- Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] resistant to multiple herbicide sites of action continues to limit winter wheat production in western Oregon. In addition to impacting wheat production, multiple resistant Italian ryegrass may threaten the market for annual ryegrass seed for cover crops if wheat fields with severe infestations are subsequently planted to annual ryegrass and the seed from these fields is sold as cover crop seed. Italian ryegrass resistant to diclofop was first documented in western Oregon in 1987 (Stanger and Appleby 1989), after less than 10 years of use. Since that time the list of herbicides Italian ryegrass populations have been observed to be resistant to has grown to include diclofop (WSSA group 1), diuron (WSSA group 7), mesosulfuron (WSSA group 2), flucarbazone (WSSA group 2), flufenacet (WSSA group 15), pinoxaden (WSSA group 1), propoxycarbazone (WSSA group 2) and pyroxsulam (WSSA group 2) . For example, in a 2013 wheat field trial, plots treated with pyroxsulam alone and flufenacet-metribuzin followed by pyroxsulam yielded 45 and 72 bu/a respectively. Untreated plots and plots where Italian ryegrass was controlled yielded 28 and 110 bu/a, respectively (Roerig et al. unpublished data). In 2014, pyroxasulfone a new group 15 herbicide that provides preemergent control of grass weeds in wheat, including those resistant to flufenacet, was registered. Previous patterns of resistance development in Italian ryegrass indicate that Italian ryegrass will likely develop resistance to pyroxasulfone as well.
An Italian ryegrass plant does not have the means to distribute its seed more than a meter or two, however harvest equipment does. McManny and Cavers (1988) found that proso millet in corn at harvest was moved in excess of 50m by combines and observed in surveys that proso millet could be spread across most of a field from an isolated patch in two years. As such, combines appear to pose the greatest risk of moving seed from an initial site of resistance development across an entire field. A combine separates harvested material into three parts: grain, straw, and chaff. The combine threshes and sifts the desirable portion of the crop along with anything that is similar in size and weight, often some weed seeds, into the grain tank. The straw is any long piece of the crop which is discarded either by spreading or dropping into a windrow for later baling. The chaff is the material that is too light to be grain and is blown out the back. In most cases the majority of weed seeds exit the combine in the chaff portion.
Some progress has been made in controlling weeds seeds in the chaff, such as use of chaff collection carts, narrow windrow burning and placing the chaff on top of the straw for baling. Additionally, the Harrington Seed Destructor has been in limited use in Australia for several years. This device routes chaff exiting the combine through a cage mill where 90-99% of weeds seeds are destroyed (Walsh et al. 2012). There has also been discussion of major equipment manufactures integrating milling or microwave systems to destroy weed seeds into the machines they manufacture. This sort of equipment may become available for growers in the Pacific Northwest in future years. Research is needed to determine efficacy of this type of system in controlling locally important weeds such as Italian ryegrass in wheat.
Any time a weed seed is prevented from entering the soil seed bank it preempts the opportunity for that seed to become a plant that will compete with the crop, produce offspring or even develop or spread herbicide resistance. However, for a practice such as weed seed removal or destruction to have value it must have a net effect of reducing the total weed seed bank or preventing the spread of new infestations. This research intends to determine the efficacy of these practices when herbicides do not adequately control the weed population. The data collected will help growers determine whether the use of these systems would be able to prevent or reduce populations of herbicide resistant Italian ryegrass and be beneficial to their farming operation.
Combine harvesters are designed to separate desirable material, such as grain, seeds, and kernels, from undesirable material such as stems, leaves, weed seeds and other foreign material while traveling through the field. The desired material is collected into the grain tank and everything else is typically discharged from the back of the combine into the field. Since this process occurs while the combine is moving, weed seeds entering the combine are typically discharged some distance from where the weed grew, potentially increasing the distance the seeds could travel beyond natural dispersal mechanisms. Studies were conducted in 2016 and 2017 to assess how far Italian ryegrass seeds travel during wheat harvest. Seed was prepared for the study by microwaving to prevent germination following the study and dyed to aid in detection and counting.
Twenty 0.74m² pans were placed 11m apart and wheat harvest was conducted straddling the pans so they remained centered under the combine. At the beginning of each pass, thin paper bags containing a total of 5.5kg of one color of Italian ryegrass seed were attached to the heads of the wheat. For each pass a different color of Italian ryegrass seed was used. The distribution of three colors was measured in 2016 and four colors in 2017. Since the colors were distinctly different it was not necessary to clean the combine between each color, allowing observations of the previous colors to be made during passes with a subsequent color at a distance of up to 873m. The first pan in the sequence had an average of 14,220 seeds and the number quickly decreased. By the end of the first pass, at 215m, the pans contained an average of 41 seeds of the color introduced at the beginning of the pass. In pans starting at 774 and up to 873m from the initial introduction, an average of 4.2 seeds per pan were collected, a rate of over 500,000 seeds/ha in the chaff row. These data provide evidence that weed seeds entering a combine during harvest can be transported great distances, readily facilitating the establishment of weed populations across a field from an isolated source from within the field.
Two additional studies assessed the effect of collecting chaff using a chaff cart during harvest on the population of Italian ryegrass in a field over time. In both trials there were four treatments: untreated, herbicide, herbicide plus chaff collection, and chaff collection alone. In one trail the entire area was planted with Italian ryegrass during the first year of the crop. In the other a strip of Italian ryegrass was planted at one end of each plot and harvest was conducted across the plot starting from the side with the strip. In both trials winter wheat was planted in years 1-3, red clover was planted in year 4 and harvested in years 4 and 5, and winter wheat was planted in year 6. In the solid planted field Italian ryegrass populations were the lowest in the herbicide plus chaff collection plots three out of the six years but the difference was never statistically significant (at p-value 0.05) from herbicide alone. It the trail where only a strip was planted Italian ryegrass populations were counted across the length of the plot. The plot was divided into ten groups. The herbicide plus chaff collection had the lowest population in each distance group, however this difference was only significant (at p-value 0.05) in the first group. These results suggest a trend towards lower Italian ryegrass populations when chaff collection is used in conjunction with herbicides than when herbicides are used alone, however further evaluation of the practice is needed to provide more conclusive results.