Palmer amaranth has many characteristics that put it in a class of its own among summer annual weeds in the Mid-South. I have heard several weed scientists describe Palmer amaranth as a “super weed” or the “perfect weed.”
Palmer amaranth has a broad emergence window, so it produces multiple flushes. It grows rapidly, which influences its competitive ability with crops. It produces an obnoxious quantity of seed, which allows the weed population to sustain itself over time.
Without exception, if glyphosate-resistant Palmer amaranth is present in a field, then that weed dictates how a herbicide program is designed.
After a few years of trying to develop an economical program for controlling glyphosate-resistant Italian ryegrass, I believe Italian ryegrass has to be close to a “perfect” winter weed.
A typical Mississippi Delta winter is the ideal environment for Italian ryegrass. The 30-year average high temperature at Stoneville, Miss., from Oct. 15 to March 15 ranges from 48 to 76 degrees F. And, the historical low temperature only reaches 32 F or lower during the first two weeks of January.
Italian ryegrass thrives where there are mild climates and fertile soils. It has a low tolerance for both hot, dry climates and harsh winter conditions. The generally mild winter temperatures and rich soils in the Delta provide a perfect niche for Italian ryegrass.
Italian ryegrass exhibits some of the same characteristics as Palmer amaranth, which helps make both of these weeds a top priority in the Delta. Peak emergence for Italian ryegrass occurs in the fall, but it continues throughout the winter with another large flush in the spring.
Italian ryegrass plants also grow rapidly. The extended emergence window and rapid growth complicate herbicide programs targeting this weed. If you treat it in the fall, you may have to spray again to control the spring flush. If you wait and spray in the spring, spray coverage may be poor because small, spring-emerging plants will be mixed in with larger, fall-emerged plants. Also, the fall-emerged plants will have extensive root systems that will help these plants recover from a spring herbicide application.
Palmer amaranth produces male and female plants, so this species must cross pollinate in order to produce seed. Italian ryegrass does not produce male and female plants, but it is an open pollinated species. This means that although it is not a requirement for successful pollination as in Palmer amaranth, two Italian ryegrass plants can cross with another. This genetic mixing between individuals and among different populations can contribute to genetic diversity, which is a prime “weedy” characteristic.
Although Italian ryegrass does not produce as much seed as Palmer amaranth, a single Italian ryegrass plant can produce 45,000 seed, which is more than enough seed to turn an isolated problem into a train wreck in one or two years.
Italian ryegrass and Palmer amaranth have similar qualities that make these species very aggressive as weeds. Barnyardgrass is another species that shares many of these “weedy” traits with Palmer amaranth and Italian ryegrass. But, the characteristic that these three species share that make them all hot-button issues in Mid-South agriculture is their tendency for developing resistance to multiple herbicide chemistries.
How do these species compare in the arena of herbicide resistance? Is one species more prone to developing resistance than another?
The Weed Science Society of America and the Herbicide Resistance Action Committee maintain a website called the International Survey of Herbicide Resistant Weeds (http://www.weedscience.org).
This website is a one-stop shop for information about herbicide resistance. It lists herbicide-resistant weeds by name, herbicide mode of action, country, and U.S. state.
Currently, Palmer amaranth is listed as resistant to four herbicide modes of action in 13 states while barnyardgrass is resistant to five herbicide modes of action in six states. Italian ryegrass is resistant to four herbicide modes of action in 10 states. These numbers are fairly similar for the three species, and they dictate that close attention be given to all three weeds.
In the southern U.S., glyphosate-resistant Italian ryegrass has been documented in Arkansas, Louisiana, Mississippi, and North Carolina. Italian ryegrass is also resistant to ACCase inhibitors (mainly Hoelon) in Arkansas, Georgia, North and South Carolina, Mississippi, Tennessee, and Virginia. Additionally, resistance to ALS inhibitors (Finesse, Osprey, Oust, and/or PowerFlex) occurs in Arkansas, Georgia, and Mississippi.
There are very few herbicides at our disposal with activity against Italian ryegrass; we are quickly running out of bullets for this weed.
Seven Mississippi Delta counties (Bolivar, Coahoma, Humphreys, Issaquena, Leflore, Tunica, and Washington) contain populations of Italian ryegrass with multiple resistance to glyphosate, ACCase inhibitors, and ALS inhibitors. That means these populations are resistant to all three chemistries.
Fortunately, not all ACCase inhibitors are created equally, and most ACCase resistance in Italian ryegrass is to Hoelon. Axial XL and Select Max are ACCase inhibitors but belong to separate herbicide families from Hoelon, so these two herbicides still control Mississippi populations of Italian ryegrass resistant to Hoelon. However, the ACCase inhibitors have a long history of resistance, so I fear it is only a matter of time before we lose Axial XL and/or Select Max.
Mississippi wheat crop
A sizeable wheat crop is presently up and growing in Mississippi. If you have wheat planted in one of the seven counties listed above, be mindful of your weed pressure. There is a good possibility that if you have Italian ryegrass, you may not be able to control it with Hoelon, Osprey, or PowerFlex. If this is the case, then the only remaining postemergence herbicide option at your disposal is Axial XL, and it may only be applied once per crop season.
In addition, if you have wheat, and are outside the above counties, do not assume the Italian ryegrass in your fields is not resistant to these herbicides.
In my wheat trials at Stoneville, I have produced excellent wheat yields when using a single application of Axial XL at spring green up. However, I rarely achieve 100 percent Italian ryegrass control with Axial XL applied in the spring.
The surviving Italian ryegrass plants produce seed that can be spread by the combine. These plants may not produce 45,000 seed growing in competition with wheat and following a herbicide application, but the seed that are produced will be there next year and will have to be managed at that time, either in wheat or during burndown.
So, Italian ryegrass plants not controlled in wheat contribute to a vicious cycle that must be broken in order to get ahead of this problem.
Herbicide resistance is on everyone’s mind. In areas with severe infestations of Palmer amaranth and Italian ryegrass, it is a never-ending grind because the growing seasons for these weeds only slightly overlap. Just because Italian ryegrass does not emerge and grow at the same time as row crops does not lessen its impact.
Be attentive. Scout for Italian ryegrass. We still have some effective herbicides to manage this weed in the burndown arena if they are used correctly in a program.
Our weed science group at Mississippi State has developed a one-page document listing programs for managing glyphosate-resistant Italian ryegrass during burndown (http://msucares.com/crops/weeds/index.html). If you have any questions about these programs, feel free to contact me at (662) 820-7794.