Late summer pigweed management critical for farmers

UNIVERSITY PARK, Pa. -- Late summer is a critical time for Pennsylvania corn and soybean growers to control the spread of invasive weeds that pose a serious threat to crop yields.

Palmer amaranth and waterhemp, two species of pigweed that are gaining a foothold in the state, soon will produce seed. As a result, producers whose fields contain these weeds should take steps to be sure that seed doesn't spread beyond the original infestation, according to extension specialists in Penn State's College of Agricultural Sciences, who will be available to offer advice and information to growers at Penn State's Ag Progress Days exposition, Aug. 16-18.

The problem is that these weeds have developed resistance to commonly used herbicides, making control difficult, noted Bill Curran, professor of weed science.

"July and August is the time of year when growers start to notice infestations of these new invasive pigweeds as they climb above the soybean canopy," Curran said. "Management options for weed 'escapes' in soybeans are limited at this point in the growing season. The goal is to prevent the spread of Palmer amaranth and waterhemp before and during harvest."

The first reaction for most growers is to look for an herbicide solution, but this can be challenging, explained Dwight Lingenfelter, weed science extension associate. "These populations are resistant to glyphosate (Roundup), and many also may be resistant to ALS, or Group 2, herbicides. Potentially effective products usually are members of herbicide Group 14, or PPO inhibitors. But these herbicides generally will not kill these large, more mature plants."

Curran pointed out that resistance to Group 14 herbicides also is on the rise. Purdue University scientists recently reported that populations of PPO-resistant waterhemp and Palmer amaranth are increasing in Indiana. Nationally, PPO-resistant waterhemp first was identified in 2001 in Kansas and now is found in at least seven states as far north as Minnesota. PPO-resistant Palmer amaranth is less common -- likely only because it is a more recent problem -- but has been reported in Arkansas, Tennessee and now Indiana.

"This is just another tale confirming that over-reliance on certain herbicide families quickly leads to herbicide-resistant weeds," Curran said. "The take-home message is to diversify the weed management program, not only rotating herbicide sites of action and using effective herbicide mixtures, but also relying on integrated weed management tactics that include effective cultural and mechanical control measures."

First detected in Pennsylvania in 2013, Palmer amaranth has been found in soybeans, corn and alfalfa, as well as in noncrop areas and at field edges, in at least 14 counties. Experts believe it exists in more counties, but that has yet to be documented. Isolated populations of waterhemp, which is a serious problem in the Midwest, have been in Pennsylvania for several years.

Research has shown that these annual weeds are capable of significantly reducing crop yields. In a Tennessee study, Palmer amaranth and waterhemp decreased soybean yields by 78 percent and 56 percent, respectively. High densities of Palmer amaranth have been shown to reduce corn yields by up to 91 percent. Experts say the two weeds also are likely to cause serious problems in vegetable and small-fruit production.

The Penn State scientists and the Pennsylvania Department of Agriculture hope to classify both pigweeds in the near future as "Pennsylvania Noxious Weeds" to help increase awareness of the problem and encourage landowners to prevent their spread.

As herbicide efficacy wanes, the Penn State specialists advise growers to look at alternative control measures. "With smaller infestations, consider walking the fields and removing the weeds by hand," Lingenfelter said. "For vegetables or crops that are harvested by early September, mow harvested fields to prevent seed production. To ensure success, two to three mowings may be necessary prior to a hard frost."

To assist in disposal of a small number of plants, the Pennsylvania Soybean Board and soybean growers provided funding to develop 40-gallon bags made from recycled paper to be used for bagging and either burning or burying large pigweed plants to prevent seed production and spread.

 
Dwight Lingenfelter pigweed bag

Dwight Lingenfelter, Penn State Extension weed management specialist, displays a bag farmers can use to dispose of pigweed plants. The bags will be available to growers who visit Penn State's Ag Progress Days, Aug. 16-18.

Image: Penn State Extension

These bags and a spiral-bound publication titled "A War on Weeds: A growers guide to minimizing the threat of Palmer amaranth and waterhemp in the Mid-Atlantic region," will be available at the J.D. Harrington Crops, Soils and Conservation Building at Penn State's Ag Progress Days expo, Aug. 16-18 on state Route 45, 9 miles southwest of State College.

"Keep a few of these bags in your pick-up or tractor," said Lingenfelter. "Do not transport plants away from the infested field. You should bury or burn plants that may contain viable seed at the field edge."

Curran added that harvesting all plant material and ensiling it also should kill some of the weed seeds that could be present as autumn approaches. "It is very important not to allow Palmer amaranth seed to spread beyond the current infestation and to try to reduce the potential for new seed production," he said.

"The potential to spread this problem at harvest via the combine is great, so anything that can be done to control the pigweeds prior to crop harvest is imperative."

More information about Palmer amaranth and waterhemp is available on the Penn State Extension Weed Management website.

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Last Updated August 12, 2016