We are starting to look at late summer pastures weeds. Yesterday, we looked at dogfennel (Eupatorium capillifolium) and mint in a Bahia pasture that is two years old. Dogfennel is normally considered to be unsightly, but research has shone that yield loss can occur in bahiagrass if dogfennel is not controlled before mid-summer. Cows don’t normally eat dogfennel, but will when forage is low in qyaility. Dogfennel has a toxin which can make cattle dehydrated.
The biggest thing with controlling dogfennel is timing or really height. It can grow from overwintering rosettes. Seeds will also germinate and sprout at 65 degree soil temperature. Though a lot about it’s biology is known, treating based on heigh is more recommended.
2,4-D and dicamba do good on dogfennel, but we obviously cannot use from most of April through the fall. In Florida, dogfennel growth will start sooner. For us, germination will occur in late April, after we put out our Weedmaster application. During the summer months, Pasturegard (fluroxypyr + triclopyr) is another option for us. The rate depends on height. When dogfennel is < 36″ tall, use 1 pt / A. When dogfennel are > 36″ tall, use 1.5 pt / A. This rate is effective on large dogfennel, even 5 ft tall.
More of this information can be found at Dogfennel: Biology and Control