Weeds in pastures and rangeland cost ranchers more than $180 million annually in Florida by reducing forage yield, lowering forage quality, and causing animal injury through toxicity or specialized plant organs (thorns and spines). Effective weed management begins with a healthy pasture. Weeds are seldom a serious problem in a well-managed, vigorously growing pasture. Good pasture management involves the proper choice of the forage species and variety, an adequate fertility program, controlled grazing management, and pest management (weeds, insects, and diseases). This publication provides practical pasture and rangeland weed management information for landowners, Extension agents, state and federal agency personnel, and others interested in managing grazing lands in Florida.
If pasture health declines, weeds will become established. Unless the management problem that caused forage decline is corrected, the grass will not reestablish and weeds will continue to re-infest the area. Bare ground is the perfect environment for establishment of weeds. Once established, weeds must be effectively controlled with mechanical or chemical methods.
Integrated weed management is both an economically and environmentally sound approach to weed management. An integrated approach involves scouting, prevention, and control (biological, cultural, mechanical, and chemical) in a coordinated plan.
Scouting pastures periodically, which is often overlooked, is the foundation of a sound weed management program. Scouting involves routinely walking or driving through pastures and identifying a weeds issue. This defines the scope of the problem and allows the best management practices to be implemented in a timely fashion. The number of weeds, the species present, and their locations are important. Note the dominant species as well as uncommon or perennial weeds. The management strategies adopted should focus on controlling the dominant species while preventing the spread of less common species. If not managed proactively, the less common weeds in a pasture may become dominant weed problems.
Proper identification of weeds is the first step toward weed control. A good example is knowing the difference between tropical soda apple (TSA) and red soda apple (cockroach berry). Of the two, only TSA is a troublesome invasive weed. However, these two species can be incorrectly identified. This costly mistake allows TSA to go uncontrolled and results in the weed spreading throughout the ranch and potentially onto neighboring ranches. If you have questions about weed identification, contact your local UF/IFAS Extension office for assistance.
Some weeds grow best in wet sites (maidencane ponds, depressional areas, ditches, etc.) while others can be found on dry sites (ditch banks, upland areas, and fencerows). Scout pastures for weeds in conjunction with other activities, such as checking calves, working cattle, and feeding. When you first discover a weed, remove it or spot treat with an appropriate herbicide. Do not allow that one plant to produce seeds and give rise to hundreds of new plants. It is less costly in terms of both time and money to control one plant than to wait and have to control hundreds of plants.
Poisonous plants (e.g., Crotalaria, black nightshade, spiny pigweed, lantana, etc.) are commonly found throughout Florida. Animals do not usually choose to graze most poisonous plants when forage is abundant; however, when quality forage is limited due to poor growing conditions or overstocking, they may graze these plants.
Prevention is any activity that keeps weeds from infesting a pasture. Most of the weeds are spread by seed. Thus, preventing the movement of weed seeds onto the ranch reduces potential weed pressure. Weed seeds can be transported in hay, harvested grass seed, sod, cattle, and mowing equipment, or dispersed by wind, water, and wildlife. Producers should avoid buying hay or grass seed that is contaminated with weed seeds. Do not purchase hay from someone who cannot provide a weed-free product. Using certified forage seed reduces weed seed contamination and is highly recommended.
Also, consider prevention of TSA while moving cattle to a new location. Cattle have been shown to excrete TSA seeds for at least 7 days after consumption. If cattle are grazing in a TSA-infested pasture, holding them in a clean area for 10 days before moving them to a new pasture is recommended. This will reduce the likelihood of transporting TSA seeds. Remember that an ounce of prevention is worth a pound of cure.
Cultural practices improve weed control by increasing the competitiveness of the forage. This involves optimizing forage production through monitoring of soil pH, fertility, and potentially irrigation management. Generally, a thick sward will prevent weed emergence, outcompete emerged weeds, and capture the majority of environmental resources (light, water, nutrients) necessary for growth. The aim of cultural practices is to modify your management program so that the sward is as competitive as possible.
Soil pH is an important factor for forage growth as well as weed establishment. UF/IFAS forage agronomists and soil scientists have determined the optimum soil pH for most forages grown in Florida. Acidic soils limit plant growth and can result in aluminum and manganese toxicity as well as magnesium, calcium, phosphorus, molybdenum, and potassium deficiency. Soil acidity may also result in poor root growth, which can reduce water and nutrient uptake. Weeds that grow under such conditions can be indicators of low soil pH. For example, flat-top goldenrod tends to be found in pastures with soil pH levels between 4 and 5, which are too low for optimum forage growth. Thus, the presence of flat-top goldenrod in your pasture may warrant a soil test and corrective action.
Mowing is one of the most often-used weed control methods in pastures. Mowing improves the appearance of a pasture, temporarily increases forage production, and, if properly timed, prevents weeds from producing seed. Mowing is generally more effective on broadleaf weeds than grass weeds and on annual weeds than perennial weeds. Carefully consider the cost of mowing and the anticipated effectiveness. As fuel prices increase, it may be more cost-effective to avoid mowing and use other forms of weed control because other weed control methods may be more effective on a given species.
Mechanical weed control does have drawbacks. Large weeds with extensive root systems will not be controlled by mowing alone. Additionally, mowing misses prostrate-growing weeds such as crabgrass, spurges, and matchweed. Mowing can also spread vegetative plant stems, allowing the plant (e.g., prickly pear) to root elsewhere. If mowing is performed after seed set, seeds can accumulate on the mowing equipment and worsen the weed problem by spreading to other pastures.
Biological control involves the use of biotic agents (e.g., plants, herbivores, insects, nematodes, and phytopathogens) to suppress weeds. Overall, biological control is still in its infancy, but great strides are being made, especially against invasive plants. Two good examples are the tobacco mild green mosaic tobamovirus (TMGMV) and the insect, Gratiana boliviana, both used for TSA control. The virus TMGMV can be sprayed to control existing TSA plants, while the beetle is used primarily for suppression.
Most biological control agents rarely provide complete weed control, but they usually suppress the weed population to a manageable level. Additionally, biological control agents are rarely fast-acting, so time is needed for the agent to suppress a given weed population. For example, the effect of Gratiana boliviana is often not seen until the year after the release of the beetle.
Chemical weed control includes the use of herbicides. Herbicides kill weeds by inhibiting plant processes necessary for growth. Herbicides should be selected based on the forage species being grown, the weed species present, the cost, and the ease of application. Application method and environmental impact should also be considered.
Proper herbicide choice and application rate are extremely important. Lower-than-recommended application rates will not provide consistent weed control, while excessive application rates may cause injury to the forage or result in only killing the aboveground portion of perennial weeds. Additionally, herbicides must be applied at the right time to be cost-effective.
Preemergence (PRE) applications are made before weeds germinate and emerge. Understanding the life cycle of the weed is important when using a preemergence herbicide. Some weed seeds germinate in the summer, while others germinate in the winter months. Always refer to the herbicide label for additional information about controlling specific weeds.
Postemergence (POST) applications are made after the weeds emerge. The most effective and cost-efficient applications are made when the weeds have recently emerged and are small (3 to 5 inches tall). For perennial weeds (regrowing from root storage organs), it is advisable to allow them to bloom before spraying. This allows sufficient leaf surface for coverage and ensures that the perennial is transporting photosynthates back to the roots.
Postemergence herbicides may be broadcast over the entire pasture or may be applied as a spot treatment to sparse weed patches. Spot treatment is less costly compared to broadcast spraying. Other application methods include wipers and mowers that dispense herbicide while mowing the weed. Carefully read the herbicide label before purchasing to determine if that herbicide controls the weeds in your situation.
Precautions When Using Phenoxy or Benzoic Acid Herbicides
- For information about growth-regulating herbicides not covered below, see https://edis.ifas.ufl.edu/wg051.
- Application of other pesticides from sprayers previously used for 2,4-D, dicamba, or other phenoxy or benzoic acid herbicides to susceptible crops may result in injury.
- Legumes in pastures or rangelands will be injured or killed by these herbicides.
- Avoid drift to susceptible crops by applying at low pressures and when wind speeds are low and blowing away from susceptible crops. The use of a drift-control additive is advisable.
- Clean the sprayer thoroughly as described on the herbicide label. If no instructions are provided, you may follow the procedure below using household ammonia:
- Flush system with water. Drain.
- Flush the system with ammonia (1 qt ammonia per 25 gallons water); let it circulate for at least 15 minutes, then flush the system again. Drain again.
- Remove screens, strainers, and tips, and then clean in fresh water.
- Repeat step 5b.
- Thoroughly rinse the tank, hoses, booms, and nozzles.
- Be sure to clean all other associated application equipment.
Not all cultivars of a particular forage species respond similarly to a given herbicide (Table 5). Argentine bahiagrass tolerates most pasture herbicides except Roundup, while Pensacola bahiagrass may be severely injured by metsulfuron-containing products, such as Cimarron and others. All herbicides may be used on stargrass and bermudagrass, with some level of injury from Velpar (hexazinone). Hemarthria, also known as limpograss, is the most sensitive to herbicide applications of all forage grasses grown in Florida.
Note that the response to an herbicide application can vary. For example, the chance for forage injury can increase or decrease as the rate of herbicide applied either increases or decreases. Additionally, environmental conditions such as high temperature and high relative humidity may increase the potential for herbicide injury. For example, we have observed little or no injury to limpograss from 8 pt/acre 2,4-D amine when applied under cooler conditions, while 4 pt/acre in warmer weather caused moderate to severe injury.
The response of forages in Table 5 is for established forage cultivars. However, 2,4-D + dicamba (2 pt/acre) can be applied to sprigged forage cultivars, except for limpograss, seven days after planting/sprigging. A forage can be considered established when at least three tillers are present on bahiagrass or at least 6 in of new stolon growth are present on sprigged forages.
Maintaining healthy, productive pastures will minimize the risk associated with weeds. Good pasture management practices such as adequate fertilization, insect control, and controlled grazing will result in healthy pastures. Unfortunately, weeds are present in pastures, and the associated loss in forage production can have serious economic implications. An integrated weed management strategy combining prevention, detection, and control is the most economical and environmentally friendly approach to pasture weed management.
Table 1. Weed control suggestions for pastures and rangeland. Contacts: Brent Sellers (firstname.lastname@example.org) and Pratap Devkota (email@example.com). This table lists registered herbicides that should be integrated with other pest management methods. Contact your local UF/IFAS Extension office for additional information (https://ifas.ufl.edu/maps/).
Table 2. Estimated effectiveness of herbicides on common broadleaf weeds in pastures and hayfields (2,4-D through Impose/Panoramic).1
Table 3. Estimated effectiveness of herbicides on common broadleaf weeds in pastures and hayfields (Milestone through WeedMaster or others).1
Table 4. Estimated effectiveness of herbicides on common grass and sedges in pastures and hayfields.1
Table 5. Tolerance of established (for at least 6 months) forage cultivars to commonly used herbicides.
Table 6. Days between herbicide application to forage or pasture and feeding, grazing, or animal slaughter.