Dogfennel Size at Application: It Does Matter

by Dr. Brent Sellers, University of Florida/IFAS

Although tropical soda apple has received much of the attention over the past decade, dogfennel is our number one broadleaf weed in pastures. Most ranchers simply mow their pastures to suppress dogfennel seed production. However, a timely herbicide application can not only suppress seed production, but increase forage production by eliminating shading from tall dogfennel plants. Furthermore, increased fuel costs are making herbicide applications more attractive economically.

An experiment was initiated in 2005 to determine the impact of dogfennel size on control with various herbicides. Dogfennel plants were sprayed when they were 15, 30, or 60 inches tall. Herbicides were applied on June 8 (15" timing), June 21 (30" timing) and September 2 (60" timing) in a spray volume of 30 gallons per acre. Dogfennel control was visually evaluated 30, 60, 90 days after application. After the 90 day evaluation, the entire experiment was mowed. Visual control was rated one last time approximately 280 days after application.

When applied to 15" dogfennel, all herbicides performed equally 30 days after treatment (DAT), with the exception of WeedMaster at 2 pt/A and Outlaw at 1.75 pt/A (Table 1). By 60 and 90 DAT, all herbicides performed equally (Tables 2 and 3). Approximately 1 year after treatment, all herbicide treatments provided >90% control except for the low rates of WeedMaster and Outlaw (Table 4).

Applying these herbicide treatments to 30" dogfennel resulted in similar trends as seen with 15" dogfennel. At 30 DAT, at least 2.5 pt/A of Pasturegard or 2 pt/A of Remedy were needed for >90% control (Table 1). However, by 60 DAT, all herbicide treatments provided >90% control except when Outlaw was applied alone (Table 2). By 90 DAT, Outlaw at 1.75 and 2.75 pt/A were the only treatments that failed (Table 3). This trend continued to 280 days after treatment (Table 4).

Herbicide applications to 60" dogfennel took longer for adequate control to occur. For example, all herbicide treatments provided <90% control 30 DAT (Table 1). By 60 DAT, WeedMaster at 2 pt/A and Outlaw at 1.75 pt/A had the poorest control of only 64 and 61%, respectively, while all other herbicide treatments provided at least 80% control (Table 2). This same trend was observed 90 DAT, with all Outlaw treatments and WeedMaster at 2 pt/A provided the poorest dogfennel control. The only treatments that fared well approximately 1 year after treatment of 60" dogfennel were Pasturegard at 3 and 4 pt/A, Remedy at 2 pt/A, WeedMaster at 3 pt/A, Remedy + WeedMaster, and Outlaw + Remedy (Table 4).

Take home message: Results from this study show that applying herbicides early in the growing season will enhance dogfennel control. Additionally, applying these herbicides early, results in more options for adequate control. Waiting until dogfennel are greater than 30" tall will result in increased herbicide costs to achieve the same amount of control with less herbicide when dogfennel is only 15" tall.

Point to ponder: These results were from last year, under good rainfall conditions. This same study has been repeated this year. It appears that the early season drought experienced by many of us in 2006 has impacted the results of this study. Stay-tuned as we will be updating this information as time progresses.