What is in this article?:
- Glyphosate-resistant ryegrass, one hard-to-kill weed
- Spray coverage
- Glyphosate-resistant biotypes of Italian ryegrass have been documented in Mississippi, Arkansas and North Carolina.
- The best timing for control of glyphosate-resistant Italian ryegrass is before it ever comes up.
- There are very few postemergence options for control of glyphosate-resistant Italian ryegrass, and once it's in crop, there's nothing producers can do.
Spray coverage is also critical, weed scientists say. Be sure to use spray nozzles such as flat fan or twin jet to ensure thorough coverage of the weed. Avoid the use of air induction nozzles with contact herbicides.
Eubank says a 6-inch tall plant with 6 tillers “is the breaking point for control. You’re not going to control it very effectively after this point. It is probably going to require a two-pass program to effectively control Italian ryegrass — either a fall residual followed by a winter application of clethodim or a winter application of clethodim followed by a spring application of paraquat.”
Eubank noted that there are very few postemergence options for control of glyphosate-resistant Italian ryegrass. “Once you’re in crop, there is nothing you can do. So it’s imperative that glyphosate-resistant Italian ryegrass is controlled prior to planting.”
Arkansas weed scientist Bob Scott said herbicide-resistant Italian ryegrass “is the No. 1, 2 and 3 problems in the state’s wheat crop.” Scott says most ryegrass populations are resistant to Hoelon and some are resistant to ALS herbicides, while glyphosate-resistant Italian ryegrass “is becoming a bigger and bigger problem.”
Alan York, a weed scientist at North Carolina State University, says glyphosate-resistant Italian ryegrass has been confirmed “in a couple of counties in North Carolina.”
According to DREC research associate Robin Bond, Italian ryegrass prefers fertile soil and mild climates. It has a low tolerance for hot, dry climates and harsh winter conditions. Its peak emergence is in the fall but it can emerge during the winter months.
Italian ryegrass germinates in six to 10 days when daytime temperatures are between 50 and 87 degrees and it flowers three weeks after head emergence.
Italian ryegrass is a slow spreader, oftentimes starting out in a corner and slowly taking over. It can also be spread by equipment, animals or flooding. Its color and texture, shiny and dark green, often make it difficult for sensing devices to pick up, making variable-rate applications more difficult.
Robin Bond noted that Italian ryegrass is not as prolific a seed producer as Palmer pigweed, “but it makes 20,000 to 45,000 seeds per plant. Italian ryegrass also exudes an unknown substance that kills surrounding plants, which allows it to be competitive and establish itself as the dominant plant. In rice, this characteristic has been known to reduce yields.”
Italian ryegrass is also a very efficient scavenger of nitrogen.
DREC weed scientist Vijay Nandula said resistance in the weed was first observed in 2005. Italian ryegrass in a field which had been planted to glyphosate-resistant soybeans for six consecutive years survived two applications of glyphosate. The biotypes produced viable seed.
By 2009, scientists confirmed glyphosate-resistant Italian ryegrass in 12 Mississippi counties. Italian ryegrass also has documented resistance to ALS and ACCase herbicides. No resistance has been documented to Command, Dual or Treflan in the United States.
Jeff Ray, a research geneticist with the USDA-ARS, pointed out that Italian ryegrass found in Mississippi shows a high degree of genetic diversity which has the potential to allow adaption to changing environments including what growers are doing to control it.