Managing resistant weeds is going to cost producers money, whether it’s plowing and cover-cropping, adding modes of action or rotating to another crop that may not be as well-suited for the soil type.
Then again, these often costly alternatives are better than not harvesting a crop at all due to a severe infestation of a resistant weed, notes Billy McLawhorn, a Cove City, N.C., consultant.
McLawhorn, participating in a panel discussion on weed resistance at the National Alliance of Independent Crop Consultants annual meeting in Orlando, Fla., says it’s not surprising that weeds are developing resistance.
“I think we all recognize the fate of any pharmaceutical or pesticide that is used extensively — it’s going to end up with resistance problems at some point. Exposure, extent of use, mode of action and reproduction method of the weed all have a lot to do with how fast that happens.”
These factors came into play with the recent development of resistance to glyphosate herbicide, noted McLawhorn. “Roundup Ready soybeans swept across the South in the late 1990s, while herbicide-resistant cotton was planted on 99 percent of Southern acreage in 2008.”
In the early 2000s, horseweed provided the first chink in glyphosate’s armor. “It first appeared in the South in west Tennessee, where there is a lot of no-till long-term. It absolutely exploded in 2008-09 and now it’s documented in 19 states. In most cases, it’s manageable, but it’s required changes in pre-emergence and burndown programs.
“And the selection is not just coming from row crops, but from Department of Transportation weed control of right of ways, which are being sprayed almost exclusively with glyphosate.”
Other weeds would follow horseweed and in 2009, 68 percent of the acreage in the Southeast and Mid-South had some kind of resistance problem, noted McLawhorn. Problem weeds include Palmer amaranth, common waterhemp, horseweed or marestail, johnsongrass, ryegrass and ragweed.
Resistant biotypes are continuing to adapt as well, notes McLawhorn. “Horseweed used to germinate for a couple of months in the spring and a couple of months in the fall. Now it germinates 10 months out of the year.”
Palmer amaranth, however, “is the game changer for us,” McLawhorn said. “It can grow 2 inches a day and produces so many seeds. So managing the seed bank is critical. If you have five Palmer amaranth escapes in a field and 50 percent of the seeds germinate, even if you had 99.9 percent control for two years in a row, by the third year, you would have 80,000 plants per acre.”
In some cases, resistant Palmer amaranth is driving the crop rotation, according to McLawhorn, “and we’re planting corn where we shouldn’t, because the atrazine is excellent on it and we have more options on corn that work.”
As part of weed resistance management strategies from consultants and companies such as Monsanto, “growers are being urged to put down at least two modes of action on soybeans and three modes of action on cotton.
“If we get pre-emergence herbicides activated, that’s the single biggest thing that helps. We don’t always do that. In 2008, we had three-tenths of an inch of rain on May 18 and the next rain we had was the third week of July. We had fields that were inundated with Palmer amaranth. Some of the fields weren’t harvested.
“This past year, in contrast, I had a field on my farm where we had had a big problem with Palmer amaranth the year before. We had spent $40 an acre chopping it. The following year, we planted soybeans, put out Reflex and Dual, got an inch of rain the second day and we sprayed it one time over the top, and had a perfectly clean field.”
LibertyLink technology is helpful for controlling some of the resistant weeds, according to McLawhorn. “Ignite is helping, and it’s a matter of the varieties catching up with the technology. Future technologies such as 2,4-D and dicamba-resistant  cotton are being worked on, and they show some promise. They certainly are inexpensive materials and are very effective on a wide range of broadleaves, but when you have high-value crops around them, the risk of drift or volatility is a serious issue.”
McLawhorn hopes that conservation tillage won’t be a casualty of weed resistance, but the plow is an option in some cases where Palmer amaranth has taken over.
“In Georgia, where there has been extreme pressure, producers and researchers have had success with running a moldboard plow and planting a rye cover crop. Growers do not want to do that, but they have gotten extremely good results out of that combination.”
General weed resistance management practices include competition from the crop, rotation and diversity of chemistry, according to McLawhorn. “We have to act when a problem develops, and we need to do a better job of picking up isolated infestations. It’s hard to convince a grower to stay with a heavy dose of pre-emergence herbicides. As consultants, our job is more about preventing disasters, and we have to do the best job we can do in resistance management.”
While Palmer amaranth and horseweed get much of the attention in cotton and soybeans, there are also troubling trends in rice, noted Dexter, Mo., rice consultant Amy Beth Dowdy .
For example, the list of herbicides for control of barnyardgrass continues to shrink. Propanil-resistant barnyardgrass was confirmed in 1990, Facet-resistant barnyardgrass was confirmed in 1999 and today, Command-resistant barnyardgrass has been confirmed, isolated in two locations in Arkansas.
Also of concern is an apparent over-dependence on Clearfield rice , which is resistant to Newpath herbicide. There is concern that shattering in Clearfield fields creates volunteer plants that will carry the Clearfield trait. Volunteers emerging at different times increase the window for out-crossing with red rice.
A stewardship program requires that growers not plant Clearfield in consecutive years on the same field.
Dowdy wonders how closely this requirement is being followed. Of 1.6 million acres planted to rice in Arkansas in 2008, 70 percent were in Clearfield varieties. “Shattering and out-crossing with red rice is occurring and Clearfield-tolerant red rice has been confirmed.”
To managed out-crossed or shattered fields, Dowdy suggests flooding them for geese, killing the first flush with tillage, and rotating Clearfield fields with soybeans, and using Roundup or a graminicide.
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