Hi Isabeau,

Thank you for reaching out to us at the UNH Extension Infoline.

I am including a few links for you below that will go over various management methods you can reference to remove/control pesty weeds in the pasture. Please be sure to refer to the pesticide label to make sure that the product you choose is labeled to target sedge (perennial weeds). The following resources will include a few different options.

We are not able to recommend one specific product or company for any kind of retail purchases through UNH Extension. I also want to make sure you are aware of the chemicals in relation to your horse(s). I know we all want to make sure they are safe and healthy! You can certainly also triple check with your local grain store, they may sell specific products in house for weed control and can help you choose the best option for horse paddocks. 

Resources:
https://extension.unh.edu/resource/integrated-weed-management-pastures-hayfields-guide-landowners
https://equine.ca.uky.edu/news-story/weed-management-plans-horse-pastures-0#:~:text=Methods%20of%20removing%20horse%20pasture,poison%20hemlock%2C%20from%20the%20pasture.

Here is also an informative video from LSU AgCenter:

https://www.extension.purdue.edu/extmedia/TURF-46-W.pdf
https://extension.psu.edu/controlling-yellow-nutsedge-in-agronomic-crops-an-integrated-approach

I hope this information is helpful! Have a wonderful rest of your day!