What you are seeing in your pasture is Parthenium ragweed (Parthenium hysterophorus L). It is also called False ragweed and Santa Maria feverfew. This is a difficult ragweed to control since it is tolerant of Gramoxone and there are many colonies showing resistance to Glyphosate. A warm weather annual, this weed reproduces only by seed. However, each plant can produce 3,000 to 45,000 seeds. Seeds germinate throughout the growing season and the plant will produce seeds whether it is mowed to 4 inches or allowed to grow to 2-3 feet.

In humans and livestock, parthenium ragweed contributes to contact dermatitis, skin irritation, nausea, giddiness, respiratory problems (bronchitis, asthma), and ingestion by livestock results in acute toxicity and often death.

This weed also has an allelopathic (a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms) reaction to many tree, crop and grass varieties.

The most effective herbicide used to date is Saflufenacil or Sharpen. From BASF, we learn, "Powered by Kixor herbicide technology, Sharpen herbicide drives burndown three to five times faster than 2, 4-D or glyphosate and offers broad-spectrum control of tough broadleaf weeds. Later in the season, Sharpen herbicide is a fast and complete harvest aid that allows growers to harvest on their schedule.". Used in conjunction with other pre and post-emergent herbicides you should be able to bring this weed to bay.

I will attach links that will give you more information.

https://www.cabi.org/isc/abstract/<personal data hidden>

If you have more questions or concerns, please contact Fort Bend County Master Gardeners at <personal data hidden>