Glyphosate-resistant Palmer pigweed is like a pain that starts up in your back when you turn 40. If you keep doing things the way you’ve always done them, it will likely get worse.
That’s why Arkansas weed scientist Ken Smith is recommending that growers include as many weed control options as they have at their disposal — cultural, mechanical and chemical — to keep the pain of pigweed manageable.
Smith spoke to farmers attending the PigPosium, in Forrest City, Ark., sponsored by the Arkansas Cooperative Extension Service and Delta Farm Press. The symposium, attended by close to 800 people, focused on the impact of glyphosate-resistant pigweed in the Mid-South.
Glyphosate-resistant pigweed “is not going away,” Smith said. “There is not going to be a silver bullet next week. We need to think about what we need to do to manage this pest. It is here to stay.”
Smith said none of the weed control tools available for farmers is without weakness. For example, while rotating from cotton to corn can allow a farmer to rotate chemistries for more effective control of pigweed, real life experience may reveal otherwise. “A farmer told me that as he was combining his corn, he was running into pockets of pigweed, and knew he was spreading them. But he never saw them until he ran up on them. They were sneaking up on him. He could see them in cotton and soybeans, but not in corn.”
Cultivation is also a consideration, according to Smith. “We had long forgotten about tillage. We don’t want to do go there, and I’m with you. But it is a consideration. For many farmers, it’s their last consideration.”
Hand hoeing is another option for growers, especially when there are isolated patches of resistant weeds. “I’ve heard costs of $3 to $100 an acre for hand hoeing,” Smith said. “Three dollars an acre is cheaper than any herbicide we put on. It’s an option and a tool we need to keep in our program.”
Chemicals can also bring fields back under control, but will add cost, and will require a proactive, deliberate approach, which could be a challenge for producers, Smith says. “We are accustomed to farming in a hurry. We need to slow down and take our time. We can’t ignore pigweed. It won’t go away.”
Smith said the huge turnout at the PigPosium is a good sign “that many producers are past the stage of denial.”
While pigweed is an ideal weed because of its high seed production and rapid growth habit, it also has a weakness, seed longevity. “It doesn’t last very long, up to six years,” Smith said. “Our data in the Delta says seed mortality is 99.9 percent after four years.”
The weakness of the seed “gives us an up on managing the soil seed bank,” Smith said. “We have a program called zero tolerance, where we do not allow a seed to be produced. We walk the field on a regular basis to chop the weeds out and make sure we don’t have seed production. We’re going to repeat this again next year on our zero tolerance fields. Hopefully, the trips across the field are going to be reduced significantly.”
Smith says the soil seed bank, or the number of pigweed seed in the germination layer, “must be stable or decreasing is order to be sustainable. This is not rocket science. This is arithmetic. If we do not decrease our soil seed bank, we cannot fight this weed. Ninety-nine percent control may not be good enough, depending on how many you started out with. If you have one per square foot, you might. But if you have a hundred per square foot, 99 percent control still leaves you one per square foot.
“The moral of the story is to not let the seed germinate. Unlike in soybean production, we do not have the option to come over the top. We must rely on pre-emerge and overlapping residual herbicides.”
Here are some of Smith’s suggestions for chemical control:
Pre-emerge herbicide necessary
If you’re going to control pigweed in the Roundup Ready system, a pre-emerge herbicide is essential. Start clean. Apply Reflex prior to planting or Cotoran, Direx or Caparol pre-emergence.
While pre-emerge herbicides are effective, little if anything will provide 100 percent control. For that reason, proper hand hoeing has to remain an option, according to Smith.
“If you’re running a hoe crew, walk behind them just to make sure they’re doing it correctly. Adventitious buds at the base of the plant will come back.”
For post-emergence applications on Roundup Ready cotton, Smith recommends applying Roundup plus Dual at 2-leaf cotton. “Don’t wait for the 3-leaf stage. If pigweeds germinate, we’re behind, so don’t let them germinate. Then, apply Roundup plus Dual at 6-8 leaf, scout and be prepared to post-direct. Apply Valor plus MSMA at layby.
LibertyLink cotton and Ignite herbicide is also an option for producers, according to Smith. But residuals must be used with this technology as well.
Smith noted that some cotton producers “are going to plant WideStrike cotton and treat with Ignite. I think you should treat it like it was Roundup Ready Flex and scout closely for weeds. But if the weeds germinate, you have to spray with Ignite. If you do this, be prepared to lose two nodes of cotton. The yield number that farmers keep telling me is 200 pounds. It doesn’t happen every time, but be prepared.”