Italian ryegrass is historically not a problem on the clay soils where most rice in Mississippi is grown. However, fields to be planted to rice that contain glyphosate-resistant (GR) Italian ryegrass should be managed aggressively.
In the typical Mississippi rice field where little to no tillage occurs in the spring, the presence of GR Italian ryegrass compromises burndown programs by reducing herbicide coverage on other species. Also, no herbicide labeled for rice is effective for controlling GR Italian ryegrass, so there are no herbicide options after rice emerges.
Among the grass herbicides labeled for rice, only Beyond controls GR Italian ryegrass more than 50%. Unfortunately, Beyond is an ALS-inhibiting herbicide (Group 2 herbicide), and Italian ryegrass is resistant to ALS herbicides in most counties in the Mississippi Delta, rendering this chemistry ineffective for ryegrass control. Therefore, it is imperative to control all GR Italian ryegrass before rice is planted.
Similar to corn, rice is also sensitive to early-season competition from GR Italian ryegrass. Research at the Delta Research and Extension Center in Stoneville has evaluated the impact of Italian ryegrass by comparing rice growth in plots with and without Italian ryegrass. The only difference between plots was the amount of Italian ryegrass residue at planting.
Where rice was planted into Italian ryegrass residue, rice seedling density was reduced 25%, height was reduced 33%, and rice maturity was delayed from 7 to 12 days compared with plots that did not contain Italian ryegrass residue. By the end of the season, rice yields were 15% lower in plots that contained Italian ryegrass residue early in the season.
Herbicide Resistance Info
For rice, 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.
Even though there is still time to safely apply clethodim in Mississippi rice fields, some GR Italian ryegrass in parts of the Delta is getting too large for effective control with clethodim. The only other reliable postemergence option for GR Italian ryegrass control is paraquat.
Large GR Italian ryegrass (12 to 24 inches) will require two applications of paraquat (3 to 4 pints of Gramoxone SL or 2 to 2.67 pints of 3-lb paraquat) spaced 10 to 14 days apart. Earlier applications are better to optimize rice growth and yield.