A wet spring followed by drier summer weather has led to a flush of summer weeds for many farms. Late summer or fall can be a good time to get a handle on some of the harder to control biennial and perennial weeds commonly found throughout pastures and hayfields. If you haven’t already done so, it might not be a bad idea to scout your pastures and hayfields in search of problem weeds as we move through late summer and into the fall season. When it comes to weed control, timing of herbicide application is critical and it is important to spray when weeds are most susceptible to achieve maximum effectiveness.
Summer annuals like cocklebur, pigweed, spiny amaranth, ragweed, and lambsquarters will germinate in early summer and complete their full reproductive cycle in one growing season. Like all annual weeds, these species are best controlled during the seedling and early vegetative stage when they are young and actively growing. Herbicide applications will be more effective if made at this stage while they are still smaller and more susceptible and will prevent them from flowering and producing seed. By this time of year, these summer annuals are already fairly mature and any herbicide applications will likely be less effective. Instead, take note of any problem areas to keep an eye on next spring and utilize tools like mowing or targeted grazing to help minimize seed production from these species.
If weedy annual grasses like goosegrass, foxtail, panicum, and Japanese stiltgrass are problematic, pendimethalin (Prowl H2O) has a supplemental label that allows for its use on established perennial pastures or hayfields grown for grazing, green chop, silage, or hay production. This product may be applied to perennial grass stands or alfalfa-grass mixed stands. Prowl H2O may be applied as a single application in the early spring, or for more complete control it can be applied as a split application with the first application in early spring and the second application after first cutting (sequential applications must be 30 or more days apart). Research has shown that split applications are able to provide better control than a single, early season application. Keep in mind, pendimethalin is a pre-emergent herbicide, meaning it will only control weeds if applied prior to germination; by this time of year, these summer annual grasses have already germinated so Prowl H2O will not be effective and there are currently no herbicides labeled to control emerged weedy grasses in grass stands or alfalfa/grass mixes. Instead, focus on mowing or targeted grazing to help minimize seed production from these species now and take note of any problem areas to plan ahead for next spring.
Biennials live for two growing seasons, with the first year consisting of only vegetative growth as a seedling and rosette and the second year consisting of vegetative growth and also reproductive growth in the form of an elongated flower stalk. Common biennial species include burdock, bull thistle, musk thistle, and wild carrot. These species are best controlled during the seedling and rosette stage and should be treated while they are smaller and more susceptible before they begin to bolt. Fall is a great time to control these species when they are in the first year of their lifecycle; at this point they are smaller and still in the rosette stage of growth as they prepare to overwinter. Once spring arrives, they will grow and develop rapidly and it becomes more difficult to achieve effective control.
Late summer or fall is also a great time to control many common perennial species like horsenettle, smooth groundcherry, multiflora rose, and Canada thistle. At this time of year, these perennial plants are translocating sugars (energy) into their root system prior to winter to prepare for next spring’s regrowth. A fall application of systemic herbicides will result in translocation of the herbicide to the plant root system as well, allowing for better control.
Horsenettle in particular seems to be especially prevalent this year. This warm-season creeping perennial grows actively from May through September and is capable of thriving in hot, dry conditions and low fertility environments due to its deep root system. In addition to seed production, horsenettle is also able to sprout new shoots from its roots, allowing new plants to emerge throughout the summer. Due to its sharp prickles, livestock typically avoid grazing horsenettle, but it can cause irritation and injury to mouth and throat tissue if consumed and the plant can be toxic if ingested in high enough quantities. Mowing can impact the growth of horsenettle, but mowing alone is likely not enough to fully control this species. Mowing horsenettle once or twice during the growing season in combination with an effective herbicide application in late summer is likely the best strategy for control.
There are a number of herbicides available for control of broadleaf weeds in pastures and hayfields. Herbicide selection should be based on the type of forage and the weed species present. The most common herbicides used for control of broadleaf weeds in grass hay or pasture are the plant growth regulator herbicides, which includes products containing 2,4-D, dicamba, triclopyr, fluroxypyr, or a mix of these (see the table below for a list of common products). These products are safe if applied to established grass forages at the labeled rates but can kill or injure desirable broadleaf forages (i.e. clover, alfalfa) in grass-legume mixes. Also, remember that adequate green leaf tissue must be present for the targeted plant to effectively absorb the herbicide. Avoid applying herbicides immediately after mowing or if plants are no longer actively growing following a frost.
Note that if forages were recently seeded and are not yet established many of these herbicides can cause severe crop injury. Most herbicide labels for cool-season perennial grasses state that the grasses should be ‘well established’ with at least 4-5 inches of growth, although some labels are more restrictive than this. In addition, some of these herbicides have haying or grazing restrictions following application. Always read and follow the guidelines listed on the product label for proper rates, timing, residual effects, and any grazing or harvest restrictions following application.
Lastly, remember that while herbicides can be a useful tool for weed management in pastures and hayfields, they are not the only option for weed control. A program that integrates several different control strategies is generally more successful than relying on a single method. For maximum results, include cultural practices such as selecting adapted species and maintaining optimum soil pH and fertility, mechanical practices such as timely mowing or clipping to suppress weed seed production, and biological practices such as utilizing livestock for controlled grazing or browsing. Most importantly, remember that weeds are opportunistic; the ultimate strategy and number one form of weed control is competition with a healthy, dense stand of desirable forage species.
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Product1
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Active Ingredient(s)
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Application Rate2
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General/Restricted Use
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2,4-D
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2,4-D
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1 to 2 qt/A
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General
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Banvel/Clarity
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dicamba
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0.5 to 2 pt/A
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General
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Crossbow
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2,4-D + triclopyr
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1 to 6 qt/A
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General
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PastureGard HL
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triclopyr + fluroxypyr
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0.75 to 4 pt/A
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General
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Prowl H2O
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pendimethalin
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1.1 to 4.2 qt/A
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General
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Remedy Ultra 4L
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triclopyr
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0.5 to 4 pt/A
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General
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WeedMaster
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2,4-D + dicamba
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1 to 4 pt/A
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General
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1Always read and follow all guidelines listed on the product label
2For use in established grass pasture or hayfields
