Early summer tends to be a primary time for weed control  in pastures, but fall can also be an appropriate time to manage certain weeds with a systemic herbicide in grass hay and pastures that have been mowed or grazed.
In particular, biennials such as common burdock and bull and musk thistles are much easier to kill while they are in the rosette stage of growth and prior to surviving a winter. (The same is true of the dandelions in your lawn.) Once these weeds awake in the spring, they grow rapidly with the goal of reproducing and it becomes more difficult to control them.
Thus, Sept. 1 into Oct. can be favorable conditions for applying weed control  herbicide. But keep in mind that with both biennial and perennials species, adequate leaf tissue must be present and it should be reasonably healthy to absorb the herbicide.
Favorable air temperatures should also be a consideration immediately before, during, and after application. In general, the warmer the better, with daytime high temperatures in the mid 50's at a minimum. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications. The more active the weeds are growing, the better the herbicide performance.
Additional strategies to keep in mind:
Inventory your pastures  for weedy trouble spots. Determine if overstocking is contributing to the problem and consider adjusting your grazing management plan to match available forage.
Identify the weeds of concern – then what will control them. Which herbicides you choose, and the recommended application rates, will vary by weed species and timing. For many weeds, a broad-spectrum herbicide with residual control will be the most cost-effective. If woody plants are also present, or are the dominant species, consider products labeled for brush control. Some products offer weed and brush control, or you can tankmix to reach the desired control spectrum. Once you’ve established what species you want to target, contact your applicator or ag chemical specialist for a specific product and timing recommendation .
Spray the right rate at the right time. Annual weeds in pastures are generally most susceptible early in the season, when they’re about 2” tall and actively growing, and when soil moisture is adequate. The lowest labeled rates will be effective then. A broad-spectrum herbicide with residual control at higher labeled rates will control weeds that germinate after spraying. Contact herbicides, such as 2,4-D, are effective only on emerged weeds and won’t effectively control weeds that sprout after application. Treat weeds while they are actively growing, but before flowering and seed production. Keep in mind that you’ll need to increase herbicide rates as the plants advance in their life cycle.
Consider mowing – not spraying – drought-stressed or mature weeds. Weeds without adequate moisture that aren’t actively growing will be difficult to control with herbicides. Don’t spray unless you’re willing to accept less control. Mowing biennial and perennial plants will set them up for fall treatment when they generate regrowth.
Follow label directions for application and mixing. For ground broadcast, apply the recommended herbicide rate in 10-20 gallons of total spray mixture per acre. For brush control, use at least 20 gallons/acre to ensure thorough coverage. For either weeds or brush, use the recommended rate of an ag surfactant to thoroughly wet the foliage. Consider a drift-control additive to reduce drift and improve deposition.
- Use herbicides with good soil residual activity carefully. They shouldn’t be used on cropland or land to be rotated to crops. Herbicide-treated grasses may, for a time, carry a residue that can be transferred to the soil by hay, livestock manure or urine. Be sure to read and observe all label precautions. Fine more information on rangeland and pasture weed control, weed identification, species-specific rate and timing recommendations here .
Here is a quick summary of common herbicide options:
2, 4-D ester 4E (1/2 to 1.5 pt/A) - 2, 4-D is a systemic herbicide that controls annual, biennial, and perennial broadleaves. The ester formulation is slightly more active than the amine and should not be used post--emergence if temperatures are greater than 80°F. 2, 4--D is commonly tank mixed with other herbicides (e.g., dicamba) to improve control and broaden weed spectrum. This is a good, inexpensive herbicide with limitations. It tends to be weak on a number of weed species including wild carrot, dock species, bedstraw, horsenettle, hemp, dogbane, common milkweed, pokeweed, brambles and most woody perennials. It has a 7 day grazing and 30 day haying restriction. Check product labels as some restrictions/uses vary.
Clarity 4S or Banvel 4S (0.5 to 4 pt/A) - Clarity/Banvel (dicamba) is a systemic herbicide that controls many annual and biennial broadleaf weeds and provides suppression or control of numerous perennials. Clarity/Banvel is commonly tank mixed with other herbicides (e.g., 2, 4-D) to improve control and broaden weed spectrum.
Overdrive 70WDG also contains dicamba in addition to diflufenzapyr (a synergist) and can be used in established grass stands (not seedlings) for control of numerous broadleaf weeds. The Overdrive use rate is 4 to 8 oz/A and can be tank mixed with numerous herbicides. Dicamba is fairly broad spectrum but tends to be weak on wild carrot, buttercup species, dandelion, milkweed, and bedstraw to name a few. At 1 to 2 pints/acre, dicamba has a 21 day grazing and 51 day haying restriction. Overdrive does not have any grazing or haying restrictions.
Cimarron 60DF (0.1 to 1 oz/A) - Cimarron (metsulfuron-methyl) is an ALS - inhibitor herbicide that controls many annual, biennial, and some perennial broadleaf weeds, depending on the rate used. It can be used in established warm or cool season grass stands. For most grass species, do not apply until one year after establishment (minimum of 6 months); timothy and fescue require a longer period. It is often tank mixed with 2, 4-D or dicamba to increase activity and weed control spectrum. This combination provides good control of weeds like Canada thistle, bull, musk, and plumeless thistle, and multiflora rose to name a few. COC or NIS must be included in the spray solution. Cimarron does not have any grazing or haying restrictions.
- Milestone 2L (3 to 7 fl oz/A) - Milestone (aminopyralid) is a newer active ingredient labeled for grass hay and pasture. Milestone controls many annual, biennial, and perennial broadleaf weeds and is effective on thistles (Canada, bull, musk, plumeless), burdock, dock species, bedstraw, horsenettle, knapweed, sowthistle, ironweed and others. It is less effective on wild carrot, hemp dogbane, common milkweed, and most brush species to name a few. Milestone can be tank-mixed with other herbicides and the addition of NIS is recommended to enhance activity. Milestone is non-volatile. ForeFront R&P 3L (1.5 to 2.6 pt/A) is a premix of aminopyralid plus 2,4-D that can also be used in grass hay and pasture to broaden the spectrum of activity. Milestone has no grazing or haying restrictions, while Forefront has a 7 day haying restriction. For both Milestone and ForeFront, special manure handling precautions are recommended to prevent injury to sensitive broadleaf plants (see label guidelines).
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