The fall can be an appropriate time to manage certain weeds with a systemic herbicide in hayfields and pastures that have been mowed or grazed. Biennials such as common burdock, 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. So the end of August (if not too hot) into October can be favorable conditions for applying herbicides for weed control.

Keep in mind that with both biennial and perennials plants you need adequate leaf area to absorb the herbicide and be effective. 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-50s at a minimum. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications too. The more active the weeds are growing, the better the herbicide works so don’t wait too long.

Here are some strategies to keep in mind:

Look at your pastures and identify trouble spots. Determine if overstocking is contributing to the problem and consider adjusting your grazing management plan to match available forage. A lot of times we bring on the problem of weeds by our grazing management practices.  

Identify the weeds and 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 they are the dominant species, consider products labeled for brush control. Some products offer weed and brush control, or you can tank mix to reach the desired control.

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 inches tall and actively growing, and when soil moisture is adequate. The lowest labeled rates will be effective then. Treat weeds while they are actively growing, but before flowering and seed production. Remember you’ll need to increase herbicide rates as the plant matures. 

Instead of spraying consider mowing especially 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.

Always follow label directions for application and mixing. When ground broadcasting, apply the recommended herbicide rate in 10-20 gallons of total spray mixture per acre. For brush control, use at least 20 gallons per acre to ensure thorough coverage. For either weeds or brush, use the recommended rate of an ag surfactant to thoroughly wet the foliage. And pay attention to drift and consider a drift-control additive to reduce it.

Use herbicides with good soil residual activity carefully. They should not 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. Always read and observe all labels.

Marty Morgan is Cooke County Ag Agent for the Texas A&M AgriLife Extension Service. Reach him at 940-668-5412 or

Recommended for you