Pollen movement could be a primary factor for the spread of resistant Palmer pigweed in the Southeast and Mid-South, although other factors such as equipment movement, gin trash and flooding can also spread the weed and its seed, according to research.
Resistant Palmer pigweed was first observed in Georgia in the early 2000s and quickly spread throughout the Southeast region.
“Up until 2007, we had it in only three counties, Crockett, Lauderdale and Lake counties in Tennessee and across the river in Arkansas,” said Larry Steckel, University of Tennessee weed scientist, speaking at a UT-sponsored conference for grain and soybean producers in Dyersburg, Tenn.
“I was still thinking we could get a handle on it. Then 2008 happened. It showed up all over the place. We confirmed it in 10 counties in west Tennessee in hundreds of fields. It really shocked me how fast this particular biotype spread.”
Steckel said resistant Palmer pigweed “was likely spread through equipment, gin trash piles moving around and the flooding that we saw. But that didn’t answer all the questions.”
Steckel cited a recent study by Georgia Extension Agronomist Stanley Culpepper which has shed some light on those unanswered questions. Culpepper planted one male, glyphosate-resistant Palmer pigweed plant in a research field and planted glyphosate resistant females from 1 meter away to 300 meters away. After evaluating plants grown from the resulting seed, Culpepper found that 60 percent of the seed from the glyphosate-susceptible females within a meter of the glyphosate-resistant male proved resistant. At 300 meters, the susceptible females produced glyphosate-resistant plants from 20 percent of their seed.
“What this tells us is that resistance is moving in the pollen,” Steckel said. “It’s the reason it really snuck up on us in 2008. It was building up in the background, then all of a sudden we had glyphosate-resistant males out there shedding a lot of pollen.
“Palmer pigweed, even competing with a cotton crop, can produce almost a half-million seeds, noted Steckel. “Twenty percent of a half million is a boatload of seed that can be glyphosate-resistant.
“It’s a very competitive plant. Just two Palmer pigweed plants per foot of row will reduce cotton yield by 23 percent. The problem is that it doesn’t come at you at two plants per foot of row. It can quickly overrun you.”
Steckel says Palmer pigweed has also been observed in 18 counties in Arkansas and two counties in Mississippi, and it’s suspected in several parishes in Louisiana. At least 30 counties or parishes in the Mid-South have glyphosate-resistant Palmer pigweed.
Waterhemp, a close cousin of Palmer pigweed, also has a resistant biotype and can confuse growers because it looks like Palmer pigweed. “You can’t tell them apart early on. Basically, you have to wait for flowering before you can tell the difference. If you can measure the flowering branches in inches, it’s waterhemp. If they’re measured in feet, it’s Palmer pigweed. If you grab the flowering branches of female Palmer pigweeds, they are sticky. That isn’t the case with waterhemp.”
Steckel says waterhemp has been confirmed in Arkansas and in Lauderdale County, Tenn. “It’s basically washing down the river on us.”
Palmer pigweed “is the king of the pigweed species as far as its competitiveness is concerned, according to Steckel. “I’m confident it will overtake the waterhemp, if not through competition, then through crossing and breeding it out.”
Steckel says the level of resistance is increasing in glyphosate-resistant Palmer pigweed. When it was first observed in west Tennessee, “greenhouse screening indicated a few glyphosate-resistant Palmer pigweed surviving 44 ounces of Roundup WeatherMax, but we could control them with 88 ounces. That’s still way off label, but it was still better than what they were seeing in Georgia. That changed in 2008. We ran into plants that survived 152 ounces of Roundup.”
In some cases, weed scientists found Palmer pigweed with more tolerance to glyphosate than the Roundup Ready soybeans in the field.
Weed scientists applied 120 ounces of Touchdown to glyphosate-resistant Palmer from one field in west Tennessee. Noted Steckel, “The 120 ounces beat up redvine in the field, but didn’t touch the Palmer pigweed. That’s when I knew we were in trouble.”
The field was in corn the previous year, which surprised Steckel, “because rotating to corn is one of my recommendations for control. We have a lot of good herbicides in corn that will manage Palmer pigweed. But going with Roundup Ready corn, and using glyphosate with a little bit of atrazine isn’t going to be enough to get control. We have to be more aggressive than that.”
Steckel said growers who occasionally see a decline in atrazine’s effectiveness on weeds might not be seeing actual resistance to the product. “Atrazine can become less effective on land where it is used more frequently.”
Studies confirm this, according to Steckel. “We applied atrazine in two fields, one in which atrazine had been applied before and another in which atrazine had not been applied before. On the non-atrazine field, the atrazine lasted about 14 days. On the atrazine field, the atrazine lasted only four days.
“What we’re seeing in these atrazine fields is that we’re building up the microbes that tear down the atrazine. That’s why we’re not seeing as much residual out of it. We’re going to have to use it more as a post application to help manage Palmer pigweed if we’re trying to go into a rotation.”
Steckel says many growers who apply glyphosate alone this season “are going to be blindsided, particularly if they missed a few pigweed last year. We’re going to need to use pre-emergence herbicides and overlap them if we have a really heavy infestation of Palmer pigweed.”
Steckel says growers should treat resistant Palmer pigweed when it’s small. “The days of catching 2-foot-tall Palmer pigweed with glyphosate and melting it to the ground are over. We’re going to have to catch it before 6 inches, preferably 3 inches to 4 inches.”
That’s a lot easier said than done with a weed that can grow up to an inch every 10 hours. “You can control Palmer pigweed with Reflex or Ignite, but two days later, it could be too large to control. Do we have the sprayer power to get around to all these fields? I don’t think so. That’s why we’re really going to have to rely on pre-emergence products, so we don’t have to be in all these fields at the same time.”
Steckel noted that several companies have rebate programs on pre-emergence products. “Look into those programs. We can get some help with the input costs on managing these weeds.”