Spatial variation in roughstalk bluegrass affects tall fescue seed yield

Authors: Mueller Warrant, George; Trippe, Kristin; JESSIE, WILLIAM - Oregon State University

Submitted to: Seed Production Research at Oregon State University
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2019
Publication Date: 4/30/2019
Citation: Mueller Warrant, G.W., Trippe, K.M., Jessie, W. 2019. Spatial variation in roughstalk bluegrass affects tall fescue seed yield. Seed Production Research at Oregon State University. 2019:9-11.

Interpretive Summary: Roughstalk bluegrass has long been recognized as a serious weed in production of tall fescue and perennial ryegrass seed, competing vigorously in the field and complicating seed cleaning. Because the best herbicide treatments currently available in these crops cause substantial injury to the crops, with yield losses often exceeding 25%, an alternate management approach has been proposed. The new approach would use knowledge of weed patterns within fields, and limit herbicide applications to those areas within fields where roughstalk bluegrass density was already high enough to be causing serious yield losses. The first step in this process is to measure the severity of roughstalk bluegrass infestations across entire fields at a resolution fine enough to be useful in deciding which areas to treat and which to leave uninjured. Weed counts taken at higher resolution spacings of 60 feet compared to coarser resolutions of 180 feet improved the quality of the maps intended to guide spatially varying applications of herbicide. However, at a site with the most variable weed densities were this approach is most likely to be of value, the coarser resolution data was nearly as useful as the finer resolution data. The coarser resolution data is also 9 times easier to obtain. Unusually good prices for grass seed in 2018 led the grower to decide to just live with the weed rather than applying spatially varying treatments. As a result, yield monitor data from the grower's combines simply measured the losses due to competition rather than combining those with damage from the herbicide. Estimates yield losses were 49% in 2018 and 20% in 2017.

Technical Abstract: The herbicide glufosinate is registered for use in seed crops of tall fescue and perennial ryegrass as a "rescue/salvage" treatment for suppression of weeds such as roughstalk and annual bluegrass when all previous treatments have failed. Yield losses in tall fescue are particular pronounced, and can exceed 30%, especially when applications are delayed into early spring. A novel approach for use of this herbicide for control of roughstalk bluegrass in tall fescue was developed involving grid-sampling of entire fields at resolutions of 0.74 to 0.083 acres per point to identify patches that could either be treated with the herbicide using a GPS-controlled sprayer or left alone based on severity of the weed. Because unusually high seed prices convinced the cooperating grower to leave his fields untreated rather than risk reductions in seed yield from the herbicide, analyses of relationships between roughstalk bluegrass severity and tall fescue seed yield are simply measurements of weed/crop competition rather than more complex interactions of competition plus crop damage in areas treated with glufosinate. Yield loss estimates for average levels of weed infestation compared to weed-free areas of the field ranged from 49% in 2018 to 20% in 2017 at the site with the most highly variable infestations. Yield losses were not measurable at the other two sites. In one case, this was because it was a very young stand with only scattered presence of very small roughstalk bluegrass plants. In the other case, roughstalk bluegrass was present and uniform throughout the field. Any spatial variability would be due to other factors.