Weeds cause problems is general appearance and visual quality of lawns. A single weed in a lawn may be considered a problem for the intended use of that area. Thus it becomes obvious that any method that helps to eliminate or decrease the population of weeds is a point of interest and it requires attention.

Due to the perennial crop cycle in most turf grasses, herbicides that are selective to different weed species are necessary. Lontrel and Confront are selective herbicides in turf species aimed to control several broad leave species. The objective of this study is to test the effectiveness of these two products controlling some broadleaves and to observe their effects on the crop itself.

This herbicide study was conducted on a mature stand of common Kentucky bluegrass at the Horticulture farm of Iowa State University from May to August of 2001. The site was located 6 miles north of Ames, Iowa.

Two different herbicides, Confront and Lontrel at two different concentrations were applied over Kentucky Bluegrass (Poa pratensis) plots. The study followed a complete randomized block design with 5 treatments including control and 4 replications. Each replication had 5 plots of 5x5 ft. separated by a 2 ft. border between them. Replications were separated by 2 ft. The area had a very uniform and dense population of White Clover (Trifolium repens) and Dandelion (Taraxacum officinale). Other weeds were present in the surrounding area but in a very limited number and not evenly distributed. Black Medic (Medicago lupulina) also appeared inside the study area but in such scarce level that no statistical analysis was performed; however data is shown at the end.

Treatments were applied using a backpack CO₂ sprayer on May 15. Lontrel was applied at 0.5 and 1.0 PT PR/A, Confront was applied at 1.0 and 2.0 PT PR/A. Control plots only received a water spray.

Visual assessment of turf and weeds was conducted at 2, 4, 6 and 10 weeks after treatment application. The variables recorded were % cover of each of the present weed species and also the turf crop. An injury index was established and recorded for the same species. This index ranged from 0 (no injury) to 10 (dead but still present).

Percent weed control was calculated using the percent cover of each of the weeds in each treatment-plot compared to the control-plot in its respective replication at the time of evaluation.

Analysis of variance and LSD were calculated for each the recorded variables. Raw data and statistical tests appear in the appendix tables at the end of this report.

Kentucky bluegrass in the study area remained actively growing during the entire study with no evidence of turf injury by treatments or environmental conditions.

The percent control of White Clover appears in table 1. Both Lontrel and Confront provided good control of white clover. Two weeks after the application of herbicides there was already 50-70% reduction on weed populations. Their effects increased during the 4 and 6 week after application surpassing the 90% of control. By the 10^th week new plants of white clover appeared although in very limited numbers. There was a significant difference between control and herbicides along the duration of the trial. But there were only significant differences between herbicides on the last two readings for Lontrel 0.5 compared to the other treatments. Lontrel 0.5 showed a lower degree of control.

(*) Percent control based on percentage of species in treated plots vs. control plots.

In contrast as shown in table 1, the effect of treatments on Dandelion were evident only after the 4th week. Even though there are some evidence of effect by the 2nd week, this was not significantly different from the control as indicated by the anova. Four weeks after application herbicides showed between 55 and 85 % of control compared to the non-treated plots and they were significantly different against control but not among them. The effect of herbicides are more clear by the 6th week where they not only show significant differences against control but also between chemicals and their rates of application.

At the lowest rate applied Confront gave better control of dandelion than Lontrel. At the highest rate Confront and Lontrel provided similar dandelion control and both resulted in about 95% of the control.

By the last reading date, Lontrel 0.5 was not significantly different than the control, however Lontrel 1.0 showed a perfect (100%) control of dandelion. Confront at both rates showed a very similar behavior as the previous reading date. However, there were already indications of new plants of dandelion growing at this time.

As it can be observed in table 2, weed populations were dense at the starting point. More than 50% of the plots were covered with weeds at the onset of the trial. Four weeks after treatment application on control plots, in average weed populations had over 80% of the area covered. This makes even more dramatic the observed effect of herbicides. The analysis of variance showed significant differences between treated and non treated plots at all four reading dates, being the difference much clearer at 6 weeks after treatment application.

There were significant differences between herbicides-rates during the 2 first readings but not at 6 or 10 weeks after treatment application. In the first reading Lontrel 0.5 showed the best control with only a 22% cover of weeds in average. Four weeks after treatment Confront 2.0 is significantly better than the rest of treatments with only 7% of weed cover. Although there are no significant differences among herbicides 6 weeks after application, Confront 2.0 showed the minimum percent cover of weeds during the whole trial. Control plots also showed a decrease in the percent cover of weeds after the 2 reading, maybe as a product of the dry conditions that were present during most of June and July.

Two major points can be extracted from this trial.

First is the effective control of dandelion and white clover by Lontrel and Confront without any injury to Kentucky bluegrass.

Second, although not statistally significant, the high rate of Confront gave the greatest level of weed control

Also it was observed that white clover was affected by herbicides some time sooner than dandelion. Dandelion was harder to eliminate and new plants were coming back at week 10. A second application later during the summer season may be considered.