Preplant Burndown Options for Glyphosate Resistant Italian Ryegrass
Kurt M. Vollmer, Ph.D.
Extension Specialist, Weed Management
Italian ryegrass is a winter annual weed found throughout the United States. This species is most problematic in small grain production, but can be a problem if not managed prior to planting corn and soybean. Uncontrolled, Italian ryegrass can reduce corn yields as much as 65% and soybean yields as much as 37% (Steckel and Bond 2018). Italian ryegrass resistant to Group 1 (ACCase-inhibiting) herbicides was first reported in Maryland in 1998 (Heap 2024). In 2017, a population of Italian ryegrass from southern Maryland exhibited resistance to both Group 1 and Group 2 (ALS-inhibiting) herbicides. In 2021, a population of Italian ryegrass from Maryland’s Eastern Shore exhibited resistance to glyphosate, clethodim, and pyroxsulam. It is recommended that Italian ryegrass be controlled in the fall with residual herbicides or tillage. However, Italian ryegrass can emergence in the spring, and residual herbicide control from fall applications may decline prior to soybean planting. Therefore, options are needed for pre-plant control of Italian ryegrass prior to planting cash crops.
A study was conducted in the spring of 2023 at the Lower Eastern Shore REC Poplar Hill facility to investigate preplant herbicides for controlling Italian ryegrass. Treatments included glyphosate, glyphosate + clethodim, glyphosate + saflufenacil, paraquat, and paraquat followed by paraquat. Herbicide applications were made on March 30 and April 13, 2023. A non-treated check was included for comparison. Weed control was rated on a visual scale from 0% (no control) to 100/% (complete control).
No treatment differences were observed 7 days after the initial treatment (DAT, Table 1). However, greater control was observed with glyphosate + clethodim 14 and 22 DAT, and by sequential applications of paraquat 22 DAT compared to other treatments (Figure 1). This study supports previous work conducted in Pennsylvania that showed glyphosate + clethodim controlled ryegrass 98% two months after treatment (Wallace and Lingenfelter 2022).
While this particular population does not appear to be glyphosate-resistant, it is important to note that several factors dictate the efficacy of ryegrass control with glyphosate. Low temperatures will affect glyphosate movement in plants. For best results, glyphosate should be applied at 1.25 to 1.5 lb. ae/A when temperatures are greater than 55°F (and remain above 45°F for 3 to 5 days). Damage to foliage using urea ammonium nitrate (UAN) and high rates (>0.25 lb. ai/A) of triazine herbicides such as atrazine, simazine, and metribuzin can also reduce glyphosate absorption and translocation. Ryegrass plants should be less than 6 in. (but no more than 8 in.) tall at the time of application. A spray grade ammonium sulfate (8.5 to 17 lb. /100 gal) should also be included in the tank to minimize any water compatibility issues.
Always consult the label before applying any pesticide.
a. Means followed by the same letter are not significantly different (α = 0.05).
b. All treatments contained ammonium sulfate (8.5 lb. /100 gal), a nonionic surfactant (0.25% v/v) was included with the glyphosate + clethodim treatment, and crop oil (1% v/v) was included with the glyphosate + saflufenacil and paraquat treatments.
c. Abbreviation: DAT, days after (initial) treatment; fb, followed by.
d. Paraquat was applied as a sequential application on March 30, 2023 and April 13, 2023.
References:
Heap, I. (2024). The International Herbicide-Resistant Weed Database. http://weedscience.org/Home.aspx. Accessed 28 March 2024.
Steckel L, Bond J (2018) Italian ryegrass management in soybeans. Take Action Herbicide Resistance Management. United Soybean Board. https://iwilltakeaction.com/uploads/files/57229-7-ta-hrm-factsheet-italianryegrass-r2-final.pdf. Accessed 28 March 2024.
Wallace J, Lingenfelter D (2022) Glyphosate is Necessary to Control Annual Ryegrass. No Till Farmer.
https://www.no-tillfarmer.com/articles/11328-glyphosate-is-necessary-to-control-annual-ryegrass. Accessed 28 March 2024.
