The adage that one change typically follows another is especially applicable to weed shifts in farming, says Eric Prostko, University of Georgia Extension weed scientist.
Peanut producers in the Southeast are starting to notice new or emerging weed species in their fields, says Prostko, and this could be due to factors such as the introduction of new chemicals or the adoption of new cultural practices.
“This isn't anything new,” he says. “If you've been farming for some time, you've seen shifts in weed species in your fields. Change is inevitable, especially in a biological system such as weeds in agriculture. Something will find the niche that we're creating.”
Weed shifts basically occur in response to whatever farmers are doing on their land, says Prostko.
“We know a lot about some of these new and emerging weeds while we don't know so much about others. Some of these weeds haven't been a problem in peanuts. And until we see fields with significant problems, it'll be difficult to get good data on controlling these weeds,” he says.
Weed shifts can occur due to the adoption of cultural practices such as reduced-tillage production, he says. “In Georgia, we've seen more reduced-tillage and less heavy plowing over the past several years, and this has affected our weed species.”
Row spacings, such as the move from single to twin rows in peanuts, also can have an impact, he adds.
“We've also changed our herbicide use patterns,” says Prostko. “If you grow cotton, you're probably using glyphosate, and that has affected how we do things.”
In listing some of the new and emerging weeds being seen in Southeastern peanut production, Prostko says tropical spiderwort would be near the top.
“This weed is causing a lot of problems in Georgia. Fortunately, we've been doing some work on this since we have such a heavy population, and we're getting some good control data on this species,” he says.
Tropical spiderwort has become a problem primarily for two reasons, notes Prostko. One is the change in tillage systems and another is the fact that with glyphosate systems, growers are not using the older herbicides on cotton such as Cotoran.
“The good news is that we have some control strategies. We rely heavily on Dual Magnum to help with this weed. In Georgia, we're also recommending a tank-mix of Gramoxone and Dual put out at cracking. Then, you can come back with Cadre or whatever else you might be using postemergence on peanuts.”
Tropical spiderwort, he says, produces both above-ground and below-ground flowers, with about 13,000 seed per plant. “It reproduces vegetatively. You can plow and cut it up, but it'll still reproduce. We've seen it in Georgia as far north as Effingham County, so it's right on the South Carolina border.”
Florida pusley, says Prostko, has been in Georgia fields for a long time, but growers now are experiencing more problems with the weed. One reason for this, he adds, is that some growers aren't using the DNA materials like Sonolan, Prowl and Treflan which control Florida pusley.
“In some situations, especially where strip-till is being used, growers are having problems due to a lack of activation. With a heavy cover crop, we're not getting that herbicide down to where it's needed. You can't let this weed get very big or you'll have problems controlling it postemergence.”
Florida pusley can be controlled with currently available products, says Prostko. But if it escapes one of the soil-applied herbicides — such as Sonolan, Prowl, Strongarm, Valor or Dual Magnum — growers will have to rely on Gramoxone at cracking. Cadre's performance has been variable, he says.
Groundcherry is another weed that's proving to be a problem for some Georgia growers, says the weed scientist. There are three species of this weed, including cutleaf (annual), smooth (perennial) and clammy (perennial).
“We don't know if this weed will respond to a material like Cadre. Based on what we know in other crops, there are some control strategies using Dual/Outlook and Ultra Blazer at postemergence.”
Copperleaf also is causing problems in Georgia, especially on the western side of the state and possibly in north Florida, says Prostko.
“The good news is that we know a lot about this weed in terms of control, and it's easy to control. Strongarm/Valor will do an excellent job pre-emergence, and Ultra Blazer works very well over-the-top.”
Eclipta also has become more of a problem in Georgia fields during the past two seasons, he says, and this could be due to wetter weather conditions.
“We also have some good control strategies. We know Strongarm and Valor have good activity pre-emergence. But postemergence gets more difficult when this weed is allowed to grow. If it gets bigger than 2 or 3 inches, the effectiveness of products like Storm, Basagran or Blazer will depend more on weather conditions. We should be able to manage this weed, as long as it doesn't get too large.”
A lot of Georgia growers also are reporting problems with carpetweed, says Prostko. “It has a small white flower and grows low to the ground. We don't know much about this weed — we don't even know if it's very competitive.”
The following soil-applied materials should have activity on carpetweed — Prowl, Sonolan, Dual, Strongarm and Spartan. For postemergence control, data from Texas shows the effectiveness rating of these materials: 2, 4-DB (58 percent), Cadre (80 percent), Storm (95 percent), Basagran (33 percent) and Blazer (100 percent).
“Another weed species we're seeing more of is purple moonflower, from the morningglory family,” says Prostko. “It's a larger morningglory species, and we're seeing it much more in the central counties of Georgia. Blazer generally is pretty good on morningglory, but we don't know about Cadre.”
Pink purslane also is being seen more in Georgia fields, he says. “Again, we don't know a lot about this weed. Taking data from other places, we feel like materials such as Sonolan, Prowl, Dual and Valor should do a pretty good job. For postemergence control, you might try paraquat plus Storm, Basagran, or 2, 4-DB.”
Georgia farmers also are seeing more spurges in their fields, says Prostko. There are several species of spurges in the Southeast, but there is limited data on controlling this weed, he says. Valor, Ultra Blazer and Storm are control possibilities.