Glyphosate resistant pigweed currently may be attracting more attention, but to an increasing number of Southwest wheat growers, herbicide resistant ryegrass poses an equally disturbing dilemma.

For several years, farmers, industry representatives and Texas AgriLife Extension specialists have noted that once-effective herbicides have failed to control some ryegrass in wheat stands.

A group of interested parties met at plots near Fairlie, Texas, to review control options for this increasingly worrisome problem.

Jim Swart, Texas AgriLife Extension integrated pest management specialist, has been looking at options for several years, especially after noticing that a control standard, Axial XL, was no longer effective in controlling some ryegrass. He said another once-effective herbicide, Hoelon, had been phased out of recommendations several years ago when ryegrass displayed resistance.

Last year Swart estimated about 10 percent of the wheat in northeast Texas showed resistant ryegrass infestations. “I think that’s closer to 20 percent this year,” he said.

Tony Driver, a tech representative for Syngenta, manufacturer of Axial XL, said the problem seems fairly localized “but must be taken seriously. This can be a serious problem if we don’t address it.”

The best solution so far is what Swart calls a “two-step program,” a resistant ryegrass management system that keeps Axial XL in play. The program calls for application of Axiom, a Bayer CropScience herbicide, at 6 ounces per acre when you can “row your wheat. We don’t want to apply until wheat is up to stand, as Axiom has some preemergence activity,” Swart said.  Following that, Axial XL is applied at 16.4 ounces per acre at the two-leaf stage of ryegrass, usually in early to mid January. “We add a half-ounce of Amber with Axial XL to control broadleaf weeds,” he said.

That combination has taken out the resistant ryegrass. “And it keeps Axial XL available,” Swart said. “Axial XL has been doing a good job and is still doing a good job in many places.” He said using Axial XL in combination with Axiom allows farmers to keep a good herbicide available when ryegrass becomes resistant. “We really have nothing to replace Axial XL,” Swart said.

He’s looked at Axiom for about 12 years but until recently could find no real fit because “Axial XL was doing such a good job. Now, we can apply Axiom early and come back with Axial XL and take out resistant ryegrass.”

Swart said a commercial field near the test plots failed last year and was overrun with resistant ryegrass. “They used the two-step program this year and will make a good wheat crop. This program is going to buy us a lot of time.”

Gary Schwarzlose, Bayer CropScience tech service representative, said Axiom also has some broadleaf activity, mostly on mustards and henbit.

Swart and other industry representatives cautioned farmers to be alert to potential resistant ryegrass infestations and to watch for “slippage” of current herbicide programs.

“Pay attention when you combine wheat,” Swart said. “If you see ryegrass at harvest following anAxial XL application, it may be resistant. And you can scatter seed all over the field with the combine.”

Russell Sutton, Texas AgriLife research agronomist, said ryegrass seed spreads easily. “We can spread it with a combine, other farm equipment” or through natural means — wind, birds, etc.

Other options

“Farmers should consider more than just herbicide for managing resistant ryegrass,” Swart said. “We always recommend crop rotation. If we can get the field out of wheat for two years, we go a long way in reducing resistant ryegrass populations.”

He said a reduced-tillage approach, using a burndown herbicide ahead of planting in a stale seedbed, also holds promise for improved control. “We may be able to knock out 80 percent to 90 percent of the resistant ryegrass with glyphosate,” he said.

Driver said it’s important for farmers “to start clean. Use a burndown herbicide in the fall to clean the fields before planting.”

Driver and Bayer representative Hap Hazzard, say the two-step program has proven an effective way to use both Axiom and Axial XL effectively. “The two-step program has worked well all the way down into the Austin area,” Hazzard said.

“We understand this is a good way to control resistant ryegrass,” Driver added.

Swart said research plots have shown that more than resistance may be the reason farmers are seeing ryegrass escapes. A graduate research project shows that resistant ryegrass may have two flushes, the second occurring after initial herbicide applications have run out.

Swart said the two-step approach takes out the first flush with Axiom. “It also predisposes the resistant ryegrass to Axial XL,” he said.

Soil type also plays into the resistance issue. Swart and others say resistant ryegrass rarely shows up on blackland soils. “We see it on poorer soils, gray land,” he said. The thinking is that farmers have few rotation choices with the lighter soils and don’t rotate as often. The wheat monoculture means reliance on the same or similar herbicides year after year. Resistant ryegrass builds up on continuous wheat.

“Farmers tend to rotate more on better cropland,” Swart said. Heavier soils provide more options, including corn and grain sorghum.

Swart walked the field day participants through herbicide trials, including wheat plots a graduate student seeded with resistant ryegrass harvested from the nearby field that failed last year. “We know this ryegrass is resistant,” Swart said.

He said the hope is that harvested plots will demonstrate the potential yield loss caused by various levels of ryegrass infestations. Plots included ryegrass seeding rates from 15 to 120 pounds per acre.

Herbicide trials

Swart also discussed herbicide trials and noted that the old standard sulfonylurea materials are no longer effective on resistant ryegrass. Axial XL and Hoelon showed typically acceptable levels of control in the Gulf ryegrass plots but mostly unacceptable control in resistant ryegrass.  “The two-step program was the best control in the study,” he said.

Swart said trials also included several combinations and options of standard herbicides, including double applications of Axial XL and stand-alone trials with Axiom. Even with two Axial XL treatments, control in resistant ryegrass was not acceptable, he said.

Axiom, at the 6-ounce rate, provided suppression of the resistant ryegrass, and higher rates, 10 ounces, tended to injure some wheat varieties. Swart said injury is mostly stunting and probably would not affect grain yield. Forage production could be reduced, however.

A new pre-emergence herbicide, Zidua, from BASF, “looks promising but might not be a stand-alone product on the resistant ryegrass,” Swart said. Registration could come as early as 2014.

Swart said wheat farmers in the area who are seeing resistant ryegrass this spring have no effective control options this late in the year. “But next year, they can incorporate the two-step program and clean them up.”