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Preserving glyphosate efficacy vital to San Joaquin Valley growers

Roundup Ready has been a boon to agriculture in terms of easy, convenient weed control, but the resistance vultures are looming, if not already feeding.

“We are in danger of losing one of the best herbicides we’ve ever had,” said University of California Cooperative Extension Tulare County Farm Advisor Steve Wright at the recent Central Coast Cotton Conference. “We need to do everything we can to keep these good herbicides viable.”

The heavy reliance on glyphosate-resistant cotton and other Roundup Ready crops is having a sweeping impact on the weed spectrum across the U.S. In cotton alone, transgenic varieties account for up to 80 percent of cotton in the U.S. and most of that is comprised of Roundup Ready varieties.

“Obviously, it’s a very effective and economical program,” Wright said. “Glyphosate-resistant varieties account for 100 percent of the acres in Southern states.”

Trailing behind, Roundup Ready cotton in California currently accounts for about 50 percent of the state’s acreage. That figure has been increasing quickly over the past few years as the technology is being bred into preferred California cotton germplasm. As a result of that slower adoption, conventional herbicides are still commonly used.

“I think that’s good because growers using these other products are helping guard against glyphosate-resistant weeds coming in,” Wright says.

It’s never been a better time to think carefully about resistance management. Glyphosate-resistant horseweed and hairy fleabane are already well documented in California. A short look toward the east is an ominous indication of what else soon could be coming.

Glyphosate resistance (or suspected resistance) is showing up in many more weed species across the U.S. It’s beginning to affect the future of farming operations in areas that have worked diligently to minimize soil erosion and reduce herbicide applications.

“In Mississippi and other Southern states, entire conservation tillage systems are in danger due to glyphosate-resistant weeds such as marestail (horseweed) and Italian ryegrass,” Wright said. “Small amounts of resistant Johnsongrass have also been noted. Glyphosate-resistant Palmer Amaranth is increasing significantly at a very fast pace. They’re also starting to see some barnyardgrass, tall waterhemp and giant ragweed.”

The list continues to lengthen, and the potential repercussions are frightening. Growers in the South are shifting back from an almost 100 percent no-till program to multiple chemical applications and intensive tillage operations to combat the ever-growing population of glyphosate resistant weeds.

“They’re in danger of losing their whole conservation tillage approach,” Wright said.

Palmer Amaranth, in particular, is threatening the existence of hundreds of thousands of acres of conservation tillage throughout the United States. It’s mostly a problem in the Southern states at this point, but no one is denying the threat to the West.

“We really don’t want to go back to the weed control programs we were practicing years ago,” Wright said.

“It was very intensive. We used ‘yellow’ herbicides at planting. We had a couple of applications of early, directed sprays for early nightshade. For nutsedge, we had three to four cultivations. Then, there was spot spraying, and we had hand-hoeing crews going through the field.”

Not only was the old system expensive, it required significantly more herbicides and pounds of active ingredient per acre to achieve adequate weed control.

“With the new weed control systems we have today, we’ve dropped the use of dinitroanilines,” Wright said.

“Glyphosate is commonly applied at one to three applications over the top. In California, in some cases, we’re tank mixing or alternating some of our layby treatments. This approach has saved us $25 to $120 an acre, and it’s allowed us to move into reduced tillage systems, especially in corn.”

Tough weeds such as nutsedge are not as problematic in California due to the Roundup Ready system in multiple crops. However, shifting weed spectrums and increasing species of glyphosate-resistant weeds in other areas bear watching, according to Wright.

Again, Palmer Amaranth is a major concern in California, particularly in Kern County.

“Palmer Amaranth is a prolific seed producer,” Wright said. “A single plant can produce up to 400,000 seeds. It grows fast. Depending upon temperatures, it can grow a quarter to 2 inches per day. It’s very competitive. Two plants per 20 feet of row can reduce yields by 23 percent.”

Palmer Amaranth is easily spread and can quickly overcome a field. “This weed has no morals,” Wright said. “The male plant will shed its seed. If you have winds over 9 mph, it will move a quarter mile away.”

It also moves though gin trash and manures. The good news is it germinates in the top inch of soil, similar to horseweed, so cultivation can minimize the problem.

In Southern states, up to 264 oz. per acre of Roundup Ready Max has been applied to Palmer Amaranth and still fallen short of complete control.

“They’re encouraging growers to go back to the pre-plant herbicides, incorporated layby applications with other herbicides, and residual herbicides, Wright said. “So they’re having to move backward to combat it. Fortunately in California, we feel that we’re moving forward and staying ahead of it by alternating some of our different programs.”

In the San Joaquin Valley, the next weeds to watch for resistance in Roundup Ready systems are the pigweeds (particularly Palmer Amaranth) and lambsquarter.

Barnyardgrass and sprangletop are also suspect. “I don’t know that we’ve had glyphosate resistance in barnyardgrass and sprangletop, but we’re certainly getting reports of reduced control,” Wright said. “Another one is annual morningglory. I don’t know if we have an issue, but it’s one of the weeds that we’re still not getting a good handle on. It may have to do with weed size.”

At times, the Roundup Ready technology turns on itself. “Roundup Ready corn in Roundup Ready cotton is something that we’re starting to see more and more,” Wright said. “Now we have to come back in with grass herbicides, and it takes two applications to control it.”

Growers need to be vigilant beyond field borders as well. Glyphosate resistant weeds growing along roadsides, in ditch banks and other crops can quickly spread into adjacent fields and create problems.

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