Image from Mississippi State University
Although it is a winter annual species, glyphosate-resistant (GR) Italian ryegrass has become a primary weed of corn in Mississippi. An early burndown targeting GR Italian ryegrass is more critical in fields to be planted to corn than those that will be planted to other crops.
Most people consider corn to be one of the more competitive crops. It usually emerges quickly, grows rapidly, and shades the middles in a much shorter time than cotton or soybean.
A general rule is that if corn can be maintained weed free for the first four to five weeks after emergence, then there will be no effects on yield due to weed competition; however, this rule applies to weeds that emerge at the same time or following corn. It does not apply to GR Italian ryegrass, which is usually 12 to 24 inches tall during corn planting season in the Mississippi Delta.
When corn is planted into standing GR Italian ryegrass, the aboveground parts of Italian ryegrass will reduce the light and space available to developing corn seedlings while ryegrass roots will limit the water and nutrients that can be taken up by corn.
Another motivation for controlling GR Italian ryegrass prior to planting is the lack of over-the-top herbicide options available in corn. Some ALS inhibitors (Accent, Resolve, Steadfast, or Stout) labeled in corn have activity on Italian ryegrass.
Unfortunately, Italian ryegrass is resistant to ALS-inhibiting herbicides in most counties in the Mississippi Delta, rendering this chemistry ineffective for Italian ryegrass control in many areas. Therefore, it is imperative to control all Italian ryegrass before corn is planted.
Research at the Mississippi State University Delta Research and Extension Center (DREC) in Stoneville documented that at least two herbicide applications are required for >90% control of GR Italian ryegrass and that the benefit to cost ratio for GR Italian ryegrass control in corn was 13:1.
This means that for every $1 per acre spent on efforts to control Italian ryegrass, $13 per acre were gained in corn yield. Other research has demonstrated that corn yield and gross economic returns were optimized when GR Italian ryegrass control was triggered at least 21 days before planting.
Herbicide Resistance Info
For corn, a spring burndown program for GR Italian ryegrass should include the maximum rate of glyphosate plus 16 ounces of Select Max or 8 ounces of 2-lb clethodim formulation applied not less than 30 days before planting.
Auxin herbicides like 2,4-D and/or dicamba may be added to the herbicide mixture if the maximum rate of clethodim is utilized. Italian ryegrass that escapes the early burndown application of clethodim should be treated with paraquat (3 to 4 pints of Gramoxone SL or 2 to 2.67 pints of 3-lb paraquat).
Earlier applications are better to optimize corn growth and yield.