Weed management is one of the important aspects for maintaining healthy, productive pastures. Any plant that emerges at an unwanted spot is called a weed. Whether a plant is considered a weed may differ from one production system to another. For example, a weed in a cattle grazing system may not necessarily be a weed in a goat grazing system, such as briers. Although goats eat a wide variety of plants that are generally discarded by large ruminants, weed infestation can be a serious problem in a goat grazing system as well.
Weeds emerge and develop voluntarily whenever and wherever they find suitable environment. Weeds compete with forage plants for nutrients, moisture, sunlight, and space, and they lower pasture productivity. Weeds are generally not eaten by grazing animals, but forages surrounding the weeds are utilized. This situation provides weeds with better growing conditions with no or minimum competition from the useful forage plants. When weeds are left on pastures, these get well established by developing strong roots and stems as well as food storage structures like rhizomes, tubers, stolons, and crowns. Moreover, weeds propagate very fast with a huge quantity of seed production if not removed before their reproductive phase (flower and seed development and maturation). Rhodes et al. (2005) found that a single plant of musk thistle, curly dock, or pigweed, respectively, can produce as many as 10,000, 40,000, or 120,000 seeds in one growing season. So if weeds are not managed properly, pasture can be badly infested with weeds in a matter of a year or two.
1. Identify weeds: Identification is the first step in weed management. One must be able to identify weeds and useful forages present in pastures so that weeds can be removed without harming the useful plants.
2. Be watchful on your pastures: Take a walk on your pasture from time to time and inspect whether any weeds are emerging. Noticing weeds early gives the producer the opportunity to control them before it is too late — that is, before they are mature and before seeds are produced and spread to infest a larger area. This saves time, money, and the environment in the long run.
3. Minimize human- or animal-mediated weed spread:
4. Manage weeds on time: When you notice weed infestation in your pastures, you need to act on time — before they flower — to manage them appropriately. There are different methods of weed management that can be used singly or in combination, depending on the weed species, available resources, associated cost, and farm conditions. Common methods of weed management are briefly presented below.
1. Cultural practices: Pasture soils need to be tested for pH and nutrient contents while establishing new pastures or maintaining existing pastures. Based on soil test recommendations, add necessary lime and fertilizers to the pasture soils to provide appropriate pH and nutrient requirements for forage growth. Also, plant suitable forage species for the given soil types and climatic conditions so that forages grow well and remain competitive. Plant an appropriate quantity of seed to a proper depth to obtain a good forage stand. Using lower seed rates leaves spaces for weeds to grow, and overseeding results in too much competition among the desirable species so they cannot perform well and weeds may invade. Similarly, planting too deep or too shallow for the given size of the forage seed will hamper the germination, eventually resulting in poor forage stand. Poor forage stand means good opportunity for weeds to germinate and flourish.
2. Crop rotation: Weed problems can be minimized by crop rotation as weeds are removed along with the existing crops and weed roots are weakened or damaged by all the tilling operations involved in planting new crops. However, it may not be a practical option for permanent pastures.
3. Grazing management: Weeds can be managed better with rotational grazing than with a continuous grazing system. In a continuous grazing system, animals are left in pastures for a whole grazing season, and managers do not have any control over where the animals go and how long they graze. Animals overgraze palatable species and undergraze less palatable species. Overgrazing depletes the sod and exposes soil/bare ground. This allows sunlight to reach the ground, which facilitates weed emergence and development. Undergrazing leaves plants to grow tall and mature; this shades the short growing forages and depletes their growth, resulting in depletion of sward thickness and promotion of weed emergence and development.
In a rotational grazing system, pastures are divided into different sections, and animals are allowed to graze one section at a time until the desirable stubble height of forages is reached, then moved to the next section sequentially based on forage availability. In this system, there are few days of the grazing period when animals are allowed to graze any section and several days of resting period when animals are taken completely off that section. Confinement of animals in a smaller portion results in uniform utilization of all forages present in that portion, so the chances of overgrazing or undergrazing are minimized. With the provision of a resting period after each grazing, forages have a chance to recover and maintain a good sod. Consequently, there is less suitable environment for weed emergence and development in a rotational grazing system compared to a continuous grazing system.
Another useful grazing strategy for weed management is to practice mixed-species grazing. This involves including different species of grazing animals with different forage preferences. For example, most weeds can be managed well when cattle and goats are grazed in the same pastures. This is because cattle prefer grass, while goats prefer browsing on brush and shrubs and grazing on broad-leaf plants including weeds and briers (Coffey, 2001; Luginbuhl et al., 2000).
4. Mechanical: This method involves removing weeds by cutting, uprooting, or mowing. Weeds should be removed when they are still in the vegetative stage. Cosgrove and Doll (1996) found that one mowing in a growing season should be enough for managing annual weeds. For managing perennial weeds, multiple mowings in an interval that allows weeds to grow 8 to 12 inches tall are required until these are killed completely.
5. Chemical: This method involves the application of appropriate herbicides at a suitable stage of weed growth. Details on herbicide use for controlling pasture weeds is found at this link:http://www.aces.edu/anr/forages/Management/WeedControl.php.
6. Integrated management: This involves the application of more than one method mentioned above.
Weed management is very important for maintaining pasture productivity and lower production cost. One needs to be very watchful to identify the emerging weeds in pastures and apply an appropriate method or adopt the integrated management technique on time to avoid weed infestation. Whichever method is used, weeds should be controlled when they are young and still in the vegetative stage. Once seeds are dropped, more weeds will come up in the next season, resulting in a waste of money and time spent for weed control. If weeds are not controlled, they gradually take over a pasture since the grazer selects against them.
Rhodes, G.N., Jr., G. K. Breeden, G. Bates, and S. McElroy. 2005. Hay crop and pasture weed management. University of Tennessee Extension.
Coffey, L. 2001. Multispecies grazing. Appropriate Technology Transfer for Rural Areas (ATTRA). http://whatcom.wsu.edu/ag/documents/other_animals/MultispeciesGrazing.pdf Accessed on Jun 14, 2012.
Luginbuhl, J.M., J.T. Green, M.H.J. Poore, and A.P. Conrad. 2000. Use of goats to manage vegetation in cattle pastures in the Appalachian region of North Carolina. Sheep and Goat Research Journal 16:124-130.
Cosgrove, D. and J. Doll. 1996. Weed control in pastures without chemicals. Agronomy Advice. Cooperative Extension Program, University of Wisconsin, Madison. http://host.cals.wisc.edu/renzweedsci/1996/10/04/weed-control-in-pasture...
Contributor: Karki, U. Cooperative Extension Program, Tuskegee University, April 2011.