The annual weed control study was designed to compare the efficacy of experimental formulations and commercial herbicides in controlling crabgrass in turf and to examine turf tolerance. Some of the products were screened at various rates and tested in formulations with fertilizer.

This study was located in an area of common Kentucky bluegrass at the Iowa State University Horticulture Research Station north of Ames, IA. The design was a randomized complete block. The soil was a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with 3.1% organic matter, 86 ppm K, 7 ppm P, and a pH of 6.5. Three replications were conducted and individual plot size was 5 x 5 ft. Experimental formulations and commercial products were screened with an untreated control and Pendimethalin 60WDG included for comparisons for a total of 22 treatments (Table 1).

Granular materials were applied using 'shaker dispensers' to ensure uniform application. Sprayables were applied using a carbon dioxide backpack sprayer equipped with TeeJet #8006 nozzles at a spray pressure of 30-40 psi. For those treatments that included both a sprayable and granular product, the granular material was applied first and the sprayable was applied over it. Preemergent materials were applied to dry foliage on April 21 before crabgrass germination and were 'watered in'. The early postemergent material (treatment 21) was applied on June 19 when the crabgrass was in the 1- to 4-leaf stage. On July 27, when the crabgrass had 1 to 4 tillers, the late postemergent material (treatment 22) was applied.

Crabgrass infestation data were taken beginning July 7 and ending on August 29 (Table 1). Crabgrass populations were estimated as the percentage area per plot covered by crabgrass. Crabgrass control was calculated as the reduction in cover per plot compared with the untreated control (Table 2). Visual quality data were taken weekly from May 2 through August 29 (Table 3). Visual quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst quality.

Data were analyzed using the Statistical Analysis System (SAS Institute Inc., 1989-1996) and the Analysis of Variance (ANOVA) procedure. Treatment effects on weed populations and visual quality were tested using Fisher's Least Significant Difference (LSD) test.

The growing season was unusual with temperatures above normal and precipitation much below normal. It was a good year for aggressive annual weeds such as crabgrass. Percentage crabgrass cover was 30% on July 7 and reached 75% by August 29 in the untreated controls (Table 1). Throughout the duration of the study, all herbicide materials except an experimental treatment (Sample A, at 0.5 lb a.i./A applied preemergently and at 1.0 lb a.i./A applied late postemergently) significantly reduced crabgrass populations as compared with the untreated control (Table 2). An experimental Dithiopyr PE2 formulation applied at 0.250 lb a.i./A (treatment 9) provided greater than 93% control for the entire test period. This level of control, however, was not statistically different from that provided by many herbicides including commonly used Dimension 1EC at 0.250 lb a.i./A and Pendimethalin 60WDG at 1.5 lb a.i./A. Mean crabgrass control data show that all materials except Sample A provided similar levels of control as compared with the untreated crabgrass. Twelve of the treatments reduced crabgrass populations by > 80% and 17 by > 70%.

There were significant differences in visual quality from May 5 through June 8 and from June 23 through June 30 (Table 3). After July 7, there were no differences in quality. Because of the high temperatures and lack of moisture, turf quality deteriorated in the entire area. Mean data show that turf treated with herbicides (except Sample A at all rates, or Dithiopyr PE1 at 0.250 lb a.i./A or Pendimethalin 60WDG at 1.5 lb a.i./A) had better quality than the untreated control.

Table 1. Percentage crabgrass cover¹ in turf treated for the 2000 Preemergent Annual Grass Study.