Semester of Graduation

Fall 2022


Master of Agriculture (MAgr)


School of Plant, Environmental & Soil Sciences

Document Type



Paraquat is the only herbicide labeled for postemergence control of Italian ryegrass in sugarcane. In 2017, sugarcane farmers in Louisiana reported Italian ryegrass control failures with paraquat applications. Seeds were collected from sugarcane fields in White Castle, St. Gabriel, and Bunkie, Louisiana where control was not achieved (reduced susceptibility). Additionally, Italian ryegrass seeds were collected from a sugarcane field in Welcome, LA with a history of paraquat use and with no reported control issues (susceptible population). A dose-response study was conducted at the Louisiana State University Agricultural Center’s Sugar Research Station in February 2021 and was repeated in November 2021. Paraquat was applied to 8- to 12- cm tall Italian ryegrass plants at 0, 1/32 1/16, 1/8, 1/4, 1/2, 1, 2, 4, 8, and 16X rates, of the maximum field use rate of 840 g ha-1, for the reduced susceptibility populations. For the susceptible population, paraquat was applied at 0, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1, and 2X rates. At 18 d after application, injury was recorded, then biomass was harvested and dried for 96 h. Dose-response curves were fitted using a four-parameter, log-logistic model in R software. The effective paraquat dose to cause 50% injury (ED50) was 77 g ha-1 for the Welcome population, whereas it required 200.4, 22.2, and 45.4 -fold more paraquat for the White Castle, St. Gabriel, and Bunkie populations, respectively. Likewise, the ED50 value required to reduce the dry weight by 50% was 44 g ha-1 for the Welcome population. To reduce dry weight biomass for 50% for the White Castle, St. Gabriel, and Bunkie populations, required rates of 13.3, 4.3, and 14.3 -fold more paraquat, respectively, when compared to the Welcome population. These results confirm the presence of paraquat-resistant Italian ryegrass in Louisiana.



Committee Chair

Orgeron, Albert