This study was designed to evaluate the spectrum of weed control provided by ZPP1560 as compared with Roundup RTU and to determine the effect of weed height (timing of application) on weed control. It was conducted at the Horticulture Research Station north of Ames, IA in old planting beds consisting of 'common' Kentucky bluegrass infested with a variety of broadleaf weed species. The soil was a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with 3.2% organic matter, 71 ppm K, 22 ppm P, and a pH of 7.95. Among the plots, there was a uniform distribution of weed species that covered approximately 20% of the area in each plot. The predominate species were dandelion and clover.

The experimental design was a randomized complete block. Three replications were conducted and individual plot size was 5 x 10 ft. An experimental formulation ZPP1560 RTU from Syngenta was screened with Roundup RTU (Table 1).

The first applications were made on May 18 when the weeds were 3 - 5 inches in height. The second applications were made on July 27 when the dandelions and clover were approximately 7- 9 inches in size. The treatments were 'ready to use' formulations and were not diluted for applications. They were applied using a carbon dioxide backpack sprayer equipped with TeeJet #8006 nozzles at a spray pressure of 30-40 psi.

The efficacy of the materials was measured by estimating the percentage area per plot covered by dead plants. The first data were taken on May 22, four days following the initial applications (Table 1). Once regrowth had begun in the treated plots, the percentage of weed cover also was recorded. For these data, estimations were made of the total area per plot covered by green weeds (Table 2). Crabgrass infestations were determined by estimating the percentage of crabgrass cover per plot (Table 3). Weed and crabgrass cover data were taken from June 28 through September 13. In addition, weed populations were recorded by species at the end of the season by estimating the percentage cover per plot (Table 4). Dandelion and clover were the predominate broadleaf species. Redroot pigweed, black medic, and spurge plants were found in two of the fifteen individual plots but the populations were less than 1% of the area.

Data were analyzed using the Statistical Analysis System (SAS Institute Inc., 1989-1996) and the Analysis of Variance (ANOVA) procedure. Treatment effects on percentage dead area and weed coverage were compared using Fisher's Least Significant Difference (LSD) means comparison test.

When applied to 3 - 5" weeds, Roundup produced faster 'kill' than ZPP1560. On May 22 (four days post treatment), there was a significantly larger area of dead plants in plots treated with Roundup than in either those treated with ZPP1560 or in untreated (Table 1). By May 29, there was total 'kill' of plant material in plots treated with the herbicides.

Weeds began to grow in the treated plots approximately four weeks post treatment and on June 21 there were significantly more weeds in the plots treated with Roundup as compared with plots treated with ZPP1560 and plots untreated. Weed coverage was similar for both herbicides after June 28 and the percentage weed cover was > 97% for July 11 through September 13. The predominate weed species covering the previously treated plots was crabgrass. Percentage crabgrass cover was similar in treated plots but was significantly higher than in untreated plots (Table 3). There were no differences in dandelion numbers on September 13 among the treated and untreated plots (Table 4). There was no clover in the previously treated plots on September 13.

When larger weeds (7-9") were treated, similar levels of plant death at four days post treatment were produced for the herbicides. Total 'kill' of all plant material in these plots was recorded on August 9.

Live weeds were first noted in the treated plots on August 21 and by September 13, there were similar levels of weed cover in the treated plots (Table 2). Percentage weed cover was 26.7% for ZPP1560 and 20.0% for Roundup. Crabgrass and dandelion populations also were similar in the treated plots on September 13 (Tables 3 and 4).

Table 1. Percentage dead area¹ in turfgrass treated for the 2001 Syngenta Weed Control Study.