to. A general principle: never let the weeds get a head start.

--Like I mentioned before, add more herbicides to the mix. Talk with a member of the PSS team about including another mode of action, or changing up your current plan.

--One of the most important practices with larger weeds is to Improve coverage of the target. This can be done through several ways:

o Apply higher spray volumes. (increase your water amount)

o Apply at slower speeds

o Use nozzles that will create a smaller range of droplet sizes

o Lower your boom height


Talk to one of our Agronomists to make sure you are prepared for a game time decision. Be flexible—and you will likely have a better weed control experience this year.


PPO Damage


We are seeing some PPO damage to soybeans this year. Just a reminder, PPO inhibitors are Group 14 herbicides. Some common group 14 herbicides we use are Authority, Envive, Fierce, Trivence, Valor, and Sharpen. We don’t often see damage from them, and they often get confused with damage from seedling diseases. Damage is more common in cool, wet soils, just like the conditions we’ve been having. Normally, a soybean seedling will just metabolize any of the herbicide it encounters, however with the current conditions, the plant is unable to keep up with the higher doses that have been brought near the germinating seed from the rains and the slower growth by the soybean seedling itself.


PPO damage to Soybeans. Photo from UNL CropWatch

PPO damage on soybeans will show up as a reddish brown hypocotyl and brown spots on the cotyledons. Check the plant, and if the cotyledons are still mainly green and the neck is firm, the seedling will be okay. Additionally, if the cotyledons are green, the neck is brown, but still fairly firm, these plants will also likely be okay. If you see girdling around the hypocotyl or the apical bud is damaged, the seedling will not likely live.


With warm temperatures and dryer conditions we should see rapid recovery of damaged seedlings. Contact a member of the PSS team to check your soybeans if you have any concerns.


Resources

https://www.farmprogress.com/issues/5-tips-corn-weed-management-start-clean-field-then-control-weeds-early-they-reach-4-inches

https://www.farmprogress.com/corn/corn-weed-control-challenges-successes-2015

https://cropwatch.unl.edu/2017/effect-excessive-rainfall-efficacy-residual-herbicides-applied-corn-and-soybean

https://www.farmprogress.com/resistance-management/managing-weeds-future

https://crops.extension.iastate.edu/cropnews/2014/07/controlling-large-weeds-do-you-feel-lucky

https://crops.extension.iastate.edu/managing-weeds-protect-crop-yields

https://cropwatch.unl.edu/2017/ppo-inhibiting-herbicides-and-soybean-seedling-injuries

https://crops.extension.iastate.edu/cropnews/2017/05/evaluating-herbicide-injury-soybean

https://www.ilsoyadvisor.com/on-farm/ilsoyadvisor/diagnosing-ppo-damage

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