- Italian ryegrass (Lolium perenne L. spp. multiflorum (Lam.) Husnot) is one of the most troublesome weeds with respect to herbicide resistance selection. Some reasons for this are the numerous documented cases of multiple and cross herbicide resistance and some of the biological characteristics of this species, such as wind cross-pollination that allows the rapid spread of some resistance traits. Oregon can be considered a unique place to study Italian ryegrass resistance management due to high production of grass seed crops in the western part of the state. The dominant grass seeds crop species grown in western Oregon are Italian/perennial ryegrass and tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.). Both crops require a high purity in the final seed lots making the management of other grass weeds vital. Seeds carrying herbicide resistance into a seed lot must be avoided.
Oregon Italian ryegrass production is divided into two types of cultivars: diploids (2x = 2n = 14) and tetraploids (2x = 4n = 28); conversely, all weedy biotypes of Italian ryegrass documented as a weed species are diploid. No studies have documented the ploidy diversity in the Willamette Valley area. A survey was conducted to quantify the frequency, distribution, ploidy diversity and herbicide resistance in populations of Italian ryegrass in the Willamette Valley. A total of 150 fields were surveyed between 2017 and 2018. Fifty percent (75 fields) of the fields had Italian ryegrass present with the majority of those located in the northern surveyed area. In these fields, 42% (32 populations) had high Italian ryegrass density levels (20 or more plants/m2). Herbicide screening tests were conducted for 11 herbicides: clethodim, pinoxaden, quizalofop-p-ethyl, glyphosate, glufosinate, paraquat, mesosulfuron, pyroxsulam, pronamide, flufenacet + metribuzin, and pyroxasulfone. For the screened populations, 88% (66 populations) were classified as having presence of resistance to at least one herbicide tested. Around 6% (5 populations) of the tested Italian ryegrass populations were tetraploids. No resistance traits were confirmed in tetraploid populations.
A combination of both multiple and cross-resistance with a frequency of 61% (46 populations) was identified in the tested populations. Plant density level was correlated with the presence of multiple resistance (considering populations with high presence of resistance level). The odds-ratio of finding cross-resistance, considering only high presence of resistance level, was higher by a factor of 3.12 in wheat fields when compared to tall fescue fields. The most frequent resistance was to the following modes of action: ACCase, ALS and EPSPs inhibitors. According to herbicide screening tests, glufosinate and pyroxasulfone are still options to control Italian ryegrass but some cases of glufosinate resistance were already documented in Oregon. Cluster patterns of multiple resistance with were identified in the surveyed area. This research was the first survey of spatial distribution, frequency of herbicide resistance and ploidy diversity of Italian ryegrass in western Oregon. These numerous cases of resistance found in western Oregon creates a need for new management approaches.
Research on rangeland areas showed that synthetic auxin herbicides can affect seed viability and can be used as a management tool to reduce the seed production of invasive annual grasses. However, no studies have been conducted on the effect to Italian ryegrass and the feasibility as a management practice in tall fescue grown for seed. Greenhouse and field trials were conducted to quantify the effects of synthetic auxin herbicides on the seed viability of Italian ryegrass and tall fescue. Two years of greenhouse trials were conducted in 2017 and 2018. Four field trials were conducted in western Oregon between 2017 and 2018. Eight synthetic auxins herbicide treatments were tested: two rates of 2,4-D, two rates of dicamba, aminopyralid, 2,4-D + dicamba, 2,4-D + clopyralid and halauxyfen-methyl. Results indicate that aminopyralid reduced seed viability of different biotypes of Italian ryegrass both in controlled and field environments. Aminopyralid reduced the viability of Italian ryegrass seeds; however, tall fescue was more sensitive to this treatment making this management method not applicable for this crop. Aminopyralid still might be used for other cropping systems and understanding the mechanism involved on the seed viability reduction could elucidate better ways to use this management practice. Thus, future studies should explore the physiological mechanism involved in this effect, quantify the recommend rates of aminopyralid and determine if other crops might be tolerant to the application of aminopyralid.