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.