This study was designed to determine the optimal intervals between application of a Novartis experimental herbicide, CGA-362, and the overseeding of creeping bentgrass with perennial ryegrass and Poa trivialis. This herbicide is a new sulfonyl urea formulation that has potential as a non-selective product for cool season grasses. This study was conducted at the Iowa State University Horticulture Research Station on a native soil green with established 'Penncross' creeping bentgrass. The soil on this green was a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with 4.9% organic matter, 4 ppm P, 85 ppm K, and a pH of 7.15.

The study was designed as a split plot with herbicide treatment as the main plot treatment and the seeding interval as the sub plot effect. Overseeding with either 'Charger II' perennial ryegrass or 'Winterplay' Poa trivialis took place at 3, 7, 14, and 21 days after treatment applications (DAT).

The size of the individual plots was chosen so they could be divided into four subplots that were the width of the Blue Bird verticut. The blade width on the Blue Bird was 18" and it was 21" between the wheels. Each individual plot was, therefore, 7 x 5 ft. Each individual plot was divided into 4 sections in such a way that on each planting date one continuous strip was verticut for planting per replication. Five-foot barrier rows were left between replications. Seeding was performed by hand using the proper amount of seed per subplot.

Perennial ryegrass and Poa trivialis were overseeded into separate plots. Ryegrass was seeded at 350 lbs/A and Poa trivialis at 150 lbs/A. Overseeding was performed on June 9, June 13, June 20, and June 27. At each event, one subplot per plot was randomly selected and was verticut, swept of debris, and hand planted. The seed was brushed into the grooves produced by the verticut.

There were six treatments including CGA-362 at 20 and 40g a.i./A, and Roundup Pro at 1 lb a.i./A on overseeded plots. In addition, CGA-362 was applied at 10 g a.i./A and 15 g a.i./A on non-overseeded plots. A non-seeded, untreated control was included. The CGA-362 formulation was mixed with a non-ionic surfactant at 0.25% V/V for all applications. Treatments were applied on June 6, 2000. It was 75º F with a slight wind. The sprayables were applied with a carbon dioxide backpack sprayer equipped with TeeJet #8006 flat fan nozzles used at 30 - 40 psi. There was no drift of materials during application. The plots were checked when still wet and coverage was uniform.

Bentgrass visual quality and percentage cover data were taken for overseeded and non-overseeded plots. Visual quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst quality. Quality data were taken from June 9 through September 5 (Table 1). Percentage bentgrass cover data were taken from June 27 through September 5 and represent the percentage area per plot covered by bentgrass. These data were converted to reflect percentage bentgrass kill per plot (Table 2).

Perennial ryegrass and Poa trivialis overseeded plots were rated for stand density beginning soon after germination. Density was assessed as the percentage germination of the overseeded species in the grooves within each plot. Germination data for both species were taken from June 22 through September 5 (Tables 3 and 4). These data also were compiled by treatment for each date by species (Tables 5 and 6).

Visual quality and percentage bentgrass cover data were analyzed using the Statistical Analysis System and the Analysis of Variance (ANOVA) procedure. The analyses for bentgrass quality and percentage cover were designed to indicate differences between treatment effects on quality and cover. The analyses for perennial ryegrass and Poa trivialis germination were constructed to test for main plot (herbicide treatment) effects and subplot (seeding interval after herbicide treatment applications) effects and the General Linear Model (GLM) procedure was used. An interaction between herbicide treatment and seeding date was examined to test for an optimum date after treatment for planting each grass species.

Significant differences in quality were found from June 9 through August 8 (Table 1). By June 13, the quality of all treated bentgrass was significantly worse than the untreated control. Bentgrass treated with Roundup Pro had the worst quality as compared with other treated and untreated bentgrass. The treated bentgrass had begun to recover by June 27 and new plants had emerged. From this date through the end of the test, the quality of the treated and untreated bentgrass was similar. Mean data indicate that Roundup Pro was the most toxic to bentgrass and was significantly different from the other treatments and the untreated control. The experimental formulation, CGA-362 was most harmful at 40 g a.i./A. The other CGA-362 treatment levels produced similar amounts of bentgrass damage.

Differences in percentage bentgrass cover were significant for the entire test period (Table 2). Treatment with Roundup Pro resulted in a 92.5% reduction in bentgrass cover on June 27 and the percentage cover remained below 25% through August 16. By September 5, recovery and regrowth had only filled in 40% of the Roundup Pro treated area and the mean percentage cover was 22.9%. Mean bentgrass cover reductions caused by CGA-362 were below 20% for the 10, 15, and 20 g a.i./A treatments. The mean cover reduction for the 40 g a.i./A treatment of CGA-362 was 38% as compared with 77.1% for Roundup Pro.

The data analyses for Poa trivialis and perennial ryegrass did not show significant date by treatment interactions (Tables 3 and 4). These data indicate that there were no differences in Poa trivialis and perennial ryegrass percentage cover among the four different overseeding dates. Differences in percentage cover for both species were found among the treatments on some of the data sampling dates (Tables 5 and 6).

Table 1. Visual quality¹ of bentgrass on plots treated with CGA-362 and Roundup Pro (June 9 through July 20).