THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Optimizing Dogfennel Control
Dr. Brent Sellers
Although tropical soda apple has received much of the attention over the past decade, dogfennel is the most commonly occurring broadleaf weed in Florida pastures. Many ranchers simply mow pastures in late summer to suppress dogfennel seed production. A timely herbicide application is worth considering, though, because it will not only suppress seed production, but also increase forage production by eliminating the weed entirely. Furthermore, increased fuel costs are making herbicide applications more economically attractive.
There are many things to consider when attempting to control dogfennel. One major factor is application timing. The time of herbicide application will affect herbicide selection and activity, forage production, and the cost of control.
Herbicide timing and selection. To properly understand application timing and selection, we need to first understand dogfennel biology. Dogfennel can begin growing from the rootstock or seed as early as February in south Florida, with new growth forming progressively later as we move northward. Stem elongation typically begins as early as March in some areas; plants can be 6 to 12 inches tall by the beginning of April, and at least 6 to 10 ft tall by August. Once we get to September, dogfennel stems tend to become woody in nature.
There is no specific month when an herbicide application for dogfennel control is recommended. Rather, it is much more important to select a herbicide program based on dogfennel height (Table 1). As you would expect, smaller plants are much easier to control than larger ones. In fact, dogfennel less than 20 inches in height are readily controlled with 2 qt/A 2,4-D amine, or 1.5 qt/A WeedMaster (dicamba + 2,4-D amine, others). Control with these herbicides tends to decline as dogfennel plants grow above 20 inches. However, it is not uncommon to reach levels of 80 to 85% control with these herbicides when plants are between 20 and 36 inches tall. Above 36 inches, the level of control is dramatically reduced and a more rigorous herbicide program is recommended.
There are several options for controlling large dogfennel (Table 1). When choosing a herbicide program, we need to consider the weed spectrum within a given pasture as well as the forage grass present. If dogfennel is the primary target weed in the pasture, Pasturegard at 3 pt/A is an extremely effective option – regardless of size at time of application. However, if other weeds such as tropical soda apple are present, we recommend applying GrazonNext at 2 pt/A in combination with Pasturegard at 1 pt/A, 2,4-D amine at 3 pt/A, or Cleanwave at 20 to 26.6 oz/A. All of these herbicide combinations can be safely applied with minimal injury to our forages in Florida. The only exception is that products containing 2,4-D should not be applied to limpograss (Hemarthria) between May 1 and November 1, as severe injury may occur. For more information concerning weed management in limpograss, please refer to the EDIS document entitled “Weed Management in Limpograss (http://edis.ifas.ufl.edu/ag344).
In addition to dogfennel height, there are environmental factors to consider. Our driest time of year, especially in south Florida, is April and May when most dogfennel is relatively small. Even though small dogfennel are typically the easiest to control, herbicide effectiveness can be severely impacted by drought conditions as plants tend to “harden-off” to limit water loss. In most cases, as long as the plant is not wilting during the day, the herbicides listed in Table 1 will provide effective control. However, it is important that application rates are not reduced. We have seen 2,4-D amine and 2,4-D amine + dicamba fail to effectively control dogfennel under dry soil conditions on several occasions.
Dogfennel infestation and forage production. There is no doubt that weed infestations have a detrimental impact on bahiagrass production. Until recently, however, the extent of the impact was not known. In an experiment conducted in 2007 and 2008, season-long production of bahiagrass was measured in response to the timing of dogfennel removal at increasing dogfennel densities.
The data indicate that a pasture with >50% infestation will yield 42% to 74% less bahiagrass than a dogfennel-free pasture, if dogfennel removal is performed in May(Figure 1). Waiting until August to remove dogfennel from pastures with > 50% infestation resulted in a more than 75% loss in bahiagrass yield. As such, we recommend that dogfennel be controlled as early as possible if more than half the pasture is infested.
Experimental pastures with < 25% dogfennel cover had relatively consistent bahiagrass yields as the growing season progressed. Even though the bahiagrass yield was lower than the plots without dogfennel, it appear that bahiagrass is somewhat tolerant of light dogfennel infestations. The same does not hold true for higher levels of dogfennel infestations. Bahiagrass yield declined as the growing season progressed in both 50% dogfennel cover plots as well as > 75% dogfennel cover plots.
From this research we concluded that when pastures are infested with < 25% dogfennel cover, herbicide applications may be delayed for almost a whole growing season, especially if dogfennel plants have become so large that more expensive herbicide mixes are needed for optimum control. Conversely, at dogfennel cover >50%, dogfennel should be controlled as early as possible to prevent substantial yield loss and potential stand reductions.
Dogfennel is an important pest that is more destructive to bahiagrass pastures than may be realized. We believe it is important to focus on this weed and remove it as infestations increase beyond 25% groundcover. Unfortunately, providing a “one size fits all” recommendation for dogfennel control is difficult, if not impossible. Each pasture is going to require some thought with regards to herbicide selection and application timing based upon both the environment and dogfennel density. We recommend you review the information in this article, along with EDIS articles:
- Dogfennel (Eupatorium capillifolium): Biology and Control (http://edis.ifas.ufl.edu/ag233),
- Tank-mix Options for control of Tropical Soda Apple and Dogfennel (http://edis.ifas.ufl.edu/ag306),
- Using Cleanwave Herbicide to Control Dogfennel (http://edis.ifas.ufl.edu/ag288),
- Weed Management in Pastures and Rangeland (http://edis.ifas.ufl.edu/wg006).
If you need assistance with your particular situation, please contact your local county agent.
Figure 1. Effect of dogfennel removal timing on season-long bahiagrass yield. Dogfennel infestations were visually assessed to be < 25% groundcover, approximately 50% groundcover, and > 75% groundcover at the beginning of the growing season.