Ethofumesate-resistant annual bluegrass (Poa annua) in grass seed production systems

Authors: VUKOVIC, VERA - Purdue University; Mattox, Clint; KOWALEWSKI, ALEC - Oregon State University; MCNALLY, BRANDON - Purdue University; BIGELOW, CALE - Purdue University; MEYERS, STEPHEN - Purdue University; BROSNAN, JIM - University Of Tennessee; PATTON, AARON - Purdue University

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2023
Publication Date: 10/13/2023
Citation: Vukovic, V., Mattox, C.M., Kowalewski, A.R., Mcnally, B.C., Bigelow, C.A., Meyers, S.L., Brosnan, J.T., Patton, A.J. 2023. Ethofumesate-resistant annual bluegrass (Poa annua) in grass seed production systems. Weed Technology. 37(6):628-636. https://doi.org/10.1017/wet.2023.68.
DOI: https://doi.org/10.1017/wet.2023.68

Interpretive Summary: Annual bluegrass is a weed that is prevalent in grass seed production systems and is particularly problematic to suppress because of herbicide resistance. Ethofumestate is an herbicide commonly used in grass seed production systems and annual bluegrass resistance to ethofumesate was reported in 1994. The objective of this study was to assess the level of annual bluegrass resistance to ethofumesate in US seed production fields, seed cleaning facilities, and seed lots. In this study, 31 populations of annual bluegrass were collected from grass seed production fields in Oregon, Minnesota, and Washington in 2019 and 2020, 18 populations were collected from seed lots from a seed testing laboratory, and 6 populations were collected from seed cleaning facilities. Ethofumesate post-emergence dose-response greenhouse experiments were carried out by comparing the 55 annual bluegrass populations to a known ethofumesate resistant and a known susceptible annual bluegrass population. All three seed sources examined contained annual bluegrass populations considered to be resistant based on the testing criteria, which indicates that annual bluegrass populations in grass seed production systems resistant to ethofumesate is common. Implications of this research are that annual bluegrass management in seed production will likely need to include integrated weed management practices to reduce the risk of herbicide resistance.

Technical Abstract: The prolific seed production and polyploidy of annual bluegrass (Poa annua L.) allow for the rapid development of herbicide resistance. Ethofumesate-resistant annual bluegrass plants were identified in the 1990s in grass seed production in Oregon but their prevalence and distribution are not well documented. Therefore, a dose-response experiment was initiated to determine the potential level of ethofumesate resistance in seed production systems. Seeds from 55 annual bluegrass populations were obtained from three sources: seed production fields (31 populations), seed cleaning process (6 populations), and seed testing lots prior to retail distribution (18 populations). Additionally, two populations, one with known resistance and one with known susceptibility, were identified in preliminary testing and used as controls in this experiment. Seed was increased from each collected population. Individual seedlings were then transplanted into separate cone-tainers, grown to a size of 2–3 tillers in the greenhouse, and then sprayed using a compressed air track spray chamber with ten doses of ethofumesate: 0, 0.56, 1.1, 2.8, 5.6, 8.4, 11.2, 16.8, 22.4, and 44.8 kg ha-1; with 0.84 to 2.2 kg ai ha-1 as the label application rates for perennial ryegrass (Lolium perenne L.). The resistant-to-susceptible ratio of populations across all sources ranged from 0.5 to 5.5. The most resistant populations found in production fields, seed cleaning, and seed testing lots had ED50 values of 12.1, 9.4, and 13.1 kg ai ha-1, respectively. Further, 68% of the populations found in production fields had ED50 higher than 6 kg ai ha-1, which indicates that annual bluegrass resistance is common in grass seed production. As such, herbicides alone will likely be ineffective at controlling annual bluegrass and integrated weed management strategies should be implemented by growers.