The presence of weeds in pastures can affect animal performance due to its negative impact on forage quality and yield. Also, some weeds can be toxic to grazing animals which compromises their health and reproductive performance. Poor pasture establishment and management practices such as seed drill not properly calibrated, low-quality seed, deep seeding, not correcting soil pH and fertility levels, choosing inadequate plants to the system, and overgrazing can make the pasture less competitive against weeds, favoring their development. In short, controlling weeds and avoiding their development is crucial to have a profitable and sustainable forage system that fits the grazing animal’s needs (Dubeux, 2020).
Weeds are defined as undesirable plants in the system, they can compete with the desirable plants for water, sunlight, and nutrients. Weeds can be annual, biannual, or perennial plants. Some plants can be weeds depending on the system they are in, such as common bermudagrass in row crop rotation. Even though animals still eat some non-toxic weeds, weeds can reduce hay market value and the forage nutritive and animal performance. Identifying weeds is not easy, especially before bloom. Make sure to take clear photographs before sending them to the local extension agents, also there are multiple cellphone apps like PictureThis® and iNaturalist® to assist with quick and precise weed identification.
Control window
| Winter | Spring | Summer | ||||||
| Activity | December | January | February | March | April | May | June | July |
| Scouting for cool-season weeds | B | B | B | B | M | |||
| Controlling cool-season weeds | B | B | B | M | M | |||
| Scouting for warm-season weeds | P | B | B | M | ||||
| Controlling warm-season weeds | B | B | P | |||||
M = Marginal month; P = Possible month; B = Best month
Weed control strategies
Weeds are an indicative that something is wrong with the forage system. Using the statement as an analogy, the weeds can be a symptom of a sick pasture. The symptoms can be controlled, the same way in a disease, but more important than controlling them is to find the real cause of the problem or the combination of them that are compromising the forage stand vigor. As mentioned before, the causes can be related to establishment failures, poor fertility, and improper grazing management. However, even in well-established and well-maintained pastures weeds can appear. Still, with a complete strategic combination of preventive and cultural with mechanical and chemical control methods, weeds will be fewer, way easier to control, and less frequent than in a poor-established and with neglected maintenance pasture.
Cultural and preventive control
Cultural and preventive management techniques will prevent the emergence and further development of weeds and improve the desirable forage’s competitiveness. Some of the techniques include maintaining adequate pH and fertility levels, not overgrazing, and keeping an adequate stubble height after cutting hay. Besides enhancing the competitiveness against weeds, these practices extend the stand life, and increase nutritional value and forage accumulation, key factors for the economic sustainability of pastures and hayfields.
Before fertilizing pastures is necessary to conduct a soil test, to determine that the field is not over or under-fertilized and limed. Maintaining a competitive pasture against weeds requires adequate soil fertility and pH levels, which will be determined after conducting soil sampling and analysis. Lime is important because it raises soil pH, which is necessary to keep the nutrients chemically available to the plants when soil pH is at its ideal level. The ideal pH range for pasture growth and yield is 6-6.5, at this level the main required soil nutrients (Potassium, phosphorus, calcium, and magnesium) are chemically accessible for plant use (Peacock et al., 2023).
It is essential to conduct adequate grazing and harvesting management in order to have adequate forage production and a healthy stand that will outcompete the weeds. When pastures are overgrazed, animals may deplete plant storage reserves and also damage growing points leading to overgrazing and compromising stand persistence (Silva, 2023). Recommendations for harvest stage, frequency, and stubble height are specific to each forage species and can be found in the South Carolina Forages Handbook (Link: https://www.researchgate.net/publication/371120650_Concepts_and_research-based_recommendations_for_forage_systems_in_South_Carolina)
Mechanical control
Mechanical control methods can comprehend all the way from hand-pulling, mowing, and even tillage. But only mowing would be desirable to use in pasture situation when not renovating it. If used at the right time it is a great tool to control annual and erect growing weeds but can also be more expensive than other operations such as spraying and removes a great quantity of desirable forage (Flynn, 2019) (Sellers & Devkota, 2023).
Tillage can be useful during pasture renovation, but prior evaluation and caution is necessary since it can spread rhizomatous species such as Bermuda grass (Cynodon dactylon L. Pers). and Johnsongrass (Sorghum halepense L. Pers.), also tilling the soil will activate a lot of seeds that are dormant in the seedbank.
Chemical control
This is always the first method to be reminded of when trying to control weeds, and to guarantee its success some previous steps such as providing adequate pH and fertility levels are needed to guarantee that the forage stand will come back after the weeds are controlled.
Most herbicides used in pastures and hayfields are synthetic plant hormones that need to be absorbed and translocated in the weeds while they are actively growing, application before bloom, and with adequate weather conditions, such as temperatures between 60 and 85ºF and few to no wind (Israel & Rhodes, 2015). Most pasture herbicides can be harmful to neighboring cash crops, avoid drift conditions and make sure that the equipment is calibrated. For detailed information on appropriated herbicides and rates for pastures and hayfields in the 2024 South Carolina Pest Management Handbook: https://www.clemson.edu/extension/agronomy/_files/pest-management-handbook-clemson-extension.pdf
Other than spraying, chemical control can be used as the impregnation technique when herbicides are applied to dry fertilizer granules and blended with dye. The method is less effective than spraying but is a viable option when spray equipment is not available and can reduce operational costs since it is going to be applied in the same operation as the fertilizer. This technique needs at least 0.25” of rainfall within 4 weeks of the application to activate the herbicide and always check the herbicide label for restrictions. For detailed information on appropriated herbicides and rates for pastures and hayfields in the 2024 South Carolina Pest Management Handbook: https://www.clemson.edu/extension/agronomy/_files/pest-management-handbook-clemson-extension.pdf
Conclusion
Combining control strategies works better than adopting just one. In addition to improving forage yield and nutritional value, preventive and cultural control methods avoid weed emergence and development, this saves money on fuel, time, labor, and herbicides by reducing the number of weeds that would need to be controlled later.
References
Dubeux, J. (2020). How Weed Infestations in Pastures affect Cattle Performance? Panhandle Agriculture. https://nwdistrict.ifas.ufl.edu/phag/2021/08/06/how-weed-infestations-in-pastures-affect-cattle-performance/
Israel, T.D. and G.N. Rhodes Jr. 2015. Buckhorn Plantain. Pasture Weed Fact Sheet. UT Extension Publication W322. Knoxville, TN.Flynn, S. (2019). Weed and brush control: Myths and mistakes. Hay and Forage Grower Magazine. https://hayandforage.com/article-2506-weed-and-brush-control-myths-and-
Morgan, S. (2023). Weed control in pastures and hayfields. UGA Forage Extension Team. https://site.extension.uga.edu/forageteam/2019/03/weed-control-in-pastures-and-hayfields/Peacock, B., Vlake, L. V., Starnes, A., LeMaster, C. W., & Burns, M. (2023, January 19). Equine Pasture Management. Land-Grant Press | Clemson University, South Carolina. https://lgpress.clemson.edu/publication/equine-pasture-management/
Sellers, B., & Devkota, P. (2023). SS-AGR-08/WG006: Weed management in Pastures and Rangeland-2023. Ask IFAS – Powered by EDIS. https://edis.ifas.ufl.edu/publication/WG006
Silva, L.S., Burns, M., Duckett, S., Aguerre, M., Bolt, B., Fischer, M., Marshall, M., Greene, J., Hersom, M., Kirk, K., Scharko, P., Smith, N. 2023. Concepts and research based guidelines for South Carolina forage systems.
Written by
Roger Furlan, Graduate Research Assistant, University of Tennessee
Dr. Liliane Silva, Forages Specialist, Clemson University
