Cogongrass control in pastures and fencerows

Brent Sellers, Jason Ferrell, and Greg MacDonald,University of Florida, IFAS

cogonrassCogongrass is a perennial grass that commonly infests disturbed areas in the Southeast. Cogongrass invades and persists through several survival strategies including an extensive rhizome system (for underground storage of energy and nutrients), adaptation to a variety of soil conditions, drought tolerance, prolific seed production, and adaptation to fire. Additionally, this weed is unpalatable to livestock because it accumulates silicates along the leaf margin, making leaves hard and razor sharp. Cogongrass also forms a dense mat of cover and quickly displaces desirable forage grasses. Due to these properties, cogongrass is one of the most difficult species to effectively remove from pasture and range settings.

Control of cogongrass has been studied for many years by researchers all over the world. During this time nearly all available herbicides have been tested on cogongrass, but few effective products have been found. For example, all of the commonly used pasture herbicides such as Cimarron, 2,4-D, Remedy, Velpar, and Weedmaster have no activity on cogongrass. Only, glyphosate (Roundup, etc.) and Stalker herbicide (imazapyr) have been found to be effective, but long-term control is rarely achieved.

Stalker is an extremely effective herbicide that controls a variety of weeds, from herbaceous to woody species. One or two applications of Stalker (2 qt/A) will often effectively control cogongrass for 18 to 24 months. However, there are several disadvantages to using this herbicide. First, Stalker will severely injure or kill forage grasses such as bermudagrass and bahiagrass. It also has a long soil half-life and will remain in the soil for several months after application. This often leads to "bare ground" for up to 6 months in the application area due to the non-selective nature of this herbicide. Stalker also has the potential to move down slopes during periods of rainfall, killing or injuring other species in the runoff area. Secondly, Stalker can only be used as a spot-treatment with no more than 10% of the pasture area treated per year.

Glyphosate is often the easiest and least expensive option. This herbicide applied at 3-4 qt/A will substantially reduce cogongrass stands, but multiple applications are needed. One drawback to glyphosate is that it is a non-selective herbicide and will control/injure all vegetation present at the time of treatment. However, glyphosate has little or no residual soil activity and crops can be planted immediately after application. But, if high rates (4-5 qt) of glyphosate are used, slight soil residual may exist in some Florida soils; therefore, a 10-14 day waiting period should be observed before reestablishing the area with tender seeds or seedlings.

To date, there are no "great" herbicide options available for control of cogongrass in pastures. For this reason an integrated control plan will be required that combines herbicides and soil amendments. Cogongrass grows extremely well over wide soil fertility and pH ranges. On the contrary, bahiagrass begins to grow poorly if soil pH becomes too low or if nitrogen fertilizer is lacking. But given a setting where bahiagrass has sufficient fertility and optimum soil pH, it may out-compete cogongrass and maintain the pasture. The most likely program to control cogongrass will require using multiple applications of glyphosate (to remove the cogongrass that is present), amending soil fertility and pH, then quickly reestablishing bahiagrass. Although this program will not eradicate cogongrass, using this integrated approach is the most effective means of managing cogongrass in grazing areas.