The most important weed management tool an Oklahoma peanut farmer should keep in his pickup just might be something as simple as a 50-cent ruler.

Timing post-emergent herbicide applications is extremely important for weed control, says Oklahoma State University Extension weed specialist Joe Armstrong.

“Peanut farmers need to treat weeds when they are four inches tall or less,” Armstrong advised during a recent production seminar at the annual Oklahoma Peanut Expo at the Quartz Mountain Resort near Lone Wolf. “When weeds average four inches high is the best time to spray,” he said.

A second essential tool is a spray rig. “Owning your own sprayer helps assure timely application. You don’t have to rely on someone else.”

Armstrong said weed control remains a challenging prospect for Oklahoma peanut producers. “We have some good pre-emergence herbicides available, but we have nothing new.”

He said a good pre-emergence herbicide program is essential for effective weed management. ‘”Pre-emergence herbicides are valuable and necessary to improve control of many weeds, including pigweed species, yellow nutsedge and morningglories.”

He said Valor SX is a good option and that applied pre-emergence “improved control of many broadleaf weeds, including pigweed and morningglory species. Valor SX must be applied to the soil surface prior to peanut emergence to avoid crop injury and should be applied with additional pre-emergence herbicides to improve control of other weeds, including grasses.”

He said pre-emergence materials also help control weeds that have developed resistance to other pre-emergence and postemergence herbicides. These include pigweed species resistant to ALS herbicides.

In 2010 herbicide trials, Valor SX showed “excellent control of many tough broadleaf weeds, including prickly sida, morningglory and pigweed. “But it offers very little grass control.” He said Valor plus Dual and Prowl provide excellent control of grasses and tough weed species.

Other options

Armstrong also looked at several herbicides not currently labeled for use in Oklahoma peanuts. “Spartan has previously had a peanut label in the Southeast,” he said, “so we looked at it for weed control, specifically pigweed, nutsedge, and morningglory. We are concerned with crop injury as high as 30percent.Weather and soil types may differ from the Southeast so I don’t think we will pursue it for Oklahoma.”

He also is looking “closely at Classic for late application morningglory control. But applying 60 days after planting is too late for morningglory in Oklahoma.”

He will test a product from BASF later this year.

Armstrong said growers and consultants are concerned about the growing threat of herbicide- resistant weeds in peanuts and other crops. They are particularly concerned about pigweed and waterhemp. “We have confirmed glyphosate-resistant marestail in Oklahoma,” he said.

With funding from the Oklahoma Peanut Commission, Oklahoma State University is conducting herbicide resistance evaluations. “It’s a free service to Oklahoma producers,” Armstrong said. “Producers can send us seed from weeds they suspect might be resistant. We will grow those out and test in the greenhouse and send back the results.”

Growers can find information at the OSU website, http://weedscience.okstatue.edu. They will find a fact sheet about the program and a form they can fill out to accompany samples.

“Our best recommendation is don’t let weeds go to seed,” Armstrong said. “We will check weeds in the field, dig them up and take them to the greenhouse to check.”