Episode 6 - Fall Weed Control

Arkansas Row Crops Radio, providing up to date information and timely recommendations on row crop production in Arkansas.

Tommy Butts: Welcome to Arkansas Row Crops Radio. My name is Tommy Butts, Extension Weed Scientist for the University of Arkansas System Division of Agriculture. 

In today's episode, I'd like to discuss fall weed control strategies. Fall weed control strategies can help us to combat several different problematic weed species in Arkansas agriculture as a part of our overall weed management programs. 

The first weed species that I'd like to discuss that tends to be problematic where fall weed control measures can help us in mitigating some of our future problems in future cropping system years is actually late germinating Palmer amaranth or Palmer pigweed. We actually saw this year some germinating pigweed all the way through the beginning of October, the first couple of weeks into October. We've seen it even at one inch tall, these late germinating Palmer plants can produce a seed head, and those seed heads still produce a thousand to a couple thousand seeds on each one of them. Now that thousand or two thousand seeds produced on that small seed head is a drastic reduction compared to the half a million to the 1 million seeds that are produced on a full season Palmer amaranth plant, but if we're still releasing a thousand to two thousand seeds back to our seed bank from some of these late-germinating Palmer plants, that still leads to future problems because of all those seeds that are still laying in our seed bank waiting for us for the next year. If we can eliminate some of those plants in the fall and reduce those seeds returning to the seed bank, it can be very beneficial for our future cropping years. 

Now, some of the research has shown that it only takes about nine days for seeds to become mature following the beginning of the reproductive stage. In about nine days once you start to see that seed head, you can have full mature seed on a Palmer amaranth plant. To get to that reproductive stage, it usually takes approximately 17 to 21 days, especially in the fall like now, to get into that reproductive form after emergence. In about 30 days, we can go from emergence out of the soil to having full mature seeds on one of these Palmer amaranth plants. For example, where we were seeing some of these plants that were already emerged at the beginning of October, let's say they've been emerged for two weeks, in two more weeks, so at the end of October, like we are right now, there should be full mature seed on those plants if they have not been controlled. It can be pretty critical to make sure we're managing those in the fall and not letting those seeds make it back to the seed bank.

Now we do have a couple of different options. We can use tillage, or we can also use some of our different herbicides. Deep tillage is an excellent option in the fall to manage some of these late germinating Palmer amaranth plants because we can bury those seeds into the soil. The one caveat to this is that the deep tillage should only be done at a maximum of about every four years. Research has shown that a high percentage of our Palmer amaranth seed can remain viable for up to three years even if its been buried in the soil. If we wait past those three years, we have effectively reduced the viability of Palmer amaranth seed by 90 to 95 percent and so there's very little of that seed that would be brought back to the surface and be viable to actually germinate and emerge.

Herbicides are also a great option. The one thing to note with fall-applied herbicides is that they tend to be a little bit less active because of the cooler temperatures and higher precipitation amounts that we are seeing, and so we might not get exactly the level of control we are expecting with some of our typical herbicides because of those situations. 

The second weed species Id like to discuss that we need to manage and through fall weed control strategies is controlling Italian ryegrass. The big thing with Italian ryegrass is that if we can destroy that first heavy flush of ryegrass it will often greatly reduce the population that emerges later and reduce our problems later on or closer to planting of next year's crop. Once again tillage is an excellent option to control that first flush of ryegrass, and it also doesn't require a residual herbicide that could affect our crop rotation the following year. Research does show, though, that if we use fall tillage without a residual herbicide it probably will require one to two post-emergence herbicide applications to achieve complete control of that ryegrass ahead of planting next years crop. That fall tillage doesn't get us all the way through to the next planting, but it does do an excellent job at reducing the bulk of that ryegrass population. Also, when we are using herbicides to manage our Italian ryegrass, please be aware that they require excellent coverage on this weed species just because of it being a grass and a little bit different leaf structure, we need to have excellent coverage to maintain our control. It's important to make sure that we're using appropriate nozzles to get an accurate droplet size, that was using appropriate spray volumes, and also the adjuvants that play a role to making sure that we achieve that excellent coverage to gain a high level of weed control. 

Some other weeds that we can see benefits from fall weed control strategies include many of our perennial weed problems, such as common pokeweed, smooth groundcherry, the bindweeds, thistles, trumpet creeper, all of those different perennial weed problems that we see. 

Fall herbicide applications are actually the best timing to apply an herbicide chemical to these weeds and achieve maximum control. And that's because of the source-sink relationship that these plants are going through in the fall. What they're actually doing is that they're sending a lot of their sugars and food and everything from the leaves to their roots or perennial structures on those plants. It's kind of a lot like a bear going into hibernation. They're taking all their food reserves and storing it in the plant parts to basically go into a dormancy stage just to survive through the winter. Well, if we can apply an herbicide in the fall while they're doing this source-to-sink relationship in sending these sugars to their roots and perennial parts, we can achieve a much higher level of control. The best option here is using some systemic herbicides like glyphosate, 2,4-D, dicamba, certain ALS inhibitors. All of those different systemic herbicides are best options because they'll translocate through the plant and move throughout to those other sinks or the roots perennial plant structures to helpfully kill those weed species out. 

The last thing I'd like to mention is some residual herbicide control options that can be applied in the fall to manage some of these problematic weed species. There are some excellent options of herbicides that can be applied in the fall, including group 15 herbicides like Dual Magnum, Outlook, Warrant, Zidua, those types of herbicides. There's also our group 3, or yellow herbicides, like Prowl and Treflan, PPO inhibitors, some ALS inhibitors. There are several different options there that can provide some residual in our soil and prolong some of our management of these tough to control weeds. However, we do need to watch out for the plant back intervals for some of these residual herbicides to our next year cropping systems. For example, some data out of Mississippi show that injury to rice could occur the following year after fall applications of Dual Magnum, Treflan, and Zidua. Some of that injury actually resulted in yield loss all the way next following fall, so there's some pretty significant injury that can occur due to this herbicide carryover from fall-applied herbicides. 

For more information on plant back information, especially specific to your situation, please see our MP519 publication, Row Crop Plant Back Intervals for Common Herbicides. Also, for more weed control recommendations, please see our MP 44 publication, Recommended Chemicals for Weed and Brush Control.

Thanks for joining me today and Id also like to thank the Arkansas Soybean Promotion Board and the Arkansas Rice Research and Promotion Board for continued funding of our weed science research here at the University of Arkansas System Division of Agriculture. This is Tommy Butts, Extension Weed Scientist, and thanks for joining us on Arkansas Row Crops Radio. 

Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information, please contact your local county extension agent or visit uaex.edu