Louisiana producers dealing with variety of weed problems

According to Donnie Miller, there are a few hot topics among Louisiana farmers. First up during his talk at a recent soybean demonstration meeting at the Dean Lee Research and Extension Center in Alexandria, La., was fall or winter herbicide applications with residual activity.

“In that realm, one product we've been researching is Valor,” said the LSU AgCenter weed specialist based at the Northeast Research Station in St. Joseph, La. “We conducted research in 2005 in cooperation with Valent, the company that sells Valor, to see the effects of winter applications on early-season crop growth and nutrient effects.”

Valor at 2 ounces was included with Roundup WeatherMAX/2,4-D. Comparisons to Roundup WeatherMAX/2,4-D and Roundup WeatherMax alone were made.

“We planted multiple crops — soybeans among them — in April. Approximately two weeks after emergence, crop height was recorded and plant nutrient analysis was conducted.”

Summarizing the results, Miller and colleagues found no difference in the nutrient analysis. Any benefits weren't from having more nutrients available to the young crops.

“People call me every year asking, ‘Should I put Valor out? Is it beneficial to put out any residual materials in the fall or winter?’

“I present both sides of the story — I don't say, ‘yes, do it,’ or ‘no, don't.’”

The materials being pushed for winter application do a very good job of controlling winter weeds — chickweed, henbit. In doing so, a lot of times farmers are able to plant into relatively clean seedbeds.

“The bad part of that is having a clean, exposed seedbed through the winter. We often receive a lot of rain during our Louisiana winters. Do you want to leave your seedbed vulnerable to erosion for that extended length of time?”

On the other hand, such residual products in the winter may keep down several weed species that could serve as hosts for things like ASR.

“It seems, over the last couple of years, I've gotten more and more calls about henbit being very difficult to control. Folks say, ‘The Roundup/2,4-D didn't do the job on my henbit.’ I've had varying levels of success with (that product combination) on henbit. I've seen much more consistent results with Gramoxone.”

But even with Gramoxone in the mix, Miller has received several calls about failures. “I wish we could pinpoint exactly why those failures are occurring.”

The second benefit to the residual products is in dealing with resistant marestail. Suspicious weeds abound, but so far there's been no confirmation of glyphosate-resistant marestail in Louisiana.

“While doing research with Valor as a potential defoliant a couple of years ago, research colleagues found that everywhere Valor was included in a defoliation plot the year before there were no marestails popping up the following spring. So it has a good residual activity. In those situations it's obviously a benefit.”

Glyphosate, soybeans

Every year Miller receives calls about unfilled bean pods and the crop not finishing out. Producers are concerned about glyphosate applications — worried they put the product out too late.

Several years ago, Miller initiated research on glyphosate applications during beans' reproductive stages. Research was done in St. Joseph, La. At the same time, weed researchers in Missouri began looking at similar research.

“We made one Roundup WeatherMax application at 12 ounces, 23 ounces, 35 ounces — all the way up to 70 ounces — at R-4 soybean reproductive growth stage.”

In 2004, the test beans yielded 43 to 45 bushels per acre. In 2005, they yielded 53 to 57 bushels. In both years, yield was similar to where no glyphosate was applied and maturity wasn't delayed.

“So the treatment changed nothing — it didn't reduce yields one bit.”

In a separate test, Miller made the same application at R-4, followed by another at R-6. As much as 140 ounces of Roundup WeatherMax was sprayed on the beans.

In 2004, those beans yielded between 41 and 48 bushels. In 2005, they yielded between 54 and 59 bushels. Again, there was no effect seen from the glyphosate application.

“In Missouri, they checked a different glyphosate formulation — Glyfos X-TRA — on two soybean varieties. They applied rates of 24 ounces, 48 ounces and 96 ounces at different growth stages than we did. They applied at either pre-bloom, bloom, pod formation or pod-fill.”

Even so, the Missouri results were very similar to Louisiana's. The Missouri yields ranged from 51 to 63 bushels for the first variety and 55 to 62 bushels for the second. Researchers could find no negative effect from the glyphosate.

Further bolstering the bottom line was more Missouri research using Glyphomax Plus, applied at the same rates and timings. That test found similar results — 55 to 61 bushels per acre.

“I still get calls from people wondering if they're causing problems with late glyphosate applications. All I can say is we haven't been able to reproduce those problems in our small plot research studies. I think going up to 140 ounces of glyphosate shows that.

“I'm not saying (those problems) are there or not. But we can't reproduce it on a small-plot basis.”

Over the past decade, much research has been conducted on the positive effects of Roundup Ready technology. Along with a graduate student, Miller wanted to put a spin on that and look at the potential negative effects.

“Obviously, with the number of acres devoted to Roundup Ready crops — cotton and soybeans — and the mild Louisiana winters, the potential exists for us to have volunteer plants survive and show up as a weed. The definition of a weed is simply ‘anything growing out of place.’ A rose bush in a soybean field is a weed.”

The researchers wanted to see how competitive such “weeds” are — a volunteer Roundup Ready cotton plant in a soybean field and vice versa. So Miller's graduate student planted side-by-side, 2 inches apart, soybeans and cotton. Then, she thinned the “weeds” to produce anywhere from 0.05 to 1.6 plants per row foot.

At the same time, “we wanted to see how long we could allow these plants to compete before having yield reductions. We chose the highest density — 1.6 plants per row foot — and allowed them to compete at weekly intervals up to eight weeks after emergence.

For cotton weeds in soybeans, a density of 0.5 and 1.0 plant per row foot reduced soybean yields 5 and 15 percent respectively. At 1.6 plants per foot of row, competition for eight weeks resulted in a soybean yield loss of 11 percent. Cotton left to compete season-long reduced soybean yields 43 percent.

As for soybean weeds in cotton, 0.5 and 1.0 plant per row foot reduced cotton yields 32 and 50 percent respectively.

“It only took two weeks of competition from soybean at a density of 1.6 plants per row foot to reduce cotton yields 11 percent. Season-long competition resulted in a cotton yield loss of 62 percent.”

Weed resistance

One topic on everyone's minds is weed resistance to glyphosate. “I was very fortunate to be invited, along with Sandy Stewart and some north Louisiana agents, to a cotton tour in North Carolina.

“One stop was Alan York's (weed scientist at North Carolina State University) work on Palmer amaranth glyphosate-resistance plots,” said Miller as he projected a photo behind him. “Look at all the healthy, 6-foot-tall Palmer amaranth. That plot received 88 ounces of WeatherMax when it was 3 inches tall, another 88 ounces at six weeks and another 88 ounces at 13 weeks. That's about 256 ounces applied for no control.”

In speaking with York, “the thing that struck me — and it's true with marestail — is those two weeds began in one county and, within a couple of years, were in multiple counties and states. It doesn't take long for them (to move).”

In Louisiana, glyphosate-resistant weeds haven't been confirmed. But there are many suspected cases.

Several years ago, there was a field of suspected glyphosate-resistant marestail in East Carroll Parish. Miller and colleagues “were all set up to do (tests) in the field. But there was some miscommunication between the farmer and pilot and Roundup/2,4-D was put out. That took care of that test.

“We were able to collect some seed from a suspected resistant Palmer amaranth in West Carroll Parish. We grew that seed out and… found out we had Sandhill amaranth, not Palmer.”

On the Sandhill amaranth, Miller found that a pound of glyphosate did a good job as long as the application was made when the plants were 1 inch tall. Once the plant gets taller than an inch, “you have to go up to 2 pounds for control. That's a tolerance issue.”

Louisiana currently has a case of johnsongrass suspected of resistance.

“We're growing it off (to check it further). To confirm a weed as resistant is a long process. You have to find the weed, grow it out, apply a toxic rate of herbicide that would normally kill it, have it survive, then harvest seed or rhizomes and grow the plants out again and have the plant survive another application. In other words, the resistance must be heritable. That's why we have a lot of suspected resistance cases and no confirmed ones.”

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