Corn is a great rotational crop from an agronomic standpoint, and has been recommended to assist with control of glyphosate-resistant pigweed.

“We can use a lot of different herbicide options and alternative modes of action in corn that aren’t available in other crops,” says Dr. Tom Barber, University of Arkansas weed scientist. “That can lead to a good, integrated management program for pigweed.”

The bad news, he says, is that the EPA is reconsidering atrazine registration. The open comment period closed in early October.

“We’re concerned that we could lose atrazine as a primary pigweed control option in corn,” Barber says. “If that happens, with all the PPO-resistant pigweed we have scattered around the state, corn will become less an option for rotation. In all of our work with PPO-resistant pigweed in 2016, atrazine was hands down one of our best options from a corn herbicide standpoint.”

Barber spoke with Delta Farm Press about the issue:

The importance of a two-shot approach…

“In general, the main story with weed control in corn is to put a pre- down to let the crop get a good head start. That can be done with something as simple as Dual II Magnum, Dual plus atrazine, as well as many other options that aren’t very expensive.

“Following that, come back with a postemergence application with something like Halex GT or Capreno plus atrazine, something with a HPPD herbicide in it. There are other options for that application, as well. The good thing with regard to corn weed control is that there are several options available, both pre- and post-, but all look better controlling pigweed when they are tank mixed with atrazine.”

Going post- only…

“The biggest thing I’ve seen in corn for the last couple of years is that growers want to go with the post- only application. So, they plant corn, let it come up to a stand, and wait until the crop is V3/V4, somewhere in there, before making their first —and sometimes only — application. That allows weeds to come up and compete with the crop early, and subsequently, we lose some yield.

“To check all this, we’ve done several trials in recent years looking at post- only options, pre- only options, or two-shot programs, pre- followed by post-. What we’ve found is that we’re gaining an average of about 15 bushels per acre with the two-shot program.

“What we’ve been doing the last couple of years is encouraging growers not to wait on the first herbicide application in corn. Go ahead and put a residual down, either at burndown, or at least at planting.

“If the residual is mixed in with the burndown application, make sure a product is used that has enough residual to carry through a couple of weeks after planting. Then, come back with a post- program between V2 and V4. Generally, from a weed control standpoint, that provides maximum yield.”

Why the reluctance for two shots?

“Those who have been waiting to spray until V4 may be getting good control — they see the weeds die back. Atrazine is a fantastic herbicide; you can mix it, at 1 quart or 1.5 quarts, with Capreno plus Roundup or Halex GT. That’s a broad spectrum program that’ll take care of a lot of difficult-to-control weeds.

“Again, the problem is that we’ve let the weeds compete with the crop for two or three weeks, and we’ve lost yield. The other risk in not applying a pre-emerge or herbicide at planting is that by the time we come back with the V3/V4 application, the weeds are too big.

“We’ve run into this the last couple of years when we’ve had wet springs and couldn’t get back into the field to spray. The pre- or at-planting application reduces early season competition and buys time and is insurance in case the weather doesn’t cooperate.”

Planting dates…

“The other side of the story is planting date. If you are able to plant in early March, there aren’t as many weeds to come up and compete with the crop in that crucial time period. The weeds just aren’t growing yet. Plant in April and there will be a lot more weed pressure.  

“A farmer may figure if he plants early, the crop won’t take a yield lick if he waits to spray until post-. He sees that one shot does a good job, and it’s the easiest route to take. Nowadays, growers farm a lot of acres, and it’s convenient to do it that way. But the weather seems to throw a kink in this plan more years than not.

“What we were seeing — both on-farm and in plots — is that if we can maintain the weed-free period for the first three to four weeks that the corn crop is trying to grow, we’ve pretty much maintained our total yield potential from a weed control standpoint. That doesn’t mean there aren’t nightmares that can occur. But using two shots adds yield benefits on the back end.”

More on the importance of atrazine…

“What we’re doing is working right now, but if the EPA takes away atrazine, it’s going to be a whole different ballgame. Atrazine has made it easier to control pigweeds.

“I don’t know which direction the EPA is leaning. I’ve heard rumors we may be able to keep atrazine, but at lower rates — maybe a pint per acre. Right now, we can apply up to 2 quarts in a single application and 2.5 quarts for the season.

“If the EPA opts for a pint of atrazine for the season, they might as well just take it off the table — you can’t get good control with that amount. So, yeah, it’s a worry, and I’ve made a public comment.

“University of Arkansas Weed Scientist Jason Norsworthy and a co-author out of the Midwest submitted a white paper on how badly atrazine is needed. And that need increased 100-fold once we found how prevalent PPO-resistance is in our pigweed populations.

“If atrazine is taken away, we’ll have to go with more dicamba or 2,4-D for pigweed control. Corn is tolerant to dicamba, but we recommend it at lower rates due to injury potential. And if that’s the route we’re forced to take, remember that the Xtend crops are waiting for registration. If those are approved, dicamba will be going onto three crops, and we know where that path leads — just more resistance.

“For our 2017 plots we’re already planning to look at several herbicides we haven’t used on corn for a long time. That’s a response to the possibility of losing atrazine, and I feel I’m already at least a year behind on that.

“There have always been whispers that ‘We’re going to lose atrazine, we’re going to lose atrazine.’ I didn’t know what to think until it checked out. One thing’s for sure: It’s going to be interesting.”