With warmer temperatures and January coming to a close, farmers will be anxious to get in their fields and do some work. It seems like burndown starts earlier every year. When you’re scouting fields to come up with a burndown herbicide program, don’t forget about Italian ryegrass.
Italian ryegrass has never been easy to control with glyphosate. However, in the early 2000s, weed scientists from the Delta Research and Extension Center in Stoneville, Miss., identified two Italian ryegrass populations from Washington County, Miss., that showed tolerance to three times the labeled rate of glyphosate. This was the first time glyphosate-resistant Italian ryegrass had ever been confirmed in a row crop setting.
A 3X tolerance is relatively low, especially compared with some of the populations of glyphosate-resistant Palmer amaranth across the Delta. But, in the five or six years since the initial confirmation, the level of resistance has increased, and we now have some populations that are resistant to much higher rates of glyphosate.
For a few years, the problem seemed localized to the west-central part of the Delta. The glyphosate-resistant Italian ryegrass spread across Washington County and into neighboring counties, especially Bolivar and Sunflower counties.
Then, for various different reasons, it exploded across the Delta in 2009.
Wet and cold conditions throughout the first few months of 2009 delayed burndown or reduced control from early spring burndown applications. Also, Mississippi planted 520,000 acres of wheat in 2007-08, but this acreage dropped to 180,000 in 2008-09. Most likely, a lot of Italian ryegrass went to seed in 2008, and this seed contributed to problems the following year.
A Delta-wide survey of Italian ryegrass was conducted by Vijay Nandula and his group from DREC in the spring of 2009. About 100 samples of Italian ryegrass were collected and screened for tolerance to glyphosate and several ALS (Osprey, PowerFlex) and ACCase (Hoelon, Axial XL) herbicides, which are the backbone of Italian ryegrass control programs in wheat.
Preliminary results showed that one-third of those 100 populations survived a labeled rate of glyphosate. Those populations cover a 12-county area in the Delta ranging from Tunica to Yazoo counties.
Some of the populations collected last year also survived applications of the ALS and ACCase herbicides. So, needless to say, the problem is no longer isolated and appears to be getting worse. Weed scientists in Arkansas, Tennessee, and most recently North Carolina, have also identified glyphosate-resistant populations.
Because of the problems in 2009, we set up a full-blown glyphosate-resistant Italian ryegrass research program for 2009-10. Work done previously had shown that the best opportunity to control glyphosate-resistant Italian ryegrass was with residual herbicides applied in the fall. Dual Magnum and Command had performed most consistently. Most of the applications in the earlier research had been made in November and required an additional herbicide be added to control Italian ryegrass that had already emerged.
So, our major objective in 2009-10 was to figure out when Italian ryegrass started emerging in the Delta and try to suggest timings residual herbicide applications that coincided with emergence.
This was a good idea, but the weather didn’t cooperate. August was unseasonably cool, and we had a huge flush of Italian ryegrass in late summer. At our research site, we found a solid stand of Italian ryegrass when the grower’s soybeans were harvested in mid-September. The Italian ryegrass continued to emerge throughout the rest of September and October but had nearly ceased by mid-November.
The most disturbing thing last fall was we had to make at least two applications of Gramoxone Inteon (3 pints per acre) to control the Italian ryegrass that was up when we applied our residual herbicides.
Now, I’ve said all that to make this one point. Italian ryegrass emerged in a lot of cases much earlier in 2009 than it had in previous years, so it is larger now than what we are accustomed to seeing in January.
Do not wait on burndown in fields that have Italian ryegrass in them. As soon as the fields dry up, go ahead and get something on it. A fair amount of field work was done in Mississippi during the dry period in November, but this was a lot less than years past.
Because so many fields will have to be worked in the spring to clean up ruts from last fall, the Italian ryegrass problem may not be as bad as last year. But, if you drive down Hwy. 61, you will see many fields that are covered in Italian ryegrass.
The best post-emergence options for controlling glyphosate-resistant Italian ryegrass that we have found so far have been Select Max at 12 ounces per acre (or another clethodim formulation at equivalent active ingredient rates) plus glyphosate or Gramoxone Inteon at 4 pints per acre. With either program, a follow-up application may be required 10 to 14 days later.
As we continue to spray plots this winter and spring, we will let you know of any new information that we find.