When glyphosate was released in the 1970s by Monsanto, Roundup became a household name for agricultural producers across the country.
Roundup was widely used for weed control in a number of crops and it worked so well that some crops were even genetically engineered to be resistant to it.
Now, some 40 years later, glyphosate is still commonly used as a weed control herbicide. However, much like the crops that were engineered to be resistant to it, so are some of the weeds it has been used to control.
Pigweed is one of those weeds, according to Doug Shoup, K-State southeast area agronomist.
“Pigweed is a general term for the amaranthus family and our predominant problem is common waterhemp,” Shoup explains. “There are a few pockets in southeast Kansas, however, that have Palmer amaranth. This one is nasty when it comes to its competitiveness and it has always been easy to control with glyphosate until you get resistance.”
The three most common pigweed species in Kansas, according to him, are redroot pigweed, Palmer amaranth, and common waterhemp. Palmer amaranth and common waterhemp are prevalent throughout Kansas. Palmer amaranth and common waterhemp can reach heights of 9 feet tall, produce as many as 500,000 seeds per plant, and are two of the most competitive weeds for nutrients and water. Redroot pigweed, on the other hand, is not very common in corn, sorghum, cotton, and soybeans in Kansas.
“One of the biggest reasons Palmer amaranth and common waterhemp are such challenging weeds to control with herbicides is because of their ability to develop herbicide resistance,” Shoup explains. “To date, Palmer amaranth and common waterhemp have developed resistance to six herbicide modes of action.”
Several of the herbicides pigweed have developed resistance to, according to Shoup, are commonly used in Kansas, including atrazine, ALS-inhibiting herbicides (Pursuit, Synchrony, FirstRate), PPO-inhibiting herbicides (Cobra, Reflex, Blazer), HPPD or “bleacher” herbicides (Callisto, Impact, Laudis), and our most widely-used herbicide, glyphosate. In Kansas, atrazine and ALS-herbicide resistance is fairly common while PPO-herbicide resistance in common waterhemp and HPPD-herbicide resistance in Palmer amaranth have only been documented on a limited number of acres.
“From a weed perspective, being resistant to six types of herbicide is a big list,” Shoup says
And glyphosate resistant pigweed is not something that is just starting to occur in this area.
“We have been dealing with glyphosate resistance for the last three years,” he says. “The problem is, farmers tend to wait until it is a big problem before they start controlling it.”
For Shoup, that reason alone is why it is paramount that producers start making a plan now for how they are going to control it.
“With such a widespread glyphosate resistance problem, now is the time to start thinking about a program or herbicides to buy to keep from having a problem this summer.”
According to the agronomist, if a farmer thinks he or she has a problem with glyphosate resistant pigweed, the best control is with a pre-emergent herbicide.
“The goal is to get the pre-emergent on before waterhemp starts germinating which is usually around May 10 to May 15,” he explains. “A lot of times this is earlier than a producer wants to start planting soybeans so you could target applying a pre-emergent tank mix with 2,4-D or Dicamba in mid-May.”
Shoup reminds producers there are plant-back restrictions for both Dicamba and 2,4-D so they need to make sure to read the labels.
For those who like to till the ground to kill weeds ahead of planting, Shoup says this is also an option.
“Tilling is an option but you still need to use a pre-emergent herbicide between tillage and soybean planting because waterhemp keeps germinating through the months of June,” he explains.
When it comes to which type of herbicide to use the agronomist says several will work.
“Some of the pre-emergent herbicides that have been very effective against waterhemp or pigweed include Prefix, Authority and Valor,” he says. “Some others, which I would say are effective to a lesser extent, include Intrro, Prowl and Treflan.”
For those producers who don’t take Shoup’s advice and start making a plan now he says there are some post-emergent herbicide options available.
“If you don’t use a pre-emergent and have pigweeds emerge in your soybeans you can tank mix a few herbicides, namely, Blazer, Cobra and Flexstar,” he says.
The problem with applying these herbicides post-emerge, according to Shoup, is that you tend to burn the beans.
“Although in many cases the beans will recover, post-emergent herbicides tend to only do good on smaller pigweeds,” he explains. “If they are six inches or less you can do some good but if they are above six inches it won’t do a very good job.”
As producers continue to heavily rely on glyphosate for weed control there will also be an increase in weeds that become resistant to it, according to Shoup.
“In addition to that, Palmer amaranth and common waterhemp have both male and female species which forces them to outcross, increasing their genetic diversity and their ability to develop herbicide resistance,” he concludes.