July is not the typical time to start discussing burndown strategies. If you have glyphosate-resistant Italian ryegrass, however, it may be worthwhile to do just that.
As of late-fall 2009, glyphosate-resistant Italian ryegrass covered a 12-county area in the Mississippi Delta from Tunica County to Yazoo County. In addition to being resistant to glyphosate, some of the populations collected in 2009 were also resistant to the ALS (Osprey, PowerFlex) and ACCase (Hoelon, Axial XL) herbicides.
Problems with glyphosate-resistant Italian ryegrass are also building in Arkansas, Louisiana, and Tennessee.
As bad as glyphosate-resistant Italian ryegrass was in 2008-09 in Mississippi, I think it was worse in 2009-10. Last August, weed scientist Tom Eubank and I developed what we thought at the time was a fairly comprehensive research program evaluating management programs for glyphosate-resistant Italian ryegrass.
Unfortunately, after collecting all the data and evaluating it this spring, I have to admit that the ryegrass won in 2009-10. Several treatments I was positive would work just completely failed for various different reasons. I attribute most of the failures to excessively high Italian ryegrass densities and to the rain and cold temperatures through most of the winter in 2009-10.
In our plots just east of Stoneville, we counted 212 glyphosate-resistant Italian ryegrass per square meter (approximately 20 per square foot) on Sept. 23, 2009. I am not sure when the Italian ryegrass began emerging at the site, but it emerged well in advance of the grower harvesting the soybean crop.
Tom saw Italian ryegrass emerged in the Stoneville area as early as July of last year.
The accompanying chart shows that Italian ryegrass emergence continued through mid-December and then resumed in February with the spring flush peaking in mid-March.
Until 2010, I had assumed that the amount of glyphosate-resistant Italian ryegrass that emerged in the spring in Mississippi was relatively low. I also was not prepared for the early date at which the glyphosate-resistant Italian ryegrass began emerging in 2009.
During July 2008, the average daily high temperature at Stoneville was 96 degrees, and there were no days with a high temperature less than 90. In 2009, the average daily high temperature at Stoneville was 89, and there were 16 days with a high temperature less than 90. The lower temperatures in July 2009 were partly due to the number of rainy days during that time.
In general, Italian ryegrass will begin emerging in six to 10 days when the daytime temperatures are less than 87. July 2009 was ideal for Italian ryegrass germination and emergence.
As of July 12, the high temperature at Stoneville in July 2010 has been less than 90 on only one day, July 1. Historically, the average high temperature at Stoneville does not fall below 87 until Sept. 14. Hopefully, 2009 was the exception rather than the rule for glyphosate-resistant Italian ryegrass emergence.
At least two postemergence herbicide applications were required to control emerged glyphosate-resistant Italian ryegrass in the fall of 2009. Work done previously on managing Italian ryegrass in Mississippi had shown that the best opportunity for control was with residual herbicides applied in the fall.
However, these residual herbicides do not control emerged glyphosate-resistant Italian ryegrass, so ryegrass that is up when the residual herbicide is applied must be destroyed either with aggressive tillage or a postemergence herbicide.
In all of our residual herbicide research last year, two applications of Gramoxone Inteon (3 pints per acre) were required to control the glyphosate-resistant Italian ryegrass that had emerged prior to the residual herbicide application.
In previous years, one application of Gramoxone Inteon had been effective when applied in the fall. I really do not have a good explanation for why two applications were required last year, but I suspect the cloudy weather that persisted through most of the fall influenced the effectiveness of the Gramoxone Inteon applications.
The reason this is so troubling is the added cost. Most of the residual herbicides that are effective against glyphosate-resistant Italian ryegrass are relatively expensive by themselves. The additional cost of two Gramoxone Inteon applications to an expensive fall residual herbicide will make this program difficult for many growers to afford.
Populations of glyphosate-resistant Italian ryegrass respond differently to management programs. Italian ryegrass, like Palmer amaranth and barnyardgrass, possesses a high level of genetic diversity. This genetic diversity is partially responsible for how quickly herbicide resistance develops in these weed species. It can also influence germination and emergence patterns as well as growth and development of individual plants.
All these can combine to complicate how we manage a particular weed. Most of our glyphosate-resistant Italian ryegrass research in 2009-10 was conducted in two fields that were approximately 0.25 mile apart. Even though these fields were close together, the best management program in one field was not necessarily the best program in the second field.
It is obviously not uncommon to see variable weed control from one field to the next. But, without delving into the specifics, we saw some major differences in the populations in the two fields last year. In addition to differences in control with herbicides, glyphosate-resistant Italian ryegrass in the two fields just looked different. That is not a scientific statement, but as I said earlier, I am just relaying some observations that I made last year.
As with glyphosate-resistant horseweed and Palmer amaranth, Italian ryegrass is here to stay. As we progress into the fall, I will pass on more information on specific management strategies for glyphosate-resistant Italian ryegrass. My major goal with this article was to get you to start thinking about Italian ryegrass again. Just like Palmer amaranth, the best management programs for it are proactive rather than reactive. If you had a problem last year, you will have a problem again this year.
If glyphosate-resistant Italian ryegrass was not completely controlled last year, then the problem will most likely be worse in 2010-11 than it was in 2009-10. Everyone is busy during the fall, and certainly burndown for next year will take a back seat to harvest for this year. But, in areas where the problem is the worst, management will have to begin in the fall.