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Controlling mature roughstalk bluegrass (Poa trivialis L.; RSBG) using bispyribac–sodium (BYS) or sulfosulfuron (SUL) often yields inconsistent results. Attempting to control RSBG shortly after emergence may eliminate or reduce it with fewer inputs and less noticeable creeping bentgrass (Agrostis stolonifera L.; CBG) phytotoxicity than if treated at maturity. The objective of these studies was to determine whether BYS or SUL controls seedling RSBG with only minimal seedling CBG cover reduction. Four separate studies on either CBG or RSBG were conducted in spring or fall of 2007 and repeated in 2008 to simulate spring or fall fairway establishment. Studies were arranged as split plots with application timing (7, 14, 21, or 28 days after CBG emergence) as main plots and subplots were herbicide treatments in a 2 × 5 factorial with BYS or SUL applied once at five uniformly increasing rates of 0, 18, 37, 55, and 74 g·ha⁻¹ a.i. and 0, 6, 13, 19, and 26 g·ha⁻¹ a.i., respectively. Plots were maintained at 1.3 cm and emergence was defined as ≈50% of the study area being populated with one- to two-leaf CBG seedlings. Spring-seeded stands of CBG were safely treated with BYS 14 or more days after emergence (DAE) at 55 g·ha⁻¹ a.i. or less, whereas SUL was not safe by 28 DAE at any tested rate. Fall-seeded CBG was generally less sensitive to BYS and SUL. Sulfosulfuron resulted in excessive damage if applied to seedling CBG before 14 DAE at rates greater than 6 g·ha⁻¹ a.i. and if applied before 21 DAE at rates greater than 26 g·ha⁻¹. Bispyribac–sodium was safely applied as soon as 7 DAE at rates of 74 g·ha⁻¹ a.i. or less. Chemical names used: {2,6-bis[(4,6-dimethoxypyrimidin-2-yl)oxy] benzoic acid} (bispyribac–sodium); {1-[4,6-dimethoxypyrimidin-2-yl]-3-[2-ethanesulfonyl-imidazo(1,2-a)pyridine-3-yl) sulfonyl]urea} (sulfosulfuron).

Roughstalk bluegrass (Poa trivialis L.) contamination is problematic on golf course fairways from the Midwest to the mid-Atlantic regions of the United States. Bispyribac–sodium and sulfosulfuron have potential to selectively control roughstalk bluegrass. Our objectives were to determine the most effective herbicide treatments for short- and long-term roughstalk bluegrass control and to determine if interseeding with creeping bentgrass (Agrostis stolonifera L.) after herbicide treatments will improve long-term control of roughstalk bluegrass or conversion to creeping bentgrass. Plots were treated with bispyribac–sodium or sulfosulfuron and then half of each plot was interseeded with creeping bentgrass in early August, 2 weeks after the final herbicide application in 2006, 2007, and 2008 in Indiana. Roughstalk bluegrass cover reduction was highest when treated with bispyribac–sodium at 56 or 74 g·ha⁻¹ a.i. applied four times or sulfosulfuron at 27 g·ha⁻¹ a.i. applied three times. Interseeding with creeping bentgrass improved long-term roughstalk bluegrass control and quickened conversion to creeping bentgrass. Furthermore, bispyribac–sodium and sulfosulfuron appeared to be more effective in the first 2 years of the study when seasonal heat stress was greater, which appeared to improve long-term roughstalk bluegrass control and promoted creeping bentgrass establishment. Chemical names used: {2,6-bis[(4,6-dimethoxypyrimidin-2-yl)oxy] benzoic acid} (bispyribac–sodium), {1-[4,6-dimethoxypyrimidin-2-yl]-3-[2-ethanesulfonyl-imidazo(1,2-a)pyridine-3-yl) sulfonyl]urea} (sulfosulfuron).