Early summer tends to be a primary time for weed controlin 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 forapplying 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.
(Source – http://beefmagazine.com/americancowman/pasture-and-range/0908-pasture-weed-control-tips)