Controlling weeds after corn harvest is a necessity in preventing resistance Palmer amaranth from spreading or getting worse in some parts of the Southeast.

In north Alabama, and in many parts of the Southeast, a lot of time passes between the end of corn harvest in August and the first frost, plenty of time for weeds like herbicide-resistant pigweed to flourish, says Charlie Burmester, Auburn University Extension agronomist.

“Again, the size of the weed is an important factor in controlling it. We have a lot of products that will go over the top and burn down pigweed. But if the weeds get more than a few inches tall, many times they’ll sprout back out. In some of our tests, we controlled the weeds really well, but some left a seedhead that’ll come back next year. It’s a hard weed to control.”

By now, everyone has seen photos of the “Godzilla”-sized pigweeds with more than 1 million seeds, says Burmester.

“You just can’t let them get started in your fields. They can be difficult to pull up, but if you have just a few in the field you can tell your crew to be observant, get off the tractor, and pull them out of the field. This will really go a long way towards reducing those populations and giving you a better shot with fall burndown treatments.”

Burmester says he gets a few calls each year from people in Alabama who are having problems killing Italian ryegrass.

“Many times it’s difficult to control this weed with glyphosate, but we haven’t seen resistant ryegrass yet in Alabama. We’re keeping our eyes open for it.”

One of the most important things in controlling resistant weeds is to scout and make your workers aware of potential problems, giving you a first line defense in the field.

“You have to be constantly vigilant and try to stop them before populations become too large.”

In about 2007 or 2008, growers started seeing the first glyphosate-resistant Palmer amaranth pigweed in Alabama, says Burmester. In the northern part of the state, it was about 2010 before escapes were seen from resistant pigweed.

In most Alabama counties, glyphosate-resistance is found in just a few fields, he says. “In north Alabama, some of the resistance came in from Tennessee, with it coming in on equipment from other areas. And in other cases, it was carried from one field to the other. A combine does an especially good job of spreading glyphosate-resistant pigweed.”