Optimizing Dogfennel Control

June, 2012
by Brent Sellers, UF/IFAS, Range Cattle REC, Ona and Jay Ferrell, UF/IFAS, Agronomy Dept., Gainesville

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 will 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 economically attractive.

There are many things one should consider when attempting to control dogfennel.  One factor to consider is application timing.  The time of herbicide application will affect herbicide selection and activity, forage production, and the cost of control. Below, we will cover each of these issues with regards to dogfennel control in pastures.

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 and 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 where we can recommend an herbicide application for dogfennel control.  Rather, it is much more important to consider herbicide selection based upon dogfennel height (Table 1).  It should be no surprise that smaller dogfennel are much easier to control than larger dogfennel.  In fact, dogfennel less than 20 inches in height are readily controlled with 4 pt/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, but it is not uncommon to reach levels of 80 to 85% control with these herbicides until plants reach 36 inches.  Above 36 inches, a more rigorous herbicide program is recommended. 

There are several options to consider when controlling large dogfennel (Table 1), and the decision may be based upon 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 (Pasturegard HL at 1.5 pt/A) is an extremely effective option.  However, if other weeds such as tropical soda apple are present, we recommend applying GrazonNext at 2 pt/A (GrazonNext HL at 1.5 pt/A) in combination with Pasturegard at 1 pt/A (Pasturegard HL at 0.5 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 or severe injury may occur. 

In addition to dogfennel height, the environment must also be considered.  Most of us realize that it is usually bone-dry, especially in south Florida, in April and May when most dogfennel is relatively small.  Weed control can be severely impacted by drought conditions as plants tend to “harden-off” to limit water loss from the plant.  In most cases, if the plant is not wilting during the day, the herbicides listed above will provide effective control.  However, it is important that application rates are not cut as we have seen several instances when dogfennel control with 2,4-D amine as well as 2,4-D amine + dicamba failed to effectively control dogfennel under dry soil conditions.

Dogfennel infestation and forage production.  There is no doubt that weed infestations have a detrimental impact on bahiagrass production.  Until recently, however, we did not know the exact impact of dogfennel infestations on bahiagrass productivity.  In an experiment we conducted in 2007 and 2008, we began removing dogfennel beginning in May and measured the season-long production of bahiagrass.  When dogfennel was removed in May, bahiagrass production was reduced by approximately 42% when plots were covered with approximately 50% dogfennel cover and  approximately 74% when dogfennel cover was at least 75% (Table 2).  This indicates that dogfennel should be controlled as early as possible when dogfennel densities are >50%. 

If you look at the data in Table 2 and focus on the column with < 25% dogfennel cover, you will notice that bahiagrass yield was relatively constant at the growing season progressed.  Even though the bahiagrass yield was lower than that in the plots without dogfennel, it is apparent that bahiagrass is somewhat tolerant of light infestations of dogfennel.  This does not appear to be the case for higher levels of dogfennel infestations.  For example, bahiagrass yield declined as the growing season progressed in both 50% dogfennel cover plots as well as > 75% dogfennel cover plots.  This indicates 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.

In closing, there is no way to provide across the board recommendations for dogfennel control in Florida.  Each pasture is going to require some thought with regards to herbicide selection and application timing based upon both the environment and the density of dogfennel.  If you need assistance with your particular situation, please contact your local county agent. 

 

Table 1.  Herbicide options for dogfennel control in pastures.  Cost of application is not included in the herbicide cost. 

Active ingredients Trade names Rate Cost Comments
2,4-D amine Many 2 qt/A $7 Apply to dogfennel less than 36 inches tall; apply higher rates as approaching 36 inches.
2,4-D amine + dicamba WeedMaster, others 1.5 qt/A $11.50 Apply to dogfennel less than 36 inches tall.
fluroxypyr+triclopyr Pasturegard Pasturegard HL 2 qt/A $12-18 Apply 2 pt/A when plants are less than 36 inches tall; apply 3 pt/A to plants > 36 inches. Applications of 3 pt/A are very effective on large dogfennel exceeding 5 ft in height.
aminopyralid + 2,4-D GrazonNext GrazonNext HL 2 qt/A $10 Apply alone to dogfennel less than 30 inches tall when pastures are also infested with tropical soda apple; when plants are > 30 inches tall, tank-mix GrazonNext with 3 pt/A 2,4-D amine, 1 pt/A Pasturegard, or see comments for Cleanwave herbicide.
fluroxypyr + aminopyralid Cleanwave 14 - 26.6 oz/A $9-16 Excellent tank mix partner for 2,4-D and GrazonNext. Tank mix 14 oz with one of these products for dogfennel < 36"; 20 oz for dogfennel between 36 and 60"; 26.6 oz for dogfennel > 60". If tank-mixing with Milestone add 20 oz Cleanwave to dogfennel < 60" and 26.6 oz to dogfennel > 60". Cleanwave is safe on limpograss.

 

Table 2. Effect of dogfennel infestation on season-long bahiagrass production. Dogfennel were removed from plots monthly beginning in May. 

Time of removal None < 25% dogfennel cover 50% dogfennel cover > 75% dogfennel cover
  - - - - - - - - - - bahiagrass production (lb/A) - - - - - - - - -
May 6,600 4,900 3,800 2,400
June 5,700 3,900 3,600 2,250
July 7,000 3,750 3,650 2,250
August 7,800 4,250 2,100 1,600
September 6,100 4,000 2,050 1,980
October 5,500 4,750 1,850 1,750

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